Heteroatom-substituted Organosilanes

Organosilanes bearing an electronegative heteroatom(s) at the a-carbon are susceptible to nucleophihc activation leading to silicon-carbon bond-cleavage, because of the electronic effect of the heteroatom. a-Heteroatom-substituted organosilanes are therefore quite valuable as protected carbon nucleophiles. The silicon-carbon bond is also readily activated by a transition metal complex. The reactivity is successfully utilized for catalytic carbon-carbon bond formation. 

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The introduction of an additional silyl-substituent in the /i-position to the heteroatom causes a further decrease in the oxidation potential of heteroatom-substituted organosilanes. The introduction of an additional silyl group results in the rise of the HOMO level. Increase in the population of the favorable conformers for the electron transfer also seems to be important. 

The utility of 2-pyridylsilanes to direct two types of coupling reactions widens the scope of organosilicon cross-couphng, both experimentally and conceptually, as it represents a new class of organosilanes that are not dependent on the more common methods of simple heteroatom substitution. 

DFT calculations combined with a distortion/interaction energy analysis showed that the anomalous Z selectivity observed in Wittig reactions of ( rt/j< -substituted benzalde-hydes is not caused by phosphoms-heteroatom interactions in the addition TS but is predominantly steric in nature. An efficient synthesis of olefins by the coupling of stabilized, semi-stabilized, and non-stabilized phosphorus ylides with various carbonyl compounds in the presence of silver carbonate has been reported. The first catalytic (in phosphane) Wittig reaction has been developed by utilizing an organosilane that chemoselectively reduces a phosphane oxide pre-catalyst to a phosphane. Sodium carbonate and A,A-diisopropyl-ethylamine have been employed as bases. The kinetic E/Z... 

The transition-metal-catalyzed [2 + 2 + 1] cycloaddition of two alkynes and heteroatom sources is a useful method for the synthesis of five-membered heterocycles. For example, a silylene species reacts with two alkynes 64 in the presence of nickel or palladium catalyst to afford substituted siloles 65 and 66. Various silylene equivalents, such as disilanes 67 [24], silacyclopropenes 68 [25], 69 [26], cyclotrisilanes 70 [27], alkylidenesilacyclopropanes 71 [28], silacyclopropanes 72, and 73 [29], have been developed as shown in Scheme 6.21. However, the utility for organic synthesis has been limited, due to the difficulty of those organosilane syntheses and the narrow alkyne scope. 

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