Spiro nonatrienes

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

Scheme 20 Formation of highly substituted spiro [4.4]nonatrienes 98 and 99 from the (T-dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 and arylacetylenes 97 [42,81]...

The reaction of silyl-P-aminovinylcarbene complex 57 with 3 equivalents of phenylacetylene results in an unexpected spirocyclization to give a mixture of spiro[4.4]nonatriene isomers 58 which differ in the position of one double bond (Scheme 30). [78] A mechanism of this unprecedented reaction has to take into account that the connectivity originating from the carbene complex 57 is lost along the spirocyclization. [79]... 

Scheme 30. Spiro[4.4]nonatrienes from P-aminoalkenylcarbene complexes.

A unique spirocyclization connecting chromium /3-silyl-aminovinylcarbene 70 and 3 equivalents of alkyne afforded spiro[4.4]nonatrienes 71 and 72. The formal [3+2+2+2]cyclization putatively involves an unprecedented triple alkyne insertion (Scheme 11.22) [46]. 

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