Rhenium and ruthenium catalyzed processes involving organosilanes 2

Recent work done by various groups explored the catalytic capabilities of rhenium carbonyl complexes towards organic reactions such as C-C bond cleavage and R" R'" R' R MeReO3Oxidizing

CHAPTER 1

Introduction

Transition metal complexes are one of the most important compounds in synthetic chemistry due to their ability to catalyse many reactions Of these many reactions, organic transformations have been the focus of many research groups in the past decades Transition metals have incomplete d-shells which are easily accessed energetically, which allow them to exhibit a variety of oxidation states and coordination numbers This property makes these metal complexes ideal candidates for catalysts

A catalyst provides an alternative pathway with lower activation energy to accelerate a reaction In theory, it should also remain chemically unchanged at the end of the reaction Catalysis allows reactions to proceed efficiently and usually reduces the wastage of chemicals, thus providing atom economy in most cases Catalysis can be classified into two main groups which are homogeneous and heterogeneous catalysis In homogeneous catalysis, all substrates, including the catalyst, are in the same phase However, in heterogeneous catalysis, at least one of the substrates is not in the same phase

One of the most common classes of catalysts is organometallic compounds Organometallic compounds are defined as complexes containing metal-carbon bonds,

of which a group of such compounds are the metal-carbonyl complexes

Metal-carbonyl complexes are useful catalysts as the Metal-carbonyl ligand can be easily characterized using spectroscopic techniques such as Infrared and 13 C-NMR Both techniques allow the identification of the symmetry, arrangement of ligands, as well as the functional groups present in the complex

Metal-carbonyl complexes have distinct carbonyl stretching frequencies within the range of 1700 – 2100 cm -1 , which is a region usually free of most other molecular vibrations By monitoring the shifting of the carbonyl stretching frequencies, one is able to deduce if processes such as ligand association and dissociation have taken place, or if the symmetry of the complex has been changed IR spectroscopy is hence a very useful tool in the investigation of reaction intermediates, and eventually the mechanism, of a metal-carbonyl-catalyzed reaction

1.1 Organometallic Compounds in Catalysis

1.1.1 Organometallics

An organometallic compound is usually defined as a substance that contains carbon bonds Nowadays, this definition is often relaxed to include other non-carbon ligands such as nitrosyls, cyanates, hydrides and metals with ligands of groups 15 and

metal-16 1-3 Besides that, main group metals such as silicon, boron, germanium, and tellurium have also been included into the vast family of organometallics

Organometallics can be sub-divided into various categories such as catalysis, synthetic organometallic chemistry and bioorganometallic chemistry, with many

established works done in these fields Industrial processes such as the Suzuki Reaction, 4 Reppe Syntheses, 5 Fischer-Tropsch Reactions 6,7 the Sonogashira Reaction, 8 and olefin metathesis 9 are but a few examples of successful application using organometallic compounds as catalysis It is thus the focus of this work to concentrate on silane transformations catalyzed by rhenium and ruthenium organometallic complexes

1.1.2 Organo-Rhenium Complexes

In the periodic table, rhenium is the last natural occurring stable element to be found in 1925 by Noddack, Tacke and Berg 10 Of the variety of rhenium complexes that were synthesized, rhenium carbonyls make up a significant part of these known complexes Re(CO) 5 Cl, Re(CO) 5 Br, Re 2 (CO) 10 and CpRe(CO) 3 are but a few examples

of commercially available rhenium carbonyl complexes Due to their cost, many rhenium complexes have not been as well studied in terms of their catalytic abilities 11 However, catalytic reactions involving oxo-rhenium complexes (MeReO 3 ) 12,13 have been reported by a few research groups (Scheme 1)

Recent work done by various groups explored the catalytic capabilities of rhenium carbonyl complexes towards organic reactions such as C-C bond cleavage and

R" R'"

R' R

MeReO3Oxidizing agent

R" R'"

R' R

O

Scheme 1 Olefin Epoxidation using MeReO 3 as catalyst

formation reactions Kusama et al 14 reported the use of Re(CO) 5 Br as a catalyst towards Friedel-Crafts acylation (Scheme 2) In normal Friedel-Crafts reactions, the Lewis Acid is usually added in stoichiometric amounts with respect to the substrates However, with Re(CO) 5 Br as a catalyst, the reaction proceeded smoothly albeit with a mixture of regio-isomers It is only until recently that Kuninobu et al 15 reported the use of Re 2 (CO) 10 as catalyst for regioselective alkylation of phenols

