Stannylenes oxidative additions

Compound 64, after reaction with 2,3-dimethylbutadiene, produces a cyclic stannylene 66 in an oxidative addition step104. 

Cyclic stannanes can be generated by reaction of stannylenes with alkynes112. For example, bis[bis(trimethylsilyl)methyl]tin reacts with cyclooctyne to provide A1,18-9,10-(distanna-9,9,10,10-tetrakis[bis(trimethylsilyl)methyl])bicyclo[6,2,0]-decene, 69. This reaction is a typical oxidative addition on stannylenes. 

A second major characteristic of the stannylenes is their ability to undergo oxidative addition with reagents such as alkyl halides, enones, and 1,2-diones (e.g., Scheme 19)552 and nitroxyl radicals (Equation (180)).553... 

Several publications about oxidative addition of metal or organometal derivatives to stannylenes describe a new and efficient way to stannyl anions38,70-74. The reaction of CpLi with Cl2Sn resulted in a mononuclear complex as colorless cubic crystals obtained in 48% yield (equation 53)70. Its structure was resolved by X-ray diffraction and shows a complete separation of the ion-pair70. 

A triaminostannyl lithium, prepared by oxidative addition of the corresponding amide to the stannylene bearing the same substituents (equation 56)73, was characterized by methylation (Mel, 71% yield) and halogenation (I2, 62% yield). 

Pentaalkylditin lithium, RjSujLi (R = Me, n-Pr, i-Pr) is present in preparations of RjSnLi reagents prepared from the trialkyltin halide and Li metal in THF. The RjSnjLi species arise by oxidative addition of RjSnLi to the stannylene R2Sn which in turn is a significant component of the equilibrium ... 

The two most characteristic properties of the stannylenes are their behaviour as Lewis acids and Lewis bases, and their ready oxidative addition to give Sn(IV) compounds. The amphoteric nature provides the model for the structure of the dimers, the distannenes, and for the oligomerisation that occurs with the less hindered examples two dative bonds are assumed to be formed between the doubly-occupied sp2 orbital and the vacant p orbital, affording a trans non-planar distannene (equation 21-9). These structures are discussed in more detail below. 

The stannylene Sn[N(TMS)2]2 undergoes oxidative addition with halides RX, where R = alkyl, allyl, allenyl, hexynyl, phenyl, or styryl, to give the Sn(IV) compounds, RSnX[N(TMS)2]2, then these can be used in Stille coupling reactions where the organo-tin(IV) compound is activated by fluoride ion 49-51... 

The other important type of reaction leading to the formation of two bonds between a metal and four-atom fragment is addition of carbenoids, i.e. germylenes and stannylenes, to a-dicarbonyl and related compounds. The process can be classified as redox (4 + 1) cycloaddition since the dicarbonyl fragment is reduced while the carbenoid is oxidized. Formally the simplest example of such reactions is the interaction of methylglyoxal with activated tin metal to give a cyclic stannylene, Sn(II) enediolate (see Scheme 34) <83CL1825>. 

Stannylenes are oxidized to keto-alcohols by dropwise addition of bromine in di-chloromethane. The reaction proceeds at room temperature at the speed of a titration. The attention of chemists who want to check their reaction by infrared (IR) examination in situ is drawn to the fact that the ketone may be chelated to tin in these conditions (vqj 1685 cm-1). The reaction is regiospecific, giving only one of the two possible keto-alcohols. It has been used in total synthesis for instance, in the synthesis of (+)-spectinomycin (described under Sec. QI.G) [12,13]. The replacement of bromine by IV-bromosuccinimide as oxidant has been reported [14]. 

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