Foiled carbenes revisited: When sigma-stabilization surpasses pi-stabilization.

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Reaction Mechanisms

Background:

• Bicyclic alkenylidenes, specifically bicyclo[3.2.1]oct-2-en-8-ylidene (9) and bicyclo[3.3.1]non-2-en-9-ylidene (17), were previously characterized as stabilized, or "foiled," carbenes.
• Carbenes are reactive intermediates with a divalent carbon atom, and their stability is a key area of research in organic chemistry.

Purpose of the Study:

• To computationally investigate the stability and reactivity of bicyclic alkenylidenes 9 and 17.
• To elucidate the rearrangement mechanisms and energy barriers associated with these carbenes.
• To compare the reactivity of these systems with related norbornene derivatives.

Main Methods:

• Density Functional Theory (DFT) calculations using the B3LYP functional.
• Second-order Møller–Plesset perturbation theory (MP2) computations.
• Analysis of reaction pathways and transition states, including energy barrier calculations.

Main Results:

• Computational results confirm prior experimental data regarding the existence of carbenes 9 and 17.
• These carbenes exhibit high reactivity, undergoing rapid rearrangement primarily through a 1,2-vinyl shift.
• The energy barriers for this rearrangement are low (1.2–5.4 kcal/mol), contrasting sharply with high barriers (up to 30 kcal/mol) for similar rearrangements in norborn-2-en-7-ylidene derivatives.
• In specific structures (17 and bicyclo[4.1.1]oct-2-en-7-ylidene (23)), the carbene center is observed to bend away from the double bond.

Conclusions:

• Contrary to previous claims, bicyclic alkenylidenes 9 and 17 are not stabilized carbenes but are highly reactive species.
• The facile 1,2-vinyl shift mechanism plays a crucial role in their rapid rearrangement.
• The observed reactivity patterns highlight significant differences in the behavior of bicyclic carbenes based on their ring structures.