Hydrogen Migration vs Carbon Migration in Dialkylcarbenes. A Study of the Preferred Product in the Carbene Rearrangements of Ethylmethylcarbene, Cyclobutylidene, 2-Norbornylidene, and 2-Bicyclo[2.1.1]hexylidene

Abstract

Ethylmethylcarbene ( 1 ), cyclobutylidene ( 2 ), 2-norbornylidene ( 3 ), and 2-bicyclo[2.1.1]-hexylidene ( 4 ) and the transition states that correspond to 1,2-H-migration, 1,3-H-migration, and 1,2-C-migration were optimized at BHandHLYP/DZP, MP2/DZP, and CCSD(T)/DZP levels of theory. The 1,2-H-migration of 1 to 2-butene has a theoretically derived barrier of 5.2 kcal/mol at CCSD(T)/DZP. The 1,2-H-shift that leads to 1-butene has a Δ G ⧧ of 8.5 kcal/mol, 1,3-H-migration of 8.3 kcal/mol and 1,2-C-migration of 18.1 kcal/mol. For 2 Δ G ⧧ for 1,2-C-migration is only 10.5 kcal/mol, which is 7.6 kcal/mol less than for 1 . This lowering of the barrier for rearrangment of cyclobutylidene is attributed to the similarity between the TS and singlet 2 which prefers a nonclassical structure. The barrier for 1,2-H-migration for 2 is 9.7 kcal/mol due to H repulsion in the TS. For 3 the process with the lowest barrier (5.2 kcal/mol, BHandHLYP/DZP) is a 1,3-H-shift that leads to nortricyclene. The preference for this rearrangement can again be explained by the similarity between the carbene geometry and that of the corresponding TS that leads to the nortricyclene. For the rearrangement of 4, which also resembles the TS for 1,3-H-migration, the corresponding TS has a Δ G ⧧ of 22.6 kcal/mol (BHandHLYP/DZP). The reason for this diverging behavior is the large amount of ring strain present in the TS for 1,3-H-migration of 4 . As a consequence, 4 is a long-lived, trappable carbene that rearranges slowly to form bicyclo[2.1.1]hex-2-ene (Δ G ⧧ = 16.2 kcal/mol), while 3 can not be trapped with pyridine.