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Torquoselectivity in the Cationic Cyclopentannelation of (2 Z )‐Hexa‐2,4,5‐trienal Acetals
Author(s) -
Iglesias Beatriz,
de Lera Angel R.,
RodríguezOtero Jesús,
López Susana
Publication year - 2000
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(20001103)6:21<4021::aid-chem4021>3.0.co;2-n
Subject(s) - chemistry , isomerization , carbocation , intramolecular force , steric effects , substituent , hexa , ab initio , double bond , stereochemistry , ring (chemistry) , computational chemistry , medicinal chemistry , catalysis , biochemistry , organic chemistry , polymer chemistry
Inter‐ and intramolecular trapping experiments and density functional theory ab initio calculations for model systems are consistent with the acid‐catalyzed rearrangement of 2‐[(1 Z )‐hexa‐1,3,4‐trienyl]dioxolanes 1 to tetrahydroalkylidenecyclopenta‐1,4‐dioxins 4 ; this involves the electrocyclic ring closure of substituted hydroxypentadienyl carbocations. The reaction, which may be considered a variant of the Nazarov cyclization, occurs much more readily than the standard Nazarov cyclization, proceeding rapidly even at −30 °C. B3LYP/6‐31G**//HF/6‐31G** calculations for models 36, 38 and 40 predict that the two alternative conrotations at the cyclization termini are associated with activation energies differing by 0.55, 0.56 and 1.60 kcal mol −1 , respectively, in favour of the R‐outwards rotation. This last value corresponds to an E ‐ 41 / Z ‐ 41 product ratio of >99:1 at −60 °C, in consonance with the experimental observation that divinylallene 1 a rearranges exclusively to E ‐ 4 a at temperatures below −30 °C. At higher temperatures the torquoselectivity of the reaction 1 a → 4 a is masked by subsequent isomerization to the Z isomer, the greater stability of which is attributable to steric interaction between the substituent at the exocyclic double bond and the bulky neighbouring t Bu group in the E isomer.