Spectroscopy and Photochemistry of Triplet 1,3-Dimethylpropynylidene (MeC₃Me)

Abstract

Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11) affords triplet 1,3-dimethylpropynylidene (MeC3Me, (3)3), which was characterized spectroscopically in cryogenic matrices. The infrared, electronic absorption, and electron paramagnetic resonance spectra of MeC3Me ((3)3) are compared with those of the parent system (HC3H) to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Quantum chemical calculations (CCSD(T)/ANO1) predict an unsymmetrical equilibrium structure for triplet MeC3Me ((3)3), but they also reveal a very shallow potential energy surface. The experimental IR spectrum of triplet MeC3Me ((3)3) is best interpreted in terms of a quasilinear, axially symmetric structure. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (|D/hc| = 0.63 cm(-1), |E/hc| = ∼ 0 cm(-1)), while theoretical analysis suggests that the methyl substituents confer significant spin polarization to the carbon chain. Upon irradiation into the near-UV electronic absorption (λmax 350 nm), MeC3Me ((3)3) undergoes 1,2-hydrogen migration to yield pent-1-en-3-yne (4), a photochemical reaction that is typical of carbenes bearing a methyl substituent. This facile process apparently precludes photoisomerization to other interesting C5H6 isomers, in contrast to the rich photochemistry of the parent C3H2 system.

Synthesis of Tetrasubstituted Allenes via Visible-Light-Promoted Radical 1,3-Difunctionalization of Alkynyl Diazo Compounds

“…Alkynyl diazo compounds are important precursors of alkynyl carbenes, and their reaction mechanism upon photolysis and thermolysis has been a subject of numerous physicochemical studies due to their unusual electronic structure . Free alkynyl carbenes are also intermediates in the chemistry of interstellar space and polymer photochemistry .…”

Ethynylhydroxycarbene (H–C≡C–C̈–OH)

“…In general, ethynylcarbenes are relevant in astrochemical processes , and can even be used synthetically. − The electronic structure of parent ground-state triplet propynylidene (HCCCH) has been explored in numerous experimental and computational studies with the result that both terminal carbons are equivalent in terms of their reactivity. − The photoreaction of propynylidene with triplet dioxygen provides another entrance channel to the C 3 H 2 O PES as shown in a study by Wierlacher et al, who observed 2 besides the corresponding dioxirane and carbonyl oxide . Other triplet ground-state derivatives of propynylidene have been investigated spectroscopically and they display similarly delocalized electronic structures. − In contrast, singlet ethynylcarbenes (such as 1 ) can best be represented by a localized carbene center in α-position to a localized CC bond …”

Infrared-Spectroscopic Study of (4-Methylpent-3-en-1-ynyl)methylthiocarbene, Its Photochemical Transformations, and Reactions in an Argon Matrix

“…Carbenesunstable compounds of divalent carbontogether with ions and free radicals are one of the main classes of highly reactive intermediates that determine the direction of chemical reactions and the structure of the resulting products . They have been well known for almost 70 years and have been a subject of numerous chemical and physicochemical studies. − In recent years, considerable attention has been paid to the study of ethynylcarbene- conjugated triplet carbon chain molecules, whose electronic structure and reactivity have been explored by different spectroscopic and computational methods. − These species are of interest due to their unusual electronic structure, as well as their participation as intermediates in many important processes in astronomy , and polymer photochemistry. − Base-induced generation of different alkynylcarbenes and their reactions with various organic substrates have been the subjects of our previous works. − Ten years back, a highly delocalized triplet carbene comprising both vinyl and ethynyl groups at the carbene center, 5-methylhexa-1,2,4-triene-1,3-diyl, was generated in a low-temperature Ar matrix upon UV photolysis of 5-ethynyl-3,3-dimethyl-3 H -pyrazole and studied by IR and ESR spectroscopies. , It was shown that two major photolytic rearrangements of this carbene represent reactions characteristic of vinyl carbenes, resulting in the formation of 1-ethynyl-3,3-dimethylcyclopropene and 3 E -2-methylhexa-1,3-dien-5-yne. A minor reaction, typical of ethynylcarbenes, leads to the formation of singlet 2-(2-methylpropenyl)­cyclopropenylidene.…”