Besides Friedel-Crafts reactions, reactions involving nucleophilic addition to unsaturated bonds were also explored Re(CO) 5 Br was reported 16 to be able to catalyze the condensation of aldehydes and activated methylene complexes with high stereoselectivity (Scheme 3) The stereoselectivity of the reaction decreases as unsymmetrical ketones are introduced as the reactant Besides addition onto carbonyl compounds, the nucleophilic addition onto C-C double and triple bonds were also reported 17

Of more interest are the reported work of Chen et al 18 and Kuninobu et al 19 on

C-H bond activation catalyzed by rhenium (I) carbonyl complexes The work by Chen et

CH3

+ Ph

OCl

Re(CO)5Br (10 mol %)reflux, 2hr

CH3Ph

O

Scheme 2 Friedel-Crafts acylation catalyzed by Re(CO) 5 Br

+Ph

al involving the coupling of an alkene with borane complexes (Scheme 4) has since then been modified by various groups using other transition metal catalysts; whereas the work by Kuninobu et al provided different insights to the reaction pathways and mechanistic studies of the insertion of unsaturated molecules into a C-H bond (Scheme 5) Other interesting reactions involving rhenium (I) catalysts like C-Si bond formation, 20 C-N bond formation 21 and C-O bond formation 22 were also reported All the reactions stated above show that rhenium (I) carbonyl complexes have huge potential in catalyzing a variety of interesting reactions

In this project, the following commercially-available, air-stable rhenium (I) compounds, Re(CO) 5 Br and Re 2 (CO) 10 were chosen to catalyze the hydrosilylation of

a wide range of carbonyls including aldehydes, ketones and esters The products formed from these reactions have wide applications in industrial processes like the production of polymeric materials 23 Rhenium (I) complexes in the form Re(CO) 5 X (X

= Br, Re(CO) 5 ) were hence chosen because it was reported to be capable of activating Si-H bonds However, their activity towards carbonyl hydrosilylation has not been fully explored 24

NH

tBu

Scheme 5 Insertion of unsaturated molecules into a C-H bond

-Due to their ability to accommodate different oxidation states and hence, different coordination number, organo-ruthenium complexes containing many different ligands such as phosphines, cyclopentadienyl, arenes and dienes have been synthesized and their applications towards catalytic transformations investigated 29 Among these transformations include bond activation and formation involving C-C, 30 C-H 31 and C- heteroatom 32 (Scheme 7), oxidation reactions 33 and hydrogenation 34

Ru

Br

Br Ru CO

CO

Ru Br THF

CO

OC Br

Ph3P

Si Et3

-Et3SiBr

Ru Br H

CO OC

Ph3P

Ru

Br

Br Ru CO

OC Br

Ph3P

PPh3

CO Br

CO

H2O

Ru Br H

CO

OC HO

Ph3P

H

Et3SiBr -H2

Ru Br Br

CO

OC

O H

Recent interest have been shown in synthesizing ruthenium complexes via oxidative

addition of the allyl halide onto Ru 3 (CO) 12 35 to obtain products of the form [Ru(3

-allyl)(CO) 3 X] (X = halide) Such complexes were reported to be useful catalysts for regioselective and steoreospecific allylation reactions under mild conditions 26 Modification of the ligands around the catalyst have also been done to better understand the mechanism of such allylation reactions towards nucleophilic attack 36

Besides allylruthenium complexes, polynuclear-ruthenium complexes have also been widely synthesized and their catalytic capabilities towards many organic reactions were extensively explored One of the most classic examples of a polynuclear-ruthenium catalysed reaction would be the Water Gas Shift Reaction, in which Ru 3 (CO) 12 was utilised as the catalyst 37 Other interesting reactions include the use of a ruthenium-phosphine dimer as catalyst for the hydrogenation of nitriles to the

Ph

CH3O

O

NMe2Si(OEt)3

R'SeSeR'+ 2 R" X

RuCl3

Zn, DMF, 60-100 °C 2 R'Se R"

Scheme 7 Examples of C-C, C-H, C-heteroatom bond activation

and formation catalyzed by organoruthenium complexes

corresponding amines 38 and also oxidation of alkanes to alcohols and ketones 39 Of

more recent interest would be the use of ruthenium dimers containing bridging halides

to catalyze the Kharasch reaction with high efficiency 40

As ruthenium metal has the ability to adopt a variety of oxidation states, many

modifications have been made to organo-ruthenium catalysts to further fine tune their

electronic and steric properties Ruthenium-NHCs 41 have recently been synthesized

and they were shown to be able to catalyze reactions such as Ring-Opening

Metathesis Polymerization, 42 and cyclopropanation reaction 41 (Scheme 8)

