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Thermal Rearrangements of 2‐Vinylcyclopropylidene to Cyclopentadiene and Vinylallene: A Theoretical Investigation
Author(s) -
Olivella Santiago,
López Núria
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010917)7:18<3951::aid-chem3951>3.0.co;2-u
Subject(s) - carbene , conformational isomerism , chemistry , basis set , singlet state , ab initio , cyclopentadiene , computational chemistry , bond length , crystallography , atomic physics , molecule , density functional theory , physics , excited state , crystal structure , catalysis , biochemistry , organic chemistry
In an attempt to clarify the favored rearrangement reaction of vinylcyclopropylidenes, the prototype thermal rearrangements of singlet 2‐vinylcyclopropylidene ( 1 ) leading to 1,3‐cyclopentadiene ( 2 ) and 1,2,4‐pentatriene (vinylallene) ( 3 ) were investigated by means of ab initio quantum‐mechanical electronic‐structure calculations. The B3LYP functional with the 6‐31G(d) basis set was employed for geometry optimization of the equilibrium and transition‐state structures relevant to the two reaction pathways and for computing their harmonic vibrational frequencies. Final energies were evaluated by single‐point calculations at the CCSD(T) level of theory with the 6‐311+G(3df,2p) basis set. The rearrangement of s ‐cis 1 to 2 is found to occur by a three‐step pathway. The first step involves the formation of a nonclassical carbene ( 5 ), which is an internal π complex between the π molecular orbital of the double bond and the empty p atomic orbital of the carbene carbon. In the second step, the nonplanar five‐membered ring geometry of 5 flattens to reach the planar structure of 3‐cyclopentenylidene ( 4 ). The last step is the 1,2‐migration of a α ‐hydrogen atom to the carbene center in 4 . The rate‐determining step for the rearrangement of s ‐cis 1 to 2 is the formation of 5 , with a predicted global Δ G ≠ (220 K) of only 0.6 kcal mol −1 . The rearrangement of s ‐trans 1 to 2 requires an initial conversion of s ‐trans 1 to the s ‐cis conformer, with a predicted Δ G ≠ (220 K) of 1.8 kcal mol −1 . The transition structure for the ring‐opening of s‐ trans 1 into s‐ trans 3 (Δ G ≠ (220 K)=4.7 kcal mol −1 ) is more energetic than that for the ring‐opening of s ‐cis 1 into s ‐cis 3 (Δ G ≠ (220 K)=2.5 kcal mol −1 ) due to larger repulsive nonbonded H⋅⋅⋅H interactions in the former transition structure. On the basis of these results, it is suggested that if the reaction of 1,1‐dibromo‐2‐vinylcyclopropane with methyllithium at −78 °C leads to the initial formation of carbene 1 , then the reaction should yield 2 as the main product together with small amounts of 3 . This theoretical prediction nicely agrees with experimental findings.