Carbonyl complexes, hydrosilylation

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

Suisse and co-workers have studied the asymmetric cyclization/silylformylation of enynes employing catalytic mixtures of a rhodium(i) carbonyl complex and a chiral, non-racemic phosphine ligand. Unfortunately, only modest enantioselectivities were realized.For example, reaction of diethyl allylpropargylmalonate with dimethylphenyl-silane (1.2 equiv.) catalyzed by a 1 1 mixture of Rh(acac)(GO)2 and (i )-BINAP in toluene at 70 °G for 15 h under GO (20 bar) led to 90% conversion to form a 15 1 mixture of cyclization/silylformylation product 67 and cyclization/ hydrosilylation product 68. Aldehyde 67 was formed with 27% ee (Equation (46)). 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

Studies on the immobilization of Pt-based hydrosilylation catalysts have resulted in the development of polymer-supported Pt catalysts that exhibit high hydrosilylation and low isomerization activity, high selectivity, and stability in solventless alkene hydrosilylation at room temperature.627 Results with Rh(I) and Pt(II) complexes supported on polyamides628 and Mn-based carbonyl complexes immobilized on aminated poly(siloxane) have also been published.629 A supported Pt-Pd bimetallic colloid containing Pd as the core metal with Pt on the surface showed a remarkable shift in activity in the hydrosilylation of 1-octene.630... 

The cobalt carbonyl complex is also an effective catalyst for the siloxy-methylation of aromatic aldehydes.110 Arylethane-l,2-diol disilylethers are obtained in good yields, resulting from incorporation of one molecule of CO and two molecules of HSiR3. Good selectivity for the siloxymethylation product is observed at 0°C in hexane. At 15°C, faster reaction rates are observed, but the selectivity for the CO-incorporated product is lower. In contrast, aliphatic aldehydes react under these conditions (1 atm CO, 0°C) to give only a small amount of CO-incorporated product, with a major product resulting from hydrosilylation. 

An excess of alkene favored the formation of the unsaturated product 44 whereas the silylated compound 45 dominated athigher silane to alkene ratios. The described reaction was also carried out in the presence of colloidal iron and led to results similar to those obtained with Fe(CO)5. Hydrosilylation of vinyltrimethylsilane 46 with chlorosilanes R3SiH using the modified iron carbonyl complex (CH2=CHSiMe3)Fe(CO)4 [44, 45]... 

The reductive elimination process that yields the hydrosilylation product has been recently observed in the decomposition of cis-alkyl (trimethylsilyl)iron carbonyl complexes 133 (212) (eq. [46]). To account for the overall retention of configuration at silicon the hydrosilylation process, the last step should occur with retention. Although not demonstrated, this is very likely, since reductive elimination of hydrosilanes in silyl hydride transition metal complexes occurs with retention of configuration (211,213). 

The proposed intermediacy of [Fe(CO)3], coupled with the high quantum efficiency observed with most mononuclear metal carbonyl complexes [54], inspired Schroeder and Wrighton to study photocatalytic olefin isomerization, hydrogenation and hydrosilylation reactions with Fe(CO)s [55], Irradiation of Fe(CO)s with near ultraviolet (Amax = 366nm) lightin the presence of excess 1-pentene furnished the thermodynamic mixture of cis and trons-2-pentene over the course of 2 h. From photolysis studies in neat olefin, it was estimated that each iron center produced... 

Ruthenium dihydride complex [Ru(H)2(CO)(PPh3)3], activated by treatment with st3rrene, catalyzes several hydrosilylation polymerization processes, such as the reaction of tereftaldehyde with 1,3-tetramethyldisiloxane and hydrosilylation of poly(l-hydrido-l,3,3,5,5-pentamethyltrisiloxane) with benzophenone (221,222). [Ru3(CO)i2] was found catalytically active in the hydrosilylation of acetophenone with HSi(OEt)3 (222). The same cluster and several ruthenium carbonyl complexes efficiently catalyze the reduction of linear and cyclic amides with trisubstituted silanes to give the corresponding amines (224). Activated diruthenium and triruthenium carbonyls catalyze the hydrosilylation of ketones, as well as that of aldehydes wdth different silanes (225). First-generation Grubbs... 

Duckett and Perutz have shown the stoichiometric reaction of the CpRh(C2H4)(SiR3)H (R = Et, i-Pr) complexes (Scheme 33) . These complexes have been found to act as precursors to the catalytically active species for the hydrosilylation of ethene with EtsSiH but are not within the catalytic cycle. The mechanism proposed in Scheme 34 for the hydrosilylation of ethene was found to be equivalent to the Seitz-Wrighton hydrosilylation mechanism catalyzed by cobalt carbonyls complexes . ... 

Piers examined the equilibria between B and B-carbonyl complexes in a study of C=0 hydrosilylation (Fig. 3c, X = O, Y = H, Me, OEt). Equilibrium constants were found to favor the carbonyl complex 3 by about 10 depending on the nature of Y [19]. However, the rates of the hydrosilylation reaction were inversely proportional to the carbonyl concentration, suggesting that complexes such as 3 inhibit the reaction. These observations led to a mechanistic interpretation involving the reversible formation of a borohydride complex 2, which is the active species in the subsequent reductive silylation process via 4 or 5, shown for carbonyl reduction (Fig. 4). Analogous processes have been invoked for the conversion of aldehydes/ketones to alkanes [20], alcohols to alkoxysilanes [21], and then to alkanes [22], and the hydrosilylation of C=C bonds [23]. 

Royo et al. prepared the NHC-Fe carbonyl complexes 183 by reaction of corresponding imidazolium salts with [Fe3(CO)j2] and tested them in the hydrosilylation of benzaldehyde and acetophenone (Figure 13.23). Related NHC-Fe carbonyl complexes were also tested in the selective hydrosilylation of esters to give aldehydes. With complex [(IMes)Fe(CO)4] in the presence of Et2SiH2 (1.1 equiv.), both activity and selectivity were enhanced, full conversion was observed after 3 h at room temperature, and only the aldehyde product was detected. 

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Fig. 2.10 Hydrosilylation catalysts of carbonyl compounds based on Cu-NHC complexes...

These complexes anchored to a solid via a ligand have been tested for a number of reactions including the hydrogenation, hydroformylation, hydrosilylation, isomerization, dimerization, oligomerization, and polymerization of olefins carbonylation of methanol the water gas shift reaction and various oxidation and hydrolysis reactions (see later for some examples). In most cases, the characterization of the supported entities is very limited the surface reactions are often described on the basis of well-known chemistry, confirmed in some cases by spectroscopic data and elemental analysis. 

One or both carbonyls in /3-diketones can be reduced, as well as the carbonyl function in acyl cyanides (210). Similar treatment of a,/3-unsat-urated ketones and aldehydes can lead to the saturated carbonyl products via selective reduction of the olefinic bond (207, 208, 210) see Eq. (51) in Section III,A,4. Some terpenes (a- and /3-ionone, pulegone) were reduced in this way (208). Platinum(II) phosphine complexes have been used for the hydrosilylation of saturated ketones and could be used for the reduction (211). 

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

Ojima and co-workers first reported the RhCl(PPh2)3-catalyzed hydrosilylation of carbonyl-containing compounds to silyl ethers in 1972.164 Since that time, a number of transition metal complexes have been investigated for activity in the system, and transition metal catalysis is now a well-established route for the reduction of ketones and aldehydes.9 Some of the advances in this area include the development of manganese,165 molybdenum,166 and ruthenium167 complex catalysts, and work by the Buchwald and Cutler groups toward extension of the system to hydrosilylations of ester substrates.168... 

---