Hydrosilylations dienes

Precious insights into a reaction mechanism can be collected through the isolation of reactive intermediates. While no reaction occurs between 1-octene (2) and bis(trimethylsilyloxy)methylsilane (3) m the absence of a suitable catalyst, mixtures of (NHC)Pt(dvtms) 15 and 1-octene (2) are unreactive even under harsh conditions (G. Berthon-Gelloz, S. Dierick, and I.E. Mark6, unpublished results). However, treatment of (ICy)Pt(dvtms) 15c with five equivalents of MD M (3), in hot and degassed toluene yields the dimeric complex 25 and the hydrosilylated diene 26 (Scheme 5.7) [16]. The structure of 25 was... 

One may envisage polymerizations analogous to the thiol-enc process using other bis- or multi transfer agents (e.g. radical-induced hydrosilylation between bis-silanes and dienes). However, none has been described or achieved significance. 

Hydrosilylation by Ziegler-type catalyst systems [e.g., Ni(acac)2/AlEt3] has been examined for the reaction of 1-octene with EtjSiH in benzene 178). Complications include competing isomerization and reduction to metal however, 1,3-dienes or terminal acetylenes are readily hydrosilylated withRC i CH, the major product is CH2 CR. CRiCHSiXj. 

Rhodium Atom-Derived Catalysts in the Hydroformylation of 1,3-Dienes and in the Hydrosilylation of Aromatic Nitriles... 

We describe here (i) the selective hydroformylation of 1,3-dienes to p,y-unsaturated aldehydes promoted by catalyst A and (ii) the efficient hydrosilylation of aromatic nitriles to A,iV-disilylamines using catalysts B and C. 

Asymmetric HydrosilYtation of Dienes 85 Tab. 3-2 Asymmetric Hydrosilylation of 1,3-Cyclohexadiene (42)... 

Hydrosilylation of dienes accompanied by cyclization is emerging as a potential route to the synthesis of functionalized carbocycles. However, the utility of cycliza-tion/hydrosilylation has been Umited because of the absence of an asymmetric protocol. One example of asymmetric cycUzation/hydrosilylation has been reported very recently using a chiral pyridine-oxazoUne ligand instead of 1,10-phenanthroline of the cationic palladium complex (53) [60]. As shown in Scheme 3-21, the pyridine-oxazoUne Ugand is more effective than the bisoxazoUne ligand in this asymmetric cyclization/hydrosilylation of a 1,6-diene. 

The reductive coupling of of dienes containing amine groups in the backbones allows for the production of alkaloid skeletons in relatively few steps [36,46,47]. Epilupinine 80 was formed in 51% yield after oxidation by treatment of the tertiary amine 81 with PhMeSiEh in the presence of catalytic 70 [46]. Notably, none of the trans isomer was observed in the product mixture (Eq. 11). The Cp fuMcTIIF was found to catalyze cyclization of unsubstituted allyl amine 82 to provide 83. This reaction proceeded in shorter time and with increased yield relative to the same reaction with 70 (Eq. 12) [47]. Substitution of either alkene prevented cyclization, possibly due to competitive intramolecular stabilization of the metal by nitrogen preventing coordination of the substituted olefin, and resulted in hydrosilylation of the less substituted olefin. 

One example of asymmetric diene silylation was reported using the binaphthalenediol-based yttrocene catalyst (Fig. 3) [49]. A variety of 1,5- and 1,6-dienes were cyclized in 70-95% yields, but with < 5-50% ees. Due to the slower cyclization of 1,6-diene substrates, PhMeSiH2 was in place of PhSiH3 to prevent hydrosilylation of the olefins. 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

It has been shown that hydrosilylation may not perform as ideally as is required when preparing co-olefinic silicone compounds from organic a,co-dienes and hydrosil(ox)anes isomerization is a concern and the chemical equivalence of the double bonds requires a large excess of the diene compound to achieve essentially monohydrosilylation. Further side reactions are discussed by Torres et al [9],... 

The Pd/MOP combination has proved active for the asymmetric hydrosilylation of cyclic alkenes and dienes. Thus treatment of norbornene with HSiCl3 at 0 °C for 24 h in the presence of 0.01 mol.% of M eO-MOIV[Pd(// -C3H5)Cl]2 gave quantitative yield of evo-2-(trichlorosilyl)norbornane oxidation produced the corresponding alcohol in 93% ee (Equation (12)). Lowering the temperature (to — 20 °C) increased this to a 96% ee. Both mono- and difunctionalization of nbd has proved possible, depending upon the quantity of trichlorosilane used (Scheme 22). In both reactions, extremely good enantioselectivities are observed 113... 

Widenhoefer has also disclosed an interesting extension consisting of hydrosilylative cyclization of a diene catalyzed by palladium. High enantioselectivity (up to 95% ee) was achieved by using palladium catalysts with Ci-symmetric pyridine-oxazoline ligands351,364 and recent mechanistic studies have confirmed the involvement of an intramolecular carbometallation step.365... 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

To date, the reductive cyclization of allenic alkenes remains undeveloped. However, the reductive cyclization of activated alkene partners in the form of 1,3-dienes and conjugated enones has been achieved using late transition metal catalysts. Indeed, the hydrosilylative dimerization of 1,3-dienes reported in 1969 appears to be the first reductive... 

C-C bond formation mediated by silane.6,6a 6f With respect to the development of intramolecular variants, these seminal studies lay fallow until 1990, at which point the palladium- and nickel-catalyzed reductive cyclization of tethered 1,3-dienes mediated by silane was disclosed. As demonstrated by the hydrosilylation-cyclization of 1,3,8,10-tetraene 21a, the /rarcr-divinylcyclopentanes 21b and 21c are produced in excellent yield, but with modest stereoselectivity.46 Bu3SnH was shown to participate in an analogous cyclization.46 Isotopic labeling and crossover experiments provide evidence against a mechanism involving initial diene hydrosilylation. Rather, the collective data corroborate a mechanism involving oxidative coupling of the diene followed by silane activation (Scheme 15). 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

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