Pseudopericyclic reactions: Eight centered transition states in reactions of acetates and related systems

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2016-05

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Abstract

Pseudopericyclic reactions are a subset of pericyclic reactions that are predicted to be thermally forbidden according to Woodward-Hoffmann rules. However, all pseudopericyclic reactions are predicted to be allowed via planar transition states irrespective of the number of electrons involved in a cyclic array of orbitals. This research was undertaken to verify this qualitative prediction of eight-centered reactions, using both computational chemistry and experimental product studies. A theoretical study was performed to understand thermal reactions of xanthates that can be derived from allylic as well as pentadienyl alcohols. In addition to a well-known [3,3]-sigmatropic rearrangement, a low barrier pseudopericyclic [3,5]-rearrangement is discussed in unconstrained acyclic as well as conformationally constrained pentadienyl systems. The B3LYP/6-31G(d,p) calculations predict that the [3,5]-sigmatropic rearrangement of xanthates occurs via a pseudopericyclic transition state and is favored over the [3,3]-rearrangement by 2.2-3.6 kcal/mol. Thermal β- and δ-elimination reactions of allylic xanthates were also studied in both cyclic and acyclic model systems. The results indicated that the [3,3]-rearrangement is favored over both β- and δ-eliminations in the case of acyclic allylic xanthates (derived from cis-3-penten-2-yl alcohol). However, in a cyclic system where δ-elimination leads to aromatization, this pathway is calculated to be favored over both β-elimination and [3,3]-rearrangement. An efficient Pd-catalyzed Suzuki cross-coupling reaction of sterically crowded 4-chlorocoumarin derivatives was developed. This methodology has been used to generate a series of novel alkyl, aryl and vinyl substituted coumarin derivatives in good to excellent yields. The (2-oxo-4-vinyl-2H-chromen-3-yl)methyl acetate (2) and (2-oxo-4-vinyl-2H-chromen-3-yl)methyl 2,2,2-trichloroacetimidate (3) derivatives were predicted to be able to undergo thermal [3,3]- and [3,5]-sigmatropic rearrangements. Preliminary pyrolysis data for acetate 2 showed no formation of other products. The DFT calculations performed suggest that both the [3,3]- and [3,5]-rearrangements are endothermic and so would not be expected to occur in acetate 2. Imidate 3 underwent conversion to a mixture of unidentified products with absence of starting material in preliminary thermolysis and FVP experiments. Calculations suggests an overall lower barrier for imidate rearrangements, and also that the [3,5]-amide product 11 and the subsequent [1,5]-rearrangement product 12 might be thermodynamically more stable than the starting imidate 3. Sigmatropic [1,j]-hydrogen shifts can occur either suprafacially or antarafacially across a conjugated system. Although a pericyclic sigmatropic [1,7] hydrogen atom migration is thermally allowed via an antarafacial pathway, qualitative theory suggests that a pseudopericyclic 1,7-hydrogen migration would be allowed via a planar pathway. The bicyclo[3.3.0]octa-1-ene-2,8-dicarbaldehyde (1) and related systems have a suitable geometry to permit an in-plane eight-centered transition state for a pseudopericyclic [1,7]-hydrogen migration. DFT calculations were performed at the B3LYP/6-31G(d,p) level of theory to study [1,7]-H shift in compounds 1, 10 and 11. The low barriers calculated and also a planar transition state, suggested a pseudopericyclic mechanism for the sigmatropic [1,7]-hydrogen migration. The design and progress toward the synthesis of the constrained homoannular bicyclic dialdehyde 1 is also discussed.

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Keywords

Pseudopericyclic reactions, [3,3]-Sigmatropic rearrangement, [3,5]-Sigmatropic rearrangement, [1,7]-Sigmatropic Hydrogen shift, Acetates, Xanthate, Trichloroacetimidate, Coumarin

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