Reactions of (1-Diazo-2-oxoalkyl)silanes with 3H-1,2,3,4-Triazaphospholes: a Route to Short-Lived 4-Imino-1,2,4(λ5)-diazaphospholes

(1-Diazo-2-oxoalkyl)silanes 1 react with a 3-phenyl-3H-1,2,3,4-triazaphosphole (3) to form dispiroannulated 1,3-diaza-2,4-diphosphetidines 4. With 3-alkoxycarbonyl-3H-1,2,3,4-triazaphospholes 5, 4-alkoxy-4-isocyanato-1,2,4(λ5)-diazaphospholes 6 are obtained. The structures of 4a and 6c were established by X-ray crystal structure analysis. It is proposed that 4-imino-1,2,4(λ5)-diazaphospholes are the immediate precursors of both types of products.