Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂ , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂ IMe, Me₂ Iⁱ Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n ; n=3, 4, 5, 6] and Ph₂ C=CPh₂ . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂ CPh₂ and cyclo-(ArP)₄ CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

A Three-Membered Cyclic Phosphasilene

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N-heterocyclic carbene (NHC). First reactivity studies show a) that the PSi₂ ring is a competent ligand to the Fe(CO)₄ fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh₃ results in the rapid head-to-head or head-to-tail dimerization of the PSi₂ unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

The first donor-acceptor complex of a silaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl₃ (NHC=N-heterocyclic carbene), was synthesized using the reaction of silyliumylidene-NHC complex [(NHC)₂ (Ar)Si]Cl with water in the presence of GaCl₃ . Conversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO₂ GaCl₂ ]₂ , free silaacetal ArSi(H)(OR)₂ , silaacyl chloride (NHC)(Ar)Si(Cl)OGaCl₃ , and phosphasilene-NHC adduct (NHC)(Ar)Si(H)PTMS unveil its true potential as a synthon in silacarbonyl chemistry.

Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes

The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHC→P(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P-P bond cleavage and P-C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol^-1 for 2 b.