All the above reported organo-ruthenium catalyzed reactions showed the versatility

of ruthenium chemistry in the progression of various fields of research In this project,

the commercially available and air-stable ruthenium carbonyl, Ru 3 (CO) 12 has been

utilised as a catalyst in the investigation of its efficiency towards catalytic cross

coupling between silanes and other heteroatoms It was previously reported 43 that

ruthenium was able to catalyse the cross coupling between C-S, C-Se and C-Te

Scheme 8 Examples of ROMP and Cyclopropanation reaction catalyzed by Ru-NHCs

However, little has been done in this area to further understand the mechanism and applications of using ruthenium for cross coupling reactions

1.2 Organosilane Chemistry

Organosilane compounds contain mainly a carbon-silicon bond and its chemistry is similar to that of normal organic compounds containing carbon-carbon bonds But as the silicon atom being more electropositive, the carbon-silicon bond is actually polar This leads to certain distinctions from normal organic compounds, ie, nucleophilic attack usually occurs at the silicon atom, silicon-oxygen, silicon-nitrogen and silicon- halide bonds are stronger than their carbon-heteroatom counterpart, and addition to olefins occur in an anti-Markovnikov manner, with the ability to hydride transfer 44

Organosilanes are formed initially by reacting elemental silicon with chlorine gas

or hydrogen chloride Subsequently, reaction with alkali metal alkyl compounds (alkyl lithium, alkyl sodium etc), with Grignard reagents 45 or hydrosilylation of an olefin under catalytic conditions would yield the desired organosilane (Scheme 9)

RSiCl3

-SiCl4, -H2

+RMgCl-MgCl2

RHC=CH2catalyst

Scheme 9 Formation of organosilanes using different methods

Of these methods, the more popular method to derive the required organosilane is

to perform a Grignard reaction with a suitable Grignard reagent while hydrosilylation of olefins under catalytic conditions would afford very efficient yields Chloro derivatives of organosilanes are perhaps one of the most useful classes of organosilanes as they are frequently utilized as precursors for silicon-heteroatom compounds such as Si-N and Si-OH These silicon-heteroatom compounds have great application in industrial processes and one of the most important processes involves silicone manufacturing, which requires the reaction of water with chlorosilanes to form a silanol precursor 44

Of the many reactions involving organosilane compounds, the hydrosilylation reaction is the only reaction that provides the most application in inorganic, organometallic, organic, bioorganic, materials and polymer chemistry 46 Especially in organic chemistry, hydrosilylation provides the most efficient and direct route of introducing silyl functional groups into unsaturated substrates The resultant products are useful intermediates that can participate in a variety of synthetically valuable organic reactions, with some of them capable of participation in one-pot processes 47 Some of these organic reactions include the Tamao-Fleming protocol, 48 which involves the steoreospecific oxidation of an organosilane into an alcohol (Scheme 10), and the Hiyama coupling, 49 which involves the cross-coupling of organosilanes with organo halides to form carbon-carbon bonds (Scheme 11)

2

H2O2, KHF2DMF, r.t.

HBF4.Et2O m-CPBA , Et3N, Et2O H Si

Besides hydrosilylation reactions, cross-coupling reactions involving organosilanes also provide many useful applications in organic and bioorganic chemistry Transition metal-catalyzed silane hydrolysis and alcoholysis involving the reaction between organosilanes and water/alcohol produces silanols, silyl ethers and siloxanes, 50 which provide useful starting materials for the manufacture of pharmaceuticals 51 and also as excellent research examples for bioactivity 52 In addition, such reactions also provide a good source of hydrogen (Scheme 12), 50 which can be utilized in a variety of reactions, with the most classic example being the hydrogenation of unsaturated compounds catalyzed by palladium (II) 53 or iridium (I) 54 complexes

Besides silicon-oxygen coupling, silicon coupling with other heteroatoms such as sulfur and nitrogen also provide many uses Although organosilanes are useful

protecting groups for carbonyl compounds, organosulfur derivatives of these silanes provide more selectivity in protecting certain functional groups like aldehydes and ketones (Scheme 13) 55 However, the methods of synthesizing these compounds require harsh conditions or air-sensitive catalysts such as B(C 6 H 5 ) 3 (Scheme 14) 56 Also, there are cases whereby sulfur poisoning of the catalyst is a problem as it caused the drop in efficiency in the system, and eventually the catalytic yield of these organosulfur derivatives of silanes 57

