Dispersion-corrected energy barriers for silylene addition to white phosphorus, a density functional investigation into substituent effects

Abstract

A density functional investigation into differently substituted silylenes with respect to the first step in the addition to white phosphorus is presented. The investigations include dispersion corrections in the density functional treatment. They become sizable for the transition state geometries for the silylenes as they become increasingly substituted by bulky groups. Hence, dispersion corrections are essential for a quantum chemical treatment of real molecules using density functional theory. The different singlet-triplet energy separations of differently substituted silylenes were also investigated and compared with calculated activation barriers for the first step in the addition reaction.

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

“…DFT calculations by Schoeller et al for the truncated compound Me NacNac′Si ( Me NacNac′ [MeNC(CH 2 )-CHC(Me)NMe] 2− ) suggested that unlike the typical reactions of P 4 with nucleophiles (e.g., alkoxides), this reaction should be described as an electrophilic attack of silylene at a P−P edge of the phosphorus tetrahedron. 280,281 The alternative nucleophilic attack at a phosphorus corner was discarded because the silicon lone pair was regarded to be inert while the direct insertion of Me NacNac′Si into the P−P bond was found to be a high energy process. Counterintuitively, electrophilic P−P bond cleavage is facilitated by about 4 kcal mol −1 through coordination of another molecule of P 4 which acts as an external nucleophile toward the silylene, such that in the transition state the silicon atom adopted a distorted trigonal bipyramidal geometry.…”

White Phosphorus Degradation with a NacNac Aluminum Carbene Analogue: The Biradical Reaction Mechanism

“…For the reaction of 1 with white phosphorus, it is mandatory to briefly consider the molecular orbital system of P 4 . Symmetry considerations , give support to the following qualitative model (Scheme ).…”

“…The latter is then ready for facile subsequent rearrangements, ending finally in an energetic sink 3 . Consequently, 1 acts overall differently than substantiated previously , for carbenes and silylenes.…”

“…In this report, we present a new perspective on the reaction mechanism for the white phosphorus degradation. It will be shown that the reaction of 1 with P 4 does not follow the insertion mechanism, as it is known for the reaction of silylenes and singlet carbenes with P 4 . , Instead, the P 4 tetrahedron is simultaneously coordinated by two molecules 1 , causing ring-opening without any energy barrier to a biradicaloid species. The latter is then ready for facile subsequent rearrangements, ending finally in an energetic sink 3 .…”