Coupling chlorosilanes

SELECTIVE PINACOL-TYPE COUPLING USING ZINC, CHLOROSILANE, AND CATALYTIC AMOUNTS OF Cp2VCl2 dl-1,2-DICYCLOHEXYLETHANEDIOL... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

The ternary system consisting of a metallic catalyst, a chlorosilane, and a stoichiometric co-reductant has been reported by us for the first time to achieve the catalytic pinacol coupling. The homo coupling of aliphatic aldehydes is catalyzed by CpV(CO)4, Cp2VCl2, or Cp2V in the presence of a chlorosilane and Zn in DME to give the 1,3-dioxolanes 1 via the coupling and acetalization (Scheme 3) [18,19]. 

The coupling reaction of aromatic aldehydes proceeds well only in water by using VCI3 as a catalyst in the presence of A1 (Scheme 9). It should be noted that this method does not require a chlorosilane as an additive [31]. 

Wurtz-type couplings have also been observed upon sonication of lithium in the presence of both organic halides (yields 36-73%) (218) and chlorosilanes or chlorostannanes (yields 42-94%) [Eq. (42)] (219). 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

The Hiyama coupling of aryltrimethoxysilane 47 and 3-bromopyridine assembled arylpyridine 48 with the aid of TBAF [57], In contrast to chlorosilanes, which are susceptible to hydrolysis, aryltrialkoxysilanes are not. 

Synthesis is typically by alkali metal-mediated coupling of dichlorosilanes or 1,2-dichlorodisilanes, although electrochemical coupling of chlorosilanes and dehydrocoupling of primary and secondary silanes also often lead to oligomeric (as opposed to high polymer) fractions. 

In 1996, Hiyama and co-workers reported on the cross-coupling of activated aryl chlorides with aryl- and alkenyl-chlorosilanes 71 (Figure 16). The high temperatures required to activate the aryl chlorides did not affect the organosilanes an added advantage that can be attributed to their relative inertness. The system could be catalyzed by a variety of phosphine-bearing palladium complexes in the presence of either KF or TBAF as promoters. 

Attachment of suitable linkers to the surface of silica can be achieved by transesterification with (3-aminopropyl)triethoxysilane, which leads to the support 2 (Figure 2.8) [198-200]. Alternatively, silica can be functionalized by reaction with alkyltri-chlorosilanes [201]. For the solid-phase synthesis of oligonucleotides, supports with a longer spacer, such as that in 3, have proven more convenient than 2 [202-206]. Supports 3, so-called LCAA-CPG (long chain alkylamine CPG [194,195]), are commercially available (typical loading 0.1 mmol/g) and are currently the most commonly used supports for the synthesis of oligonucleotides. For this purpose, protected nucleosides are converted into succinic acid monoesters, and then coupled to LCAA-CPG. CPG functionalized with a 3-mercaptopropyl linker has been used for the solid-phase synthesis of oligosaccharides [207]. 

When a copper anode is used instead of platinum, the resulting chlorosilane is subsequently reduced to a Si—Si coupling product in a one-pot reaction (equation 49)57. Interestingly, when a mixture of hydrosilane and chlorosilane is electrolyzed using a copper anode and a platinum cathode, a Si—Si coupling product is obtained in 64% yield on the basis of the sum of both reagents used. Thus, the paired electrolysis of hydrosilane on the anode and chlorosilane on the cathode proceeds to give disilane (equation 50)57. 

This process has advantages in that the reaction is homogeneous and avoids the heterogeneous Na Wurtz coupling procedure, commonly used to polymerize chlorosilanes. 

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