Rhodium silyl complexes

In 1998, Mitchell and Tilley described the synthesis of a new rhodium silyl complex <19980M2912>. Reaction of (Me3P)3RhCl with (THF)2LiSiHMes2 (Mes = 2,4,6-trimethylphenyl) in toluene resulted in formation of a light yellow solution, from which colorless crystals of compound 5 were isolated after work-up. The presence of both Rh-H (5 =—9.90) and Si-H (5 = 5.76) resonances in the NMR spectrum and five separate methyl signals suggested the structure 5 shown in Equation (31). 

Rhodium silyl complexes have also been investigated over the review period. As previously described in Section 4.19.9.4, Mitchell and Tilley have reported the reaction of (Me3P)3-RhCl with (THF)2SiHMes2 (Mes = 2,4,6-trimethylphenyl) in toluene to form the metallated species (Equation 31) <19980M2912>. 

Direct alkyne insertion into a Rh—Si bond has been observed for the intermediate rhodium silyl complex (dtbpm) Rh[Si(OEt)3] (PMe3) [dtbpm = di(ferf-butyl)phosphino methane] in the hydrosilylation of 2-butyne with triethoxysilane catalyzed by the rhodium alkyl complex (dtbpm)RhMe(PMc3). The crystal structure of (dtbpm)Rh[Si(OEt)3j (PMes) shows that the coordination around the Rh metal is planar with a Rh—Si bond length [2.325(2) A] similar to that found for the complex (Me3P)3RhH(C6F5) Si(OEt)3 (Table ll) . The proposed mechanism for the hydrosilylation reaction of 2-butyne with HSi(OEt)3 yielding mainly the E-isomer of MeCH=C(Me)Si(OEt)3 is outlined in Scheme 36. 

The mechanistic and synthetic puzzle of alkyne hydrosilylation opened more fully with the discovery that rhodium will catalyze the /r.mr-hydrosilylation of terminal alkynes.22 There is much work extant in this area, and good summaries of the various catalytic systems exist.11 A trans-addition process to give (Z)-j3-silane products G is well precedented with trialkylsilanes (Table 3), for both rhodium and mixed rhodium-cobalt complexes (entry 4).22,26 However, the selectivity erodes significantly upon switching to Me2PhSiH (entry 5), and, due to the mechanistic requirements for equilibration of the /3-silyl vinylrhodium intermediate, electron-poor silanes react exclusively to give CE)-/3-silane products B (see entries 6 and 7). 

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]. 

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/silylformylation of enynes that invokes several of the same intermediates proposed for the rhodium-catalyzed cyclization/hydrosilylation of enynes (Scheme 7). Silylmetallation of the G=G bond of the enyne followed by / -migratory insertion of the pendant G=G bond into the resulting Rh-G bond could form rhodium cyclopentyl complex Illf. a-Migratory insertion of GO into the Rh-G bond of Illf followed by silane-promoted reductive elimination from the resulting rhodium formyl complex rVf could release the silylated cyclopentane carboxaldehyde with regeneration of silylrhodium hydride complex If (Scheme 7). 

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

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. ... 

It has been reported that the chiral NMR shift reagent Eu(DPPM), represented by structure 19, catalyzes the Mukaiyama-type aldol condensation of a ketene silyl acetal with enantiose-lectivity of up to 48% ee (Scheme 8B1.13) [29-32]. The chiral alkoxyaluminum complex 20 [33] and the rhodium-phosphine complex 21 [34] under hydrogen atmosphere are also used in the asymmetric aldol reaction of ketene silyl acetals (Scheme 8BI. 14), although the catalyst TON is quite low for the former complex. 

By contrast the silyl complexes are important catalysts in a variety of hydrosilylation reactions.20 They can be prepared by the similar oxidative addition of hydrosilanes to rhodium(I) complexes, either directly or in solution (equation 189).935 However, in the presence of both additional base and triphenylphosphine a hydridorhodium(I) complex results (equation 190).936 Alternatively in the presence of a large excess of hydrosilane the monohydrido complex is transformed into a dihydrido complex.922... 

The catalytic hydrosilylation of 1-aza- and 1,4-diaza-1,3-dienes R N=CH-CH=CHR [R = Me,Ph R = McjCH, McjC, (Me2CH)2CH] and R N=CH-CH=NR were performed with rhodium(I) complexes. Under mild conditions N-silylated enamines, such as R N(SiR )CH=CHCH2R or R NCH=CHN(SiR )RS were formed as the main products. ... 

The steps involved are (I) oxidative addition of hydrosilane to the rhodium(I) complex (5) to give 6 (ii) insertion of the carbonyl into the resulting silicon-rhodium bond of 6 to form diastereomeric a-siloxyalkylrhodium hydride intermediate 8, and (3) formation of an optically active silyl ether of sec-alcohol by reductive elimination ... 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetals (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table 6). The study of Matsuda and coworkers suggests that use of the rhodium complex Rlu(CO)i2 (39 at 2 mol %) in benzene at 100 C gives best results for the formation of adduct (38 Table 6, entries 1-7). There is negligible diastereoselectivity in most cases. Various cationic ihodium complexes such as (40) also catalyze the reaction. Reetz and Vougioukas have found that this aldol reaction proceeds well with the more reactive ketene silyl acetals, (37) for R = OMe or OEt, in CH2CI2 at room temperature (Table 6, entries 8-13). The intermediacy of an ti -O-bound rhodium enolate, such as (41), in the catalytic cycle is like-... 

The four most common methods for the synthesis of late transition metal enolates are oxidative addition to halocarbonyl compoxmds, ligand metathesis with main group enolates or silyl enol ethers, nucleophilic addition of anionic metal complexes to halocarbonyl electrophiles, and insertion of an a,3-imsaturated carbonyl compoimd into a metal hydride. Examples of these synthetic routes are shown in Equation 3.47-Equation 3.50. Equation 3.47 shows the synthesis of a palladium enolate complex by oxidative addition of ClCHjC(0)CHj to Pd(PPh3), Equation 3.48 shows the synthesis of a palladium enolate complex by the addition of a potassium enolate to an aryl Pd(II) halide complex, and Equation 3.49 shows the synthesis of the C-bound W(II) enolate complex in Figure 3.7 by the addition of Na[( n -C5R5)(CO)jW] to the a-halocarbonyl compound. Finally, Equation 3.50 shows the synthesis of a rhodium enolate complex by insertion of but-l-en-3-one into a rhodium hydride. This last route has also been used to prepare enolates as intermediates in reductive aldol processes. - ... 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

A few studies of isolated metal-silyl complexes and ttie computational study of rhodium-sUyl complexes illustrate the insertion of olefins into metal-silicon bonds. Wrighton studied the photochemical reaction of iron-silyl complexes witti ettiylene (Scheme 9.13). Photolysis of Cp FefCOl fSiMej) in the presence of ettiylene forms Cp Fe(CO)(CjHJ(SiMej). This complex appears to insert ethylene, but ttie 16-electron insertion product is unstable and forms the corresponding vinylsilane and iron hydride complexes as products. Photolysis of Cp Fe(CO)j(SiMe3) in the presence of ethylene and CO forms ttie p-silylaDcyl complex containing two CO ligands. 

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