title: Peralkynylated buta-1,2,3-trienes: exceptionally low rotational barriers of cumulenic C=C bonds in the range of those of peptide C--N bonds.
abstract: A variety of 1,1,4,4-tetraal kynylbutatrienes and 1,4-dialkynylbutatrienes was synthezized by dimerization of the corresponding gem-dibromoolefins. Both (1)H and (13)C NMR spectroscopy indicated that the di- and tetraalkynylated butatrienes are formed as a mixture of cis and trans isomers. Variable temperature NMR studies evidenced a facile cis-trans isomerization, thus preventing the separation of these isomers by gravity or high-performance liquid chromatography (HPLC). For 1,1,4,4-tetraalkynylbutatrienes, the activation barrier deltaG( not equal ) was measured by magnetization transfer to be around 20 kcal mol(-1), in the range of the barrier for internal rotation about a peptide bond. Unlike the tetraalkynylated [3]cumulenes, 1,4-dialkynylbutatrienes are more difficult to isomerize and could, in one case, be obtained isomerically pure. Based on experimental data, the rotational barrier DeltaG( not equal ) for 1,4-dialkynylbutatrienes is estimated to be around 25 kcal mol(-1). The hypothesis of a stabilizing effect of the four alkynyl substituents on the proposed but-2-yne-1,4-diyl singlet diradical transition state of this cis-trans isomerization is further supported by a computational study.
authors.affiliation: Laboratorium für Organische Chemie, ETH-Hönggerberg, HCI, 8093 Zürich, Switzerland.
authors.forename: Audrey
authors.initials: A
authors.lastname: Auffrant
authors.forename: Bernhard
authors.initials: B
authors.lastname: Jaun
authors.forename: Peter D
authors.initials: PD
authors.lastname: Jarowski
authors.forename: Kendall N
authors.initials: KN
authors.lastname: Houk
authors.forename: François
authors.initials: F
authors.lastname: Diederich
mesh_terms: D000475:Alkenes / Q000138:chemical synthesis / Q000737:chemistry*; D000480:Alkynes / Q000737:chemistry*; D040761:Biomimetic Materials / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D010455:Peptides / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics
chemical_list: D000475:Alkenes; D000480:Alkynes; D010455:Peptides