Transient Stannylenes

Like the Sn(III) radicals, the Sn(II) stannylenes are familiar both as persistent species of unlimited life and as transient, highly reactive intermediates. Many of the compounds that, in the older literature, were... 

In M14 = Mi4 compounds (M, M = Si, Ge, Sn), only >Si = Sn< derivatives have not yet been physicochemically characterized (a transient compound of this type has been postulated by Lappert et al. as possible intermediate in the reaction between a silylene and a stannylene leading to a silylstannylene118). 

A major problem in postulating silylenoid metal complexes as intermediates in the redistribution reactions is simply that good model compounds are lacking, and the decomposition mechanisms of silyl transition metal complexes have not been systematically investigated. While there is evidence for transient R2Si species produced by thermal or photochemical means (80-83), there are no known monomeric silylene metal complexes. Several monomeric stannylene and germylene complexes are... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

The older literature contains references to long-lived simple monomeric diorganotin species, R2Sn . Subsequent work has shown that, unless the groups R are cyclopentadi-enyl or are very bulky (see Chapter 21), these stannylenes exist only as transient inter-... 

Stannyl radicals R3Sn can be identified by ESR spectroscopy, but there is no equivalent sensitive and selective technique for observing the transient simple (singlet) stannylenes, R2Sn . Their formation usually has to be inferred from their reactions, and it is not always unambiguous as to whether monomeric R2Sn has existed as a kinetically free entity the same problem of course occurs with the carbenes R2C . 

Another indication for the Lewis acidic properties of NHSns is provided by the reaction of diben-zotetraazafulvalenes with stannylenes. Electron-rich enetetramines can be cleaved into N-heterocyclic carbenes. Pioneering work by Lappert has demonstrated that this cleavage, in the presence of an electrophilic metal center, leads to NHC complexes. Hahn and coworkers have used the electrophilic tin center in the NHSn 4 to cleave the dibenzotetraazafulvalene 5 with the formation of the zwitterionic stannylene-carbene adduct 6 (Scheme 2.8.5). Similar carbene adducts of di-aminogermylenes and diaminostannylenes have been described by Schumann et al. and Lappert et al. and even transient diorganogermylenes have been stabilized by formation of an adduct with an N-heterocyclic carbene. ... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tinatoms. Such complexes are less stable for more substituted stannylene acetals, e.g., those derived from carbohydrates. Addition of nucleophiles to solutions of stannylene acetals in non-polar solvents has been found to markedly increase the rates of reaction with electrophiles and transient 1 1 complexes of this type are... 

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