Other than silicon-sulfur compounds, silicon-nitrogen compounds also provide many useful applications in organic chemistry The formation of iminosilanes 58a , silazanes 58b and diaminosilanes 58c has been reported recently by coupling silanes such

as trichlorosilane and trimethylsilane with various amino substrates like aminofluorosilanes Besides employing expensive or dificult to synthesize substrates, harsh reducing agents like butyl lithium were required However, these reactions were significant as they provide much insight into the role of amine ligands in processes such as alkene polymerisation 59 and alkene metathesis 60

1.3 Main Objectives

1.3.1 Catalytic hydrosilylation of carbonyls via Re(CO)5Cl photolysis

It was reported that the following commercially-available rhenium (I) complexes, Re(CO) 5 X (X = Br, Cl, Re(CO) 5 ) are capable of activating Si-H bonds However, their

O R

OSiR"3R

Scheme 13 Protecting groups containing a Silicon-Sulfur bond used

for protecting aldehydes and ketones

catalytic capabilities towards organosilane-related reactions such as hydrosilylation has not been explored 18 As it was also reported 19 that the air and moisture-stable Re(CO) 5 Cl dissociates a CO ligand readily upon photolysis (quantum yield of 0.06 to 0.44 from 366nm to 313nm), it shows that Re(CO) 5 X compounds have the capability

to generate a vacant site for ligand/substrate association

As it is desirable for any catalyst to be able to perform its catalytic task in the presence of air and moisture, it is the aim of this part of the project to photoactivate Re(CO) 5 Cl and other rhenium carbonyl derivatives in order to carry out hydrosilylation reactions on a variety of carbonyl substrates at room temperature The relative efficiency of the catalysts towards hydrosilylation of different carbonyls will also be investigated and identification of possible key catalytic intermediates will be done Eventually, the mechanism of the hydrosilylation reaction will also be elucidated using a combination of IR and NMR spectroscopy as well as computational studies

1.3.2 Ruthenium carbonyl-catalysed Si-X coupling (X = S, O, N)

In this part of the project, the following air-stable, commercially-available ruthenium carbonyl compound, Ru 3 (CO) 12 , will be used as a catalyst for the coupling reaction between silanes and various substrates such as thiols, amines and alcohols It was previously reported that Ru 3 (CO) 12 can be used to activate Si-H bonds 31 and in the process form dimeric ruthenium silyl species of the form (R 3 Si)Ru-Ru(SiR 3 ) The silanes used in this case range from chlorosilanes to alkylsilanes However, their catalytic capabilities were not explored Ru 3 (CO) 12 was also known to be able to

activate benzylic C-H bonds in the presence of silanes to afford benzylsilanes, 32 while silylation of silanols with vinylsilanes catalyzed by Ru 3 (CO) 12 to afford alkenes and siloxanes was also reported 33

It is hence the aim of this part of the project to utilize Ru 3 (CO) 12 as a coupling catalyst for Si-heteroatom (S, N and O) coupling The scope of the catalysis as well as the kinetics and detection of reactive intermediates will be investigated using FTIR, NMR and ESI Computational chemistry using the Gaussian suite of programs will also be employed for further elucidation of the reaction mechanism

1.4 References

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[2] Crabtree, R H The Organometallic Chemistry of the Transition Metals, 4 th ed.; Wiley-WCH, Weinheim, 2005

