Hydrosilylation palladium complexes

Concerning enantioselective processes, Fujihara and Tamura have proved that palladium NPs containing (S)-BINAP (2,2 -bis(diphenylphosphino)-l,l -binaphthyl) as chiral stabiliser, catalyse the hydrosilylation of styrene with trichlorosilane, obtaining (S)-l-phenylethanol as the major isomer (ee = 75%) [42]. In contrast, the palladium complex [Pd(BINAP)(C3H5)]Cl is inactive for the same reaction [43]. 

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 mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Hydrosilylation of butadiene using palladium complexes supported on inorganic materials such as silica and alumina has been carried out (77, 72) however, the supported catalyst is not stable and it is difficult to compare with the soluble catalysts. 

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... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

The reaction is carried out under a dry nitrogen atmosphere. To a mixture of 7.32 g (40 mmol) of ( )-bromophenylethcnc and 0.20 mmol of the palladium complex are added 100 mL (80 mmol) of a 0.8 M solution of [a-(trimethylsilyl)benzyl]magnesium bromide in diethyl ether at —78 °C. The mixture is allowed to warm and stirred at 0 "C for 2 d and then hydrolyzed with 10% HCI at 0 C. The organic layer and ether extracts from the aqueous layer are combined, washed with aq NaHCG3 and then water, and dried over anhyd MgS04. The solvent is evaporated and the product isolated by distillation yield 10.1 g (93% ) bp 135-139 JC/0.9 Torr [a]p° —43.9 (c = 1.0, benzene) 95% op (determined by hydrogenation and direct comparison with an authentic sample prepared via asymmetric hydrosilylation and correlated with 1,3-diphenyl-t -propanol). 

An interesting hydrosilylation reaction catalyzed by supported palladium complexes occurs between trimethylsilanol and butadiene ... 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

Asymmetric hydrosilylation of styrene with HSiCl3 catalyzed by a palladium complex of a chiral ferrocenylphosphine attached to cross-linked polystyrene support at 70 °C gives PhMeC HSiCl3 in quantitative yield with only 15.2% ee65. 

Figure 22-4 Mechanism for the hydrosilylation and dehydrogenative silylation of 1-alkenes catalyzed by cationic palladium complexes Pd represents [(phen)Pd]+. The palladium alkene complex A is the resting state of the cycle. Cycle I denotes the hydrosilylation cycle, Cycle II describes the dehydrogenative silylation reaction.

A related mechanism has been established for hydrosilylations catalyzed by cationic palladium complexes and is shown in Fig. 22-4. The mechanisms for both... 

Application of various chiral palladium complexes to the stereoselective hydrosilylation of dienes results in the formation of 3-silylcvcloalkenes irrespective of the isomer of the diene used (in the case of cyclohexadienes). When complexes containing menthyldiphenylphosphine (MDPP) and neomenthyldiphenylphosphine (NMDPP) are used, the chiral products with an excess of the (S)-enantiomer are always formed1011. 

Stereoselective hydrosilylation of 1,3-cyclopentadiene giving (f )-3-trimethylsilyl-1 -cydopen-tene [(/ )-13] can be achieved using ferrocenylphosphine-palladium complexes as catalysts15. 

Asymmetric Hydrosilylation of Alkenes. The palladium complex PdCl2[(/ )-(5)-PPFA] catalyzes the asymmetric hydrosilylation of norbornene, styrene, and 1,3-dienes (eq 3). The hydrosilylation of 1-phenyl-1,3-butadiene with Trichlorosilane proceeds regioselectively in a 1,4-fashion to give (Z)-1-phenyl-1-silyl-2-butene of 64% ee. 

Palladium complexes [Pd(PPh3>4], [PdCl2(PPh3)2] and IPdCl2(PhCN)2] are not as reactive as H2PtCl6 toward hydrosilylation but they are more selective selective 1,4-addition to 1,3-dienes occurs to give aliylsilanes, and for asymmetric hydrosilylation with (R)-(S)-PPFA-Pd see Section 3.12.7. 

The reason for the high selectivity for formation of Z-olefins in the palladium-catalyzed hydrosilylation can be explained by formation of a cisoid complex of type 2 [Eq.(2)], which after hydride addition undergoes a reductive elimination which is faster then syn-anti isomerization [5]. 

Several workers have reported that low-valent palladium complexes are effective catalysts for the hydrosilylation of double bonds. The reaction of hexene-1 with trichlorosilane to give 1-trichlorosilylhexane has been catalyzed by a range of compounds (Table IV). It is particularly... 

The hydrosilylation of butadiene with low-valent palladium complexes gives an alkylsilane formed by addition of 2 molecules of butadiene to 1 molecule of silane (271, 272) [Eq. (95)]. Compounds Pd(maleic anhydride)(PPh3)2 and Pd(p-benzoquinone)(PPh3)2 were particularly effective catalysts for this reaction. Halopalladium(II) compounds and Pd(PPh3)4 were less effective as catalysts. The proposed mechanism is... 

This volume begins with two procedures in the area of catalytic asymmetric synthesis. The first procedure describes the synthesis of (R)-2-Dl PH ENYLPHOSPHI NO-2 -METHOXY-1,1 -BINAPHTHYL (MOP), a chiral ligand that has proven very useful in palladium-catalyzed hydrosilylation of olefins and palladium-catalyzed reduction of allylic esters by formic acid. The next procedure describes the catalytic asymmetric synthesis of nitroaldols using a chiral LANTHANUM-LITHIUM-BINOL COMPLEX, illustrated by the synthesis of (2S,3S)-2-NITRO-5-PHENYL-1,3-PENTANEDIOL. 

Recently, a palladium complex coordinated with an axially chiral, monoden-tate phosphine ligand, MeO-MOP (7a) or its analogs [21], has been reported to be highly effective for the enantioselective hydrosilylation of alkyl-substituted terminal olefins (Scheme 4) [22,23]. Simple terminal olefins 8 were transformed efficiently into the corresponding optically active 2-alkanols 11 with enantiose-lectivities ranging between 94% and 97% ee by the catalytic hydrosilylation-ox-... 

---