Silyl stannylation

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

FIGURE 56. Preparation of silyl plumbyl and silyl stannyl complexes of iron. Reproduced from Reference 168 by permission of Elsevier Sequoia S.A. 

A thermodynamically stable (silyl)(stannyl)palladium(n) complex is synthesized by an oxidative addition of the Si-Sn linkage to palladium(O) (Scheme 63).267 The complex has the square-planar geometry with a m-arrangement of the silicon and tin atoms. An alkyne reacts with the complex to afford a silastannated product as a mixture of cisjtrans stereoisomers (10 1). 

Mechanistically, a (silyl)(stannyl)palladium initially formed undergoes regioselective silylpalladation to the alkyne moiety of the enyne (Scheme 66). Then, two possible pathways are conceivable for addition to the alkene moiety, that is, stannylpalladation and carbopalladation. It has not been established that which pathway operates. 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

In a similar way, but in a one-pot experiment (equation 27)42, a stable silylated stannyl anion was prepared in 44% yield and isolated as white crystals. X-ray crystallography showed a distorted tetrahedral tin atom with a Sn-Li distance (2.87 A) which is shorter than those published for [Li(THF)3.Sn (4-MeC6H4)NSiMe2 3CH] (2.89 A) and [Li(PMDETA).SnPh3] (2.87 A), but longer than the Li-Mi4 distances in the Ge and Si analogues (2.67 A) and (2.64 A), respectively. 

Hydride abstractions (equations 166190 and 167192) and oxidative additions to form mixed silyl/stannyl complexes388 (equation 168) are also reported. 

The scope and value of the benzannulation reaction is further increased by the substitution pattern of the arene ring, which can be modified by the incorporation of allcynes bearing additional functional groups such as silyl, stannyl, or boryl substituents. These functional groups have been used in various palladium-catalyzed (cross)-coupling reactions [63, 64]. Further structural elaboration may be based on benzannulation followed by nucleophilic aromatic addition [63b]. 

Selective silyllstannyl exchange. Regioselective aldol condensation of methyl ketones can be effected by generation of the a-stannyl derivative via silyl-stannyl exchange... 

Anionic nucleophiles provide valuable and versatile routes for the preparation of C-glycosides. However, the most extensively used technologies lie within the chemistry of Lewis acid-mediated reactions of carbohydrates with unsaturated hydrocarbons and derivatives thereof. In the next series of examples, reactions of sugars and sugar derivatives with olefins and their silyl-, stannyl-, and aluminum derivatives are discussed. 

Addition of chlorophosphane to the mixed (silyl)(stannyl)diazo derivative 10 affords the nitrilimine 12 (70 % yield), which was unobtainable using the silylated lithium salt 5, demonstrating the superiority of stannyl diazo compounds over diazolithium salts for the synthesis of stable nitrilimines [13, 14] (Scheme 3). 

Protonation with Bronsted acids proceeds with retention (Table 9). Alkylation, silylation, stannylation, carbon dioxide, and carbon disulfide addition (entries 3-10,12,25) are accompanied by inversion. This is also true for the reaction with acid chlorides (entry 13), but esters react with clean retention (entries 14-16). Aliphatic aldehydes and ketones add to the Hthium compound 216a with complete retention of configuration. A notable exception was found for benzaldehyde it is added with inversion (entry 23). 

Nucleophilic radicals with low-lying unoccupied orbitals, such as acyl, oxyacyl, silyl, stannyl, and germyl radicals providing high regiocontrol and efficient methods for the synthesis of heterocycles 07ACR303. 

Preparation A common synthetic approach to alkynyliodonium salts involves the reaction of an electrophilic X -iodane with a terminal alkyne or its silylated, stannylated, or lithiated derivative. In the early 1980s, Koser and coworkers found that [hydroxy(tosyloxy)iodo]benzene 75 reacts with terminal alkynes 344 upon gentle heating in chloroform or dichloromethane to form alkynyliodonium tosylates 345 in moderate to low yield (Scheme 2.98) [199,471,476]. 

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