[3] Hunt, L B Platinum Metals Rev 1984, 28, 76

[4] Chemler, S R.; Trauner, D.; Danishefsky, S J Angew Chem., Int Ed Engl

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4980

[6] Maitlis, P M J Organomet Chem 2004, 689, 4366

[7] Herrmann, W A Angew Chem., Int Ed Engl 1982, 21, 117

[8] Ljungdahl, T.; Bennur, T.; Dallas, A.; Emtena H.; Martensson, J

Organometallics 2008, 27, 2490

[9] Hong, S H.; Sander, D P.; Lee, C W.; Grubbs, R H J Am Chem Soc 2005,

127, 17160

[10] Kuninobu, Y.; Takai, K Chem Rev 2011, 111, 1938

[11] Hua, R.; Jiang, J -L Maitlis, P M Curr Org Synth 2007, 4, 151

[12] Kühn, F E.; Santos, A M.; Herrmann, W A Dalton Trans 2005, 2483

[13] Espenson, J H Adv Inorg.Chem 2003, 54, 157

[14] Kusama, H.; Narasaka, K Bull Chem Soc Jpn 1995, 68, 2379

[15] Kuninobu, Y.; Matsuki, T.; Takai, K J Am Chem Soc 2009, 131, 9914

[16] Zuo, W -X.; Hua, R.; Qiu, X Synth Commun 2004, 34, 3219

[17] Kuninobu, Y.; Kawata, A.; Yudha, S S.; Takata, H.; Nishi, M.; Takai, K Pure

Appl Chem 2010, 82, 1491

[18] Chen, H.; Hartwig, J F Angew Chem Int Ed 1999, 38, 3391

[19] Kuninobu, Y.; Kawata, A.; Takai, K J Am Chem Soc 2005, 127, 13498

[20] Zhao, W -G.; Hua, R Eur J Org Chem 2006, 5495

[21] Ouh, L L.; Müller, T E.; Yan, Y K J Organomet Chem 2005, 690, 3774

[22] Hua, R.; Tian, X J J Org Chem 2004, 69, 5782

[23] Ojima, Y.; Yamaguchi, K.; Mizuno, N Adv Synth Catal 2009, 351, 1405

[24] Hua, R.; Jiang, J Cur Org Synth 2007, 4, 151

[25] Murahashi, S -I Ruthenium in Organic Synthesis, Wiley-VCH:Weinheim 2004

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2006, 10, 115

[27] Bruneau, C.; Dixneuf, P.H Ruthenium Catalysts and Fine Chemistry,

Springer: Berlin, 2004

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[29] Naota, T.; Takaya, H.; Murahashi, S -I Chem Rev 1998, 98, 2599

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[31] Trost, B M.; Toste, D.; Pinkerton, A B Chem Rev 2001, 101, 2067

[32] Zhao, X.; Yu, Z.; Yan, S.; Wu, S.; Liu, R.; He, W.; Wang, L J Org Chem

2005, 70, 7338

[33] Murahashi S -I.; Komiya, N.; Hayashi, Y.; Kumano, T Pure Appl Chem

[36] Mbaye, M.D.; Demerseman, B.; Renaud, J.-L.; Toupet, L.; Bruneau, C

Angew Chem Int Ed., 2003, 42, 5066

[37] Ford, P C Acc Chem Res 1981, 14, 37

[38] Grey, R A.; Pez, G P.; Wallo, A J Am Chem Soc.1981, 103, 7536

[39] Neumann, R.; Khenkin, A M.; Dahan, M Angew Chem., Int Ed Engl 1995,

34, 1587

[40] Quebatte, L.; Solari, E.; Scopelliti, R.; Severin, K Organometallics 2005, 24,

1404

[41] Delaude, L.; Demonceau, A.; Noels, A F Cur Org Chem 2006, 10, 203

[42] Maj, A M.; Delaude, L.; Demonceau, A.; Noels, A F J Organomet Chem

2007, 692, 3048

[43] Beletskaya, I P.; Ananikov, V P Chem Rev 2011, 111, 1596

[44] Auner, N.; Weis, J Organosilicon Chemistry IV, Wiley-VCH: Weinheim 2000

[45] Arkles, B Grignard Reagents and Silanes, reprinted from: Silverman, G.S.;

Rakita P.E Handbook of Grignard Reagents, Marcel Dekker: New York 1996

[46] Roy, A K Adv Organomet Chem 2008, 55, 1

[47] Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluć, P Hydrosilylation, A comprehensive review on recent advances, Springer Science+Business Media B V

2009

[48] Jones, G R.; Landais, Y Tetrahedron 1996, 55, 7599

[49] Hatanaka, Y.; Hiyama, T J Org Chem 1988, 53, 918

[50] Ison, E A.; Corbin, R A.; Abu-Omar, M M J Am Chem Soc 2005, 127,

11938

[51] Srimani, D.; Bej, A.; Sarkar, A J Org Chem 2010, 75, 4296

[52] Dai, X.; Strotman, N A.; Fu, G C J Am Chem Soc 2008, 130, 3302

[53] Tour, M J.; Cooper, J P.; Pendalwar, S L J Org Chem 1990, 55, 3452

[54] Wang, D W.; Wang, D S.; Chen, Q A.; Zhou, Y G Chem Eur J 2010, 16,

[57] Chauhan, B P S.; Boudjouk, P Tetrahedron Lett 2000, 41, 1127

[58] a) Jendras, M.; Klingebiel, U.; Niesmann, J Organosilicon Chemistry V, Wiley

VCH, 2000, 264; b) Abele, S.; Becker, G.; Eberle, U.; Oberprantacher, P.; Schwarz,

W Organosilicon Chemistry V, Wiley VCH, 2000, 270; c) Oprea, A.; Mantey, S.; Heinicke, J Organosilicon Chemistry V, Wiley VCH, 2000, 277

