Silicon and Tin Compounds

The silicon-silicon bond is oxidatively cleaved in phenylpenta-methyldisilane, which is converted by peroxybenzoic acid into phenyldi-methylsilyl trimethylsilyl ether in 86% yield (equation 608) [1203]. 

The oxidative cleavage of the carbon-tin bond takes place in the reaction of alkylstannanes with lead tetraacetate. Phenoxymethyltributyl-stannane, when refluxed in dichloromethane with an equimolar amount of lead tetraacetate for 32 h, is converted into phenoxymethyl acetate in 58% yield (equation 609) [1204]. 

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By an analogous route, the cyclization of chiral (z-amino imines 197, bearing the proper tw-functionality in the o -alkyl chain, will lead to 3-substituted 1,2-diaminocycloalkanes 198. The intermolecular and intramolecular addition of allylic silicon and tin compounds to a-amino azomethine compounds should also be investigated. 

In contrast to the transition metals, where there is often a change in oxidation level at the metal during the reaction, there is usually no change in oxidation level for boron, silicon, and tin compounds. The synthetically important reactions of these three groups of compounds involve transfer of a carbon substituent with one (radical equivalent) or two (carbanion equivalent) electrons to a reactive carbon center. Here we focus on the nonradical reactions and deal with radical reactions in Chapter 10. We have already introduced one important aspect of boron and tin chemistry in the transmetallation reactions involved in Pd-catalyzed cross-coupling reactions, discussed... 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency Computation in the early 1990s could not provide a reliable answer. Arshadi et al bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds ( Si chemical shift vs Sn chemical shift). Since reliable calculations were available for the Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected Sn chemical shifts of trialkylstannylium ions, which proved to be 5 ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al as the tetrahedral part of the equilibrium with pentacoordinate species. 

Jesi and /csn couphngs have been used by Wrackmeyer et a/. in order to follow the reaction between 1-boraadamantane and di(l-alkynyl)silicon and -tin compounds of the type (Me2M(C = C)R)2, M = Si or Sn, and to characterise some novel 7-metalla-2,5-diboranorbornane derivatives obtained as products. 

As is also true for silicon and tin compounds, the high stability of boronates, particularly cyclic esters, allows them to be incorporated into and carried through as substituents in a range of reaction types, such as the synthesis of pyrazole boronates for cross couplings (see 4.2.7.4). 

Germanium is an element in group 14 of the periodic table and many compounds of germanium similar to silicon and tin compounds have been synthesized [1]. However, germanium is a highly expensive metal compared with silicon or tin, and the amount used is small. Now, the research and development in the field of pharmaceuticals have been energetically conducted. Its other uses are as catalysts for isomerization of sugars, etc. 

The addition of HX compounds in which X has no unshared electron pairs as it occurs in silicon and tin compounds is similar to that of H2 i.e. one CO-ligand is expelled per mol of HX. 

As a result of these facile elimination reactions, silicon and tin compounds are becoming very useful in synthetic chemistry. Vinylsilanes and vinylstannanes, when attacked by electrophilic species, often undergo elimination of the silyl or stannyl substituent ... 

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