Laser Flash Photolysis Studies of Oxygen and Sulfur Atom Transfer Reactions from Oxiranes and Thiiranes to Singlet Carbenes¹

Abstract

Laser flash photolysis (UV-LFP) studies of benzylchlorodiazirine (1a), phenylchlorodiazirine (1b), phenylmethoxydiazirine (1c), 2,2-dimethoxy-5,5-dimethyl-∆ 3 -1,3,4-oxadiazoline (6a), 3,4-diaza-2,2-dimethoxy-1-oxa[4.3]spirooct-3-ene (6b), 5′,5′-dimethoxyspiro[adamantane]-2,2′-[∆ 3 -1,3,4-oxadiazoline] (6c), and diazofluorene (DAF) in the presence of oxiranes and thiiranes are reported. These compounds, upon irradiation, afford benzylchlorocarbene (2a), phenylchlorocarbene (2b), methoxyphenylcarbene (2c), dimethylcarbene (8a), cyclobutylidene (8b), adamantylidene (8c), and fluorenylidene (FL), respectively. Absolute rate constants for the transfer of oxygen and sulfur atoms to these carbenes have been determined in both acetonitrile and cyclohexane solvents. These carbenes abstract oxygen and sulfur atoms with bimolecular rate constants ranging from 10 4 to 10 10 M -1 s -1 at 22°C. Ylides from attack of carbenes onto heteroatom donors were not observed for any of the heteroatom transfer reactions. It was found that the magnitudes of the rate constants for heteroatom transfer are dependent on the philicity of the carbene intermediate, and trends in the kinetic data suggest that oxygen and sulfur atom transfers occur by concerted mechanisms through ylide-like transition states. The reactions of dimethoxycarbene (14) with cyclohexene oxide and propylene sulfide have been studied by the thermolysis of 6a, in benzene, at 110°C. It was found that dimethoxycarbene also abstracts oxygen and sulfur atoms, albeit in low yields. It is concluded that, for singlet carbenes, carbene electrophilicity is important in these heteroatom transfer processes.S0002-7863(97)03469-0 CCC: $15.00

Deoxygenation and Desulfurization of Oxiranes and Thiiranes by Carbenes: A Theoretical Study

“…[7,8] Indeed, this is consistent with results based on spin conservation, which predict that a singlet carbene is required to produce singlet abstraction products. [27] Additionally, as noted earlier, all of the carbenes 1 ± 6 considered in this study have either a singlet ground state or a triplet ground state with a relatively small singlet ± triplet energy gap.…”

“…Our model prediction is consistent with experimental observations, in which ylides from the reaction of carbenes with oxiranes were not detected