AbstractThe Peterson olefination reaction has been investigated as a route to novel P‐substituted alkenes. The reaction between [(nPr2P)2(Me3Si)C]Li and paraformaldehyde cleanly gives the symmetrical vinylidene phosphane (nPr2P)2C=CH2 (4) in good to excellent yields. In contrast, reactions between [(R2P)(Me3Si)2C]Li (R = Me, nPr) and paraformaldehyde yield complex mixtures of products which do not contain the expected vinylidene species. However, the unsymmetrical vinylidene phosphanes [nPr2P(BH3)](Me3Si)C=CH2 (6), (Ph2P)(iPr2P)C=CH2 (7), [nPr2P(S)](Me3Si)C=CH2 (10), [Ph2P(S)][iPr2P(S)]C=CH2 (11) and [Ph2P(S)][Me2P(S)]C=CH2 (12) are readily accessible by the Peterson olefination reaction {compound 7 is obtained by deprotection of the corresponding borane adduct [Ph2P(BH3)][iPr2P(BH3)]C=CH2 with pyrrolidine}. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)