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While Berke et al 8 has reported the use of different rhenium (I) catalysts for the hydrosilylation of aldehydes and ketones, one of the more interesting processes is the CpFe(CO) 2 Me-catalysed reaction between silanes and dimethylformamide (DMF) reported by Pannell et al 14 As shown in Scheme 1, hydrosilylation across the C=O bond of DMF first afforded a silyl ether which further reacted with another molecule

of silane to give siloxane and trimethylamine as final products Cutler et al 15 has used

a manganese complex (CO) 5 MnC(O)CH 3 to reduce the much less reactive ester to ether via a silyl acetal intermediate Silyl esters are another important class of silicon- containing compounds as they find wide use in the production of polymeric materials 16 Mizuno et al 16 reported that a range of ruthenium catalysts enables the conversion of carboxylic acids to silyl esters to take place Furthermore, Yamamoto’s group showed that a boron catalyst, B(C 6 F 5 ) 3 , reduces carboxylic acids further to the corresponding alkanes as depicted in Scheme 1 17 However, most of the reported carbonyl hydrosilylation catalysts require careful handling as they are relatively unstable under ambient conditions

Although the following commercially-available rhenium (I) complexes, Re(CO) 5 X (X = Br, Cl, Re(CO) 5 ) have been reported to activate Si-H bonds, their activity towards carbonyl hydrosilylation has not been explored fully 18 It was previously reported 19 that the air and moisture-stable Re(CO) 5 Cl dissociates a CO ligand readily upon photolysis (quantum yield of 0.06 to 0.44 from 366nm to 313nm) In light of efforts in utilizing energy from light rather than from fossil fuel sources, much work would have to be carried out to further explore light-driven catalytic processes As it

is also desirable for the catalysts to be air and moisture-stable and easy to handle, we have photoactivated Re(CO) 5 Cl and other rhenium carbonyls in order to carry out hydrosilylation reactions on a variety of carbonyl substrates at room temperature The objectives of the work are to determine the relative efficiency of the catalysts towards different carbonyls, identify possible key catalytic intermediates and explore the mechanism of hydrosilylation using a combination of IR and NMR spectroscopy as well as computational studies

r.t., h 

2.2 Results and Discussion

2.2.1 Hydrosilylation of silanes with aldehydes and ketones

Preliminary experiments have been carried out to assess the efficiency of UV photolysis of Re(CO) 5 Cl towards carbonyl hydrosilylation using Et 3 SiH and acetone

as substrates (Table 1, Entry 1) The quantum yields for Re(CO) 5 Cl dissociation at

313 nm and 366 nm have been determined to be 0.44 and 0.06 respectively 19 The light source used in our work is broadband, ranging from 300nm to 800 nm Since Re(CO) 5 Cl absorbs in the 300-350 nm region, the quantum yield would also be expected in the similar range as quoted 1 H NMR analysis of the mixture after 4 hours

of photolysis showed isopropoxytriethylsilane, Et 3 SiOCH(CH 3 ) 2 (1a) as the sole

Figure 1 1 H NMR peaks of (1a)

product (Figure 1), with a turnover number determined to be 70 to 80 with respect to

Re(CO) 5 Cl (1% mol loading)

The reaction appears to proceed most efficiently if an excess of silane is used to achieve full conversion of acetone to the product A ratio of silane : carbonyl of 3:1 has been used for further comparison with other carbonyl substrates However the

silyl ether (1a) does not undergo further reaction with the excess silane Such is also

the case in the Re(CO) 5 Cl-catalysed reactions of Et 3 SiH with other carbonyl substrates (Table 1, Entries 2-6) When the mixture was heated from 30 to 120°C in the absence of light, the yield of the resultant silyl ether was less than 10% No hydrosilylation product was observed when the mixture was photolysed in air or upon removal of the rhenium catalyst

Re 2 (CO) 10 has been found to be an effective catalyst as well (Table 1, Entries 8-9) with similar TON ratio to Re(CO) 5 Cl However hydrosilylation proceeded only sluggishly when HRe(CO) 5 was used (Table 1, Entries 10-11) Compare to previously reported catalysts 7 such as [Re(CH 3 CN) 3 Br 2 (NO)] (TON = 99 for aldehydes/ketones), similar efficiency has been observed for Re(CO) 5 Cl However, hydrosilylation with

substrates such as 1,3-diketones, anthraquinones and acetylacetone did not occur even after extended photolysis of up to 12 hours

-1 )

OSiEt3 (1d)

O H

(1g)

