Diene complexes hydrosilylation

Ferrocenyl ligands, via zinc reagents, 9, 120 Ferrocenylmethyl phosphonium salts, with gold(I), 2, 274 Ferrocenylmonophosphine, in styrene asymmetric hydrosilylation, 10, 817 Ferrocenyl oxazolines, synthesis, 6, 202 Ferrocenylphosphines with chromium carbonyls, 5, 219 in 1,3-diene asymmetric hydrosilylation, 10, 824-826 preparation, 6, 197 various complexes, 6, 201 Ferrocenylselenolates, preparation, 6, 188 Ferrocenyl-substituted anthracenes, preparation, 6, 189 Ferrocenyl terpyridyl compounds, phenyl-spaced, preparation 6, 188 Ferrocifens... 

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. 

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. 

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. 

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... 

The asymmetric cyclization-hydrosilylation of 1,5-dienes has been also reported by use of a chiral yttrocene complex (Scheme 32).87 Highest enantioselectivity (50% ee) was observed for 3,3-dimethyl-l,5-hexadiene with diphenylsilane. 

It is Hkely that the diene also remains coordinated during the hydrosilylation reactions of phenylacetylene with HSiEts, catalyzed by [Ir(diene)(NCMe)(PR3)]BF4 complexes [PR3 = P Pr3, PMe3 diene = 1,5-cyclo-octadiene, tetraflorobenzobarre-lene (TFB)]. However, detailed studies on the [Ir(COD)(NCMe)(PMe3)]BF4 complex show that cyclo-octadiene carmot be considered, in this case, as an ideal innocent Hgand because it transform into complexes containing cyclo-octadiene, cyclo-octadienyl or cyclo-octenyl Hgands in a variety of coordination modes, due to an... 

The complexes of the composition [ Ir( t-X)(diene) 2], where X = halogen, OH, OMe (e.g. [IrCl(CO)(cod)]) appeared to be very effective catalysts for the hydrosilylation of allyl chloride by trialkoxy- and alkylalkoxy-silanes [22]. Other iridium complexes have been subsequently reported as catalysts for the synthesis of silane... 

Scheme 7.8 Mechanism of heterogeneous catalysis of hydrosilylation by a surface rhodium (diene) siloxide complex.

Like rare-earth metal hydride and alkyl complexes [141, 206, 207], silylamide derivahves catalyze the hydrosilylation of alkenes and dienes with phenylsilane [208-210]. Accordingly, materials [Ln N(SiMe3)2 3] AS-380.7oo (12a-d, Table 12.3) featuring monopodal bis(silylamide) surface complexes have been exploited as catalysts for the reaction of 1-hexene and styrene with PhSiH3 (Scheme 12.20) [118]. 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

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)). 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Dienes. Hydrosilylation of conjugated dienes often leads to a complex mixture of products composed of regio- and stereoisomers of 1 1 addition, 1 2 addition products, and oligomers. Dihydrosilanes may form silacycloalkanes as well. In contrast... 

Nickel catalysts exhibit similar activity but are usually less selective than palladium 435-438 In addition to chlorosilanes, however, alkylsilanes also add in the presence of nickel catalysts. Nickel vapor showed an exceptionally high activity and selectivity in hydrosilylation.435 The photocatalytic hydrosilylation of 1,3-dienes with a chromium(O) complex occurs at room temperature to afford regioisomeric 1,4-adducts in quantitative yield.439... 

The diplatinum complexes [Pt(jU-H)SiR3(PCy3)]2 catalyze the addition of silanes R3SiH (R = Me, Et, PhCH2) Ph, OEt, Cl) to pentene-1, hexene-1, styrene, allyl chloride and 2-methylpropene. The relative reactivity order in silane is given, and the catalyst substrate ratio is 10-4-10-6 1. The majority of the reactions are carried out at ambient temperature and are exothermic. Dienes are also hydrosilylated.223 The complexes also catalyze the hydrosilylation of butyne-1, phenylacetylene, butyne-2 and diphenylacetylene in 70-90% yield. The... 

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