Carbenes with Reduced Heteroatom Stabilization: A Computational Approach

Abstract

High-level DFT calculations, coupled with appropriate isodesmic reactions, are employed to investigate the effects of monoheteroatom substitution, cyclization, and unsaturation on the stability, multiplicity, and reactivity of amino-, oxy-, silyl-, phosphino-, and thioalkylcarbenes. The results of these calculations are compared to those of di-tert-butylcarbene, 2,2,5,5-tetramethylcyclopentanylidene, and 2,2,5,5-tetramethylcyclopent-3-enylidene as the reference molecules. The calculated singlet-triplet energy gaps (DeltaE(S-T)) demonstrate the following trend: (amino approximately = oxy) > thio > phosphino > alkyl > silyl. In contrast to the previous reports, isodesmic reactions show that pi-donor/sigma-acceptor amino substituents stabilize not only the singlet but also the triplet states. The stabilization of the triplet states by amino substitution is much less than the singlet states. The DeltaE(S-T) values of all the carbenes are increased through cyclization, while the introduction of unsaturation causes small and rather random changes. These changes are carefully probed by means of isodesmic reactions for the singlet and triplet states, separately. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity issues showing amino- and phosphinoalkylcarbenes to be more nucleophilic, more basic, and less electrophilic than oxy- and thioalkylcarbenes, respectively. This detailed study offers new insights into the chemistry of these novel carbenes.

Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pKa Values for Twenty Triazolium Salts in Aqueous Solution

“…20 Remarkably, despite the widespread use of azolium ion precursors to NHCs in organocatalysis and elsewhere, there have been relatively few literature reports of the solution kinetic or thermodynamic Brønsted acidities of these precatalysts, [21][22][23][24][25][26][27][28][29][30][31][32] although there have been studies of NHC nucleophilicities 33 and gas phase proton affinities. [34][35][36][37][38][39][40][41][42] Washabaugh and Jencks reported on the C2-H/D exchange and aqueous pK a values of a range of N-alkyl thiazolium ion analogues of thiamine, that are precursors to thiazol-2-ylidenes 1. [27][28][29] Amyes et al reported aqueous pK a values for the conjugate acids of N,N-dialkylimidazol-2-ylidenes 2 (R = R' = H or Me) and two benzoannelated variants.…”

From acyclic dialkylcarbene to the unsaturated cyclic heteroatom substituted ones: a survey of stability

“…Considering the wide variety of heteroatom substituted carbenes, it seems necessary to thoroughly probe the effects of heteroatom substitution, cyclization, and unsaturation on the stability of parent form of dialkylcarbenes leading to information on the structures, characteristics and behaviors of these species. Hence, following our interest in the novel stable singlet carbenes,45, 46 the parent acyclic diethylcarbene is selected to probe the effect of π‐donor/σ‐acceptor (NH and O), π‐donor/σ‐donor (PH and S), and π‐acceptor/σ‐donor substitution (SiH 2 ) on the singlet–triplet energy gap (Δ E S–T ) (Fig. 1).…”

Stable silylenes with acyclic, cyclic, and unsaturated cyclic structures: Effects of heteroatoms and cyclopropyl α-substituents at DFT

“…This can be attributed to n/p interactions of the heteroatoms with the vacant p-orbital of the silylene center which competes exclusively with the interaction of the occupied Walsh orbital (W/p). Due to the conjugation of heteroatom lone pairs with the double bond in unsaturated cyclic silylenes 3 Y , the cyclopropyl plays a more important role in singlet state stabilization [41]. For example the singlet state of 3 N is stabilized by cyclopropyl twice the singlet state of 2 N (11.0 vs. 5.8 kcal mol À1 , Table 4).…”

“…Increasing DE SeT of dimethylcarbene upon simultaneous cyclization and unsaturation [41,42], made us probe such effects on our silylenes (3 Y , Fig. 1).…”