24

When the substrate was changed from acetone to benzaldehyde, an increase in the amount of product formed over the same period was observed (Table 1, Entries 1 and 2) This can be attributed to the increased reactivity of aldehydes when compared to ketones When different silanes such as Ph 2 SiH 2 and Ph 3 SiH were used, a decrease in the corresponding silyl ether yield was observed (Table 2) The relative lower rates towards hydrosilylation of acetone appear to bear a strong relation to the steric hindrance of the silane In addition, when the carbonyl substrate was changed from aliphatic to a hindered aromatic carbonyl, i.e benzophenone, acetophenone and benzaldehyde, a decrease in the TOF was observed

2.2.2 Hydrosilylation of esters and carbonates

In the presence of Re(CO) 5 Cl, esters and carbonates have been reduced with silanes, although at a lower rate compare to the aldehydes Under our experimental conditions, the reaction of ethyl acetate and Et 3 SiH gave ethoxytriethylsilane as the main product with trace amount of diethyl ether Acetaldehyde has been detected as

an intermediate whereby its signal intensity decreases upon further photolysis while that of ethoxytriethylsilane continues to increase (Figure 2) Similar results have been obtained for methyl phenylacetate and methyl formate

Table 1 The reactions of Et 3 SiH with various aldehydes and ketones

[a] Product was obtained after 4hrs of photolysis in vacuo, with 1% mol loading of catalyst

Ratio of silane to carbonyl substrate is 3:1

Entry Substrate Silane Time Product TOF

[b] (hr - 1 ) [a]

H OSiPh3

(2a)

OSiPh2H H

O

H OSiMe2Ph

H

Figure 2 1 H NMR intermediate studies on the amount of intermediates and

products produced during the reaction of ethyl acetate and Et 3 SiH

The reaction of Et 3 SiH with diethyl carbonate afforded a variety of products with

Et 3 SiOEt (1) and EtOCH 2 OSiEt 3 (2) being the major ones A trace amount of ethyl formate EtOCHO (3) has been detected When the reaction mixture was left overnight,

1

H NMR peaks belonging to (EtO) 2 CH 2 (4) were detected along with a decrease in (1) and (2)

2.2.3 Comparison of hydrosilylation rates for different carbonyls

By comparing the ratio of the products within the same photolysis period, the following general relationship has been determined for the relative rate using Re(CO) 5 Cl and Et 3 SiH : Aliphatic Aldehydes > Aromatic Aldehydes > Aliphatic Ketones > Aromatic ketones ~ Esters (Table 3) Comparing within each functional group, the alkyl analogues are more reactive than their aryl counterparts Using pyruvic acid which has both C=O and O-H groups, it was found that Et 3 SiH preferentially reacted with the O-H group to produce the silyl ester

Entry Substrate Product Ratio [a],[b]

Table 3 Ratio of hydrosilylation products formed Relative rates were determined by adding 3:1:1 ratio of Et 3 SiH with various carbonyl substrates and acetone

[a] Ratio with respect to Acetone

[b] Yield calculated on the basis of the integration of the 1H NMR spectra using toluene as standard

OSiEt3

OSiEt3

OSiEt3

OSiEt3Ph

OSiEt3Ph

OSiEt3Ph Ph

OSiEt3

OSiEt3O

O

Et 3 SiH

EtO

OSiEt 3 H

(2) (1)

(3) (4)

(5)

EtO

O H

2

Step 1

Step 2

+

Scheme 2: Proposed reaction scheme of the step-wise reaction

silane However our focus is on the addition reaction, hence the complex reaction pathways for the diethylcarbonate case are highlighted as an example (Scheme 2)

The products of the reaction between Et 3 SiH and diethylcarbonate were observed and identified in 1 H NMR (Experimental Section) and were attributed to triethylethoxysilane (1), ethylformate (2), ethoxymethoxytriethylsilane (3), diethoxymethane (4) and hexaethylsiloxane (5) The formation of these products led us

to postulate the following mechanism: An unstable silylacetal intermediate is first

formed, which undergoes ‘condensation’ to afford (1) and (2) With excess silane, the

ethylformate (2) can be further reduced to (3) This explains the relatively little

amount of formate left in the mixture In addition, the experimental data also

suggested that compound (1) undergoes further reaction with (2) to generate the ether (4) and siloxane (5)

