α-Borylated Phosphorus Ylides (α-BCPs): Electronic Frustration within a C-B π-Bond Arising from the Competition for a Lone Pair of Electrons.

The concept and the first example for electronic frustration within a C-B π-bond arising from the competition for a lone pair of electrons is reported. The α-borylated phosphorus ylide (α-BCP) Ph3 PC(CH3 )BEt2 (1) has been synthesized, characterized, and investigated with the aid of density functional theory calculations. These show the presence of a highly polarized C-B π-bond, induced by the electron-withdrawing Ph3 P+ substituent. This competition for the C-based lone pair of electrons leads to an FLP-type reactivity (FLP=frustrated Lewis pair), which has been demonstrated by reacting 1 with various small molecules: the dimeric insertion products [Ph3 PC(CH3 )COBEt2 ]2 (2) and [Ph3 PC(CH3 )(CO2 )BEt2 ]2 (5) result from the reaction of 1 with CO and CO2 , respectively, whereas monomeric products of the general formula [Ph3 PC(CH3 )(XYZ)BEt2 ] were obtained by treating 1 with COS (3), CS2 (4) or the heterocumulenes PhNCO (6) and PhNCS (7).