Stannylenes insertion reactions

Insertion reactions of stannylenes, even of unstable ones, into metal-metal bonds have attracted considerable attention 156 158>. In this context, it is very astonishing that the reaction (28) between the alkyl-substituted stannylene 14 and Fe2(CO)9 does not lead to a product of type 66 (X2Sn Fe(CO)4)2 (an X-ray structural analysis indicates an Sn2Fe2-ring167)) but to a three-membered ring, as determined by elemental analysis and from IR-spectral data133). 

Insertion reactions using CO and CH2N,2. Radical cyclization reactions of azaenynes in the presence of butyl stannanes in carbonylation conditions furnished /3-lactams <2003JA5632>. Azocanone 150 (Scheme 62) was prepared in good yield from enyne 149. The reaction occurred via a-stannylmethylene lactam of type 154 (Scheme 63). This free radical-mediated stannyl carbonylation is quite versatile and provides a general [ -pl]-type annulation leading to 4-exo and 8-exo systems. The yield of the isolated stannylene lactam intermediate 154 was reported as 61%, while the destannylation to lactam 150 was quantitative. 

There are examples of electron-deficient divalent metal derivatives called heavy analogs of carbene with the general formula R2M , including silylenes, "" germylenes, "" " stannylenes, "" "" and plumbylenes. When compared to carbenes, however, these compounds exhibit a significantly decreased reactivity in C-H insertion reactions. Theoretical studies have indicated that the heavier the metal, the larger the activation barrier and the less exothermic the reaction. Electropositive substituents, however, may increase reactivities. " ... 

The stannylenes from either source will insert into the Sn- Sn, Sn-R, or Sn-H bonds of organotin compounds, and react with alkyl halides, disulfides, or peroxides as shown in the reaction scheme below, but only the stannylenes that are generated photolytically will react with carbonyl compounds, and it appears that the stannylenes may exist in two forms, perhaps related as singlet and triplet, or a com-plexed and uncomplexed species. 

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Tin(II) chloride behaves as a stannylene by inserting into the Ni—Ni bond of [CpNi(PEt3)]2 to form 108, in which both chlorine atoms can be replaced by alkyl groups in reactions with LiR (R = Me, Bu)322. 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

A number of reactions are known which appear to involve the formation of oligostannanes by the insertion of stannylene units into a bond to tin, but most of these have little preparative value. For example, hexamethylditin reacts with trimethyltin chloride or dimethyltin dichloride to give, amongst other products, oligodimethylstannanes, (Me2Sn) , apparently by insertion of Me2Sn into SnSn or SnCl bonds.67-68... 

Except for 14, the reactivity of acyclic distannenes reflecting the Sn=Sn double-bond character has not been investigated because of their equilibrium in solution, and these distannenes react as the stannylene in many cases (see Section 2.8). Although some [2-1-2]-cycloaddition reactions were reported (Scheme 2.9.4),these cycloadducts could be formed either by [2-1-2]-cycloaddition or via the insertion of the second stannylene unit into the initially formed three-membered ring compound. 

Although CS2 insertion into the metal-carbon bond is rarely used to synthesize dithiocarboxylate salts of Group 14 elements, divalent tin salts of the dithiocar-boxylates 70 and 71 are synthesized by this method (Fig. 18). They are obtained by the stepwise insertion of CS2 into two Sn-C bonds of a diarylstannylene, Sn-[2,4,6- t-Bu) C L2 2 [128]. Reaction of another stannylene Sn 2,4,6-[(Me3Si)2-CH]3C6H2 [2,4,6-(z-Pr)3C6H2] with CS2 produces the stannylene-CS2 ylide 72, which is trapped by methyl acrylate to afford cyclic stannyl dithiocarboxylate 73 (Scheme 17) [129]. 

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