As one of the main objectives of this work is to elucidate the mechanism of hydrosilylation, FTIR spectroscopy has been utilised for the detection of any metal carbonyl intermediates present in the catalytic mixture Upon photolysis of Et 3 SiH

with Re(CO) 5 Cl, a dimeric rhenium carbonyl species with a bridging hydride

(henceforth known as complex (A)) has been identified in the mixture (Figure 3) The

identification of the complex was based on the IR spectral resemblance to rhenium complexes 20 of formula HRe 2 (CO) 9 (SiR 3 ) Its bridging hydride signal at -9.03ppm has also been recorded in the 1 H NMR spectrum The diphenylsilyl analogue, complex (B)

has also been prepared upon Re(CO) 5 Cl photolysis in the presence of diphenylsilane

Other than (A), another hydride signal at -5.77 ppm found in the reaction mixture has

been attributed to HRe(CO) 5

The formation of complex (A) can be rationalised as follows (Scheme 3) Upon UV

photolysis of Re(CO) 5 Cl, a CO ligand dissociates to form the 16-electron Re(CO) 4 Cl intermediate followed by Et 3 SiH coordination Upon reductive elimination, either HCl

or Et 3 SiCl would be formed together with the corresponding Et 3 SiRe(CO) 4 or HRe(CO) 4 species A free CO molecule coordinates back to HRe(CO) 4 to give

Figure 3 IR and 1 H Hydride NMR Spectrum of (A) obtained from photolysis of Re(CO) 5 Cl with Et 3 SiH

IR peaks (cm -1 ): 2102, 2094, 2085, 2030, 2026, 2020, 2007, 2000, 1978

Lit Values 13 for HRe 2 (CO) 9 SiCl 3 (cm -1 ): 2150, 2095, 2085, 2047, 2019, 2012, 1999, 1978

1 H NMR peaks (ppm): -9.03

1950 1965 1980 1995 2010 2025 2040 2055 2070 2085 2100

2115

1/ c m -0

Re H Re CO

CO OC

CO

CO

SiEt3CO CO

SiEt3CO CO

Re OC CO

H CO CO

CO OC

CO

CO

SiEt3CO CO

SiEt3CO CO

Re OC CO

H CO CO

When (A) was isolated and tested for aldehyde hydrosilylation, the silylether was

generated about two to three times faster compared to Re(CO) 5 Cl UV irradiation is

still essential for the catalysis to occur The IR signals of (A) persisted even after the end of catalysis, with a recovery of about 70-80%

Interestingly, complex (A) can also be generated upon Re 2 (CO) 10 photolysis in triethylsilane, which would explain the similarity in the reaction rate to Re(CO) 5 Cl

From these observations, there are reasons to believe that complex (A) acts as a

resting state in carbonyl hydrosilylation One of the crucial steps in the catalysis

involves the photocleavage of (A) to form HRe(CO) 5 and a 16-electron rhenium

carbonyl Et 3 SiRe(CO) 4 As it was shown earlier that catalysis with HRe(CO) 5 is sluggish, we believe that the rhenium silyl species is most likely the active catalytic species instead

A mechanism for the hydrosilylation of carbonyl compounds is proposed to account

for the experimental observations (Scheme 4) Upon photolysis, (A) dissociates to

afford Et 3 SiRe(CO) 4 Then the carbonyl substrate undergoes coordination onto the vacant site and facilitates the silyl ligand shift onto the oxygen atom This process results in the formation of an alkyl ligand bound to the Re centre (Steps 1-2) Another silane undergoes coordination via a 2

silyl-complex 4,7 or a sigma (σ H ) silyl-complex

(Step 3) The H atom migrates from the silane to the alkyl group, thus regenerating the catalyst and releasing the silyl ether product (Step 4) When either the carbonyl or silane has been depleted, the R 3 SiRe(CO) 4 would coordinate back to HRe(CO) 5 and

becomes part of the resting state (A)

The activity of (A) in the catalytic cycle can be tested by using its Ph 2 SiH 2

analogue, (B) to catalyse the hydrosilylation of acetone and Et 3 SiH Upon completion

of catalysis, 1 H NMR analysis showed the presence of iPrO(SiEt 3 ) as the expected main product with a small amount of iPrO(SiHPh 2 ) More importantly, the IR

spectrum has changed from that of (B) to (A) These observations show that silyl exchange has occurred between (B) and the free Et 3 SiH during catalysis and lent

support to (A) participating in the catalysis (Scheme 5)

H

Re CO

CO OC

CO OC

H Re

Re OC

CO

SiEt3CO O

H

CO R

Re OC

Re OC

H SiEt3

H SiEt3

Re OC

CO

SiPh2H CO

- iPrOSiPh2H Re

OC

CO

SiEt3

CO CO

Scheme 5: Silane exchange from (B) to (A) during reaction