A Highly Delocalized Triplet Carbene, 5-Methylhexa-1,2,4-triene-1,3-diyl: Matrix IR Identification, Structure, and Reactions

• Published in: Journal of the American Chemical Society Vol. 131; no. 41; pp. 14688 - 14698
• Main Authors: Boganov, Sergey E, Faustov, Valery I, Shavrin, Konstantin N, Gvozdev, Valentin D, Promyslov, Vladimir M, Egorov, Mikhail P, Nefedov, Oleg M
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 21.10.2009; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; DENSITY; ELECTRONIC EXCITED-STATES; VINYLCARBENES; AB-INITIO; SMALL RING COMPOUNDS; PHOTOCHEMISTRY; GENERATION; CYCLOPROPENE; REARRANGEMENTS; SINGLET
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja901508c

The first representative of highly delocalized triplet carbenes bearing both vinyl and ethynyl groups at the formal carbene center, 5-methylhexa-1,2,4-triene-1,3-diyl, has been generated in a low-temperature Ar matrix upon UV photolysis of 5-ethynyl-3,3-dimethyl-3H-pyrazole and detected by FTIR spectroscopy. The transformation of 3H-pyrazole into the carbene proceeds in two stages via intermediate 3-diazo-5-methylhex-4-en-1-yne. According to DFT PBE/TZ2P calculations, 5-methylhexa-1,2,4-triene-1,3-diyl possesses an effective conjugation along the five-carbon chain and shows the same type of the bond length alternation as the HC4m+1H-type polyacetylenic carbenes. The carbene readily reacts with molecular oxygen, producing carbonyl oxides, which undergo further transformations typical of this type of compound upon irradiation in the UV−visible region. Two major photolytic rearrangements of 5-methylhexa-1,2,4-triene-1,3-diyl represent reactions characteristic of vinyl carbenes and resulting in the formation of 1-ethynyl-3,3-dimethylcyclopropene and 3E-2-methylhexa-1,3-dien-5-yne. A minor reaction is that typical of ethynylcarbenes; this leads to the formation of singlet 2-(2-methylpropenyl)cyclopropenylidene. Fragments of singlet and triplet potential energy surfaces of the C7H8 system have been explored in DFT PBE/TZ2P calculations.