Silicon-Based Cross-Coupling Reactions

In a simple strategy to biaryl formation, Han et al.89 showed that silicon-directed ipso-substitution and concomitant cleavage from supports could be used for formation of functionalized biphenyls. For this they used a tethered silyl aryl bromide in a Suzuki cross-coupling reaction, followed by the ipso-substitution/cleavage step (Scheme 39). A variety of boronic acids were coupled in this manner. The only difficulty occurred with electron-deficient nitrophenylboronic acid where the desired product was formed under anhydrous conditions in only 33% yield (the remainder being starting material). Reversion to the more usual conditions of aqueous base-DME (i.e., those used by Frenette and Friesen)70 improved the yield to 82%. 

Denmark SE, Kallemeyn JM (2005) Synthesis of 3,4,5-trisubstituted isoxazoles via sequential [3+2] cycloaddition/silicon-based cross-coupling reactions. J Org Chem 70 2839-2842... 

Sequential [3+2] cycloaddition/silicon-based cross-coupling reactions allowed for the synthesis of 3,4,5-trisubsti-tuted isoxazoles. Regioselective 1,3-dipolar cycloaddition reactions between alkynyldimethylsilyl ethers 400 and ethyl or phenyl nitrile oxides, generated in situ from 1-nitropropane and A -hydroxybenzene carboximidoyl chloride, respectively, gave as predominant products after hydrolysis isoxazol-4-ylsilanols 401, converted into 4-arylisoxazoles 402 by cross-coupling with a variety of aryl iodides (Scheme 97) <2005JOC2839>. 

The same reaction conditions were applied to the coupling reaction of Trimethyl(vinyl)silane [Eq.(3)] with limited success [3], Thus, the mechanism of these silicon-based reactions appears to be different from those of the cross-coupling reactions using the main-group metal reagents. 

The organo.silicon-based cross-coupling reactions discussed here are summarized in Eq.(42). The number of fluorine atom on silicon depends on the substituent which participates in the coupUng reaction. 

New, silicon-based, cross-coupling reactions with the participation of silatranes, cyclic polysiloxanes, and Si-derivatives of heterocycles 02ACR835. 

Background of Silicon-Based Cross-Coupling Reactions... 

The combination of spectroscopic studies and kinetic analysis has provided an invaluable insight into the mechanism of silicon-based cross-coupling reactions. Interestingly, several different silicon precursors can operate under a common mechanistic regime. However, a change in the promoter employed causes a change in the mechanism, reflecting radically different modes of transmetallation. 

Direct Silylation of Heteroarylcarbonyl Compounds. Under ruthenium catalysis vinyltrimethylsilane reacts to ortho silyl-ate heteroaryl carbonyl compounds directly in good yields (eqs 32 and 33). The reaction only works with heteroaromatic systems. The resulting aryltrimethylsilanes can be used to introduce electrophiles regioselectively through electrophilic desilyla-tion. The reaction also works with vinyltriethoxysilane, opening the possibility of silicon-based cross-coupling reactions. 

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