A Highly Delocalized Triplet Carbene, 5-Methylhexa-1,2,4-triene-1,3-diyl: Matrix IR Identification, Structure, and Reactions
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 spec...
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Published in | Journal of the American Chemical Society Vol. 131; no. 41; pp. 14688 - 14698 |
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Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
WASHINGTON
American Chemical Society
21.10.2009
Amer Chemical Soc |
Subjects | |
Online Access | Get full text |
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Summary: | 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. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0002-7863 1520-5126 |
DOI: | 10.1021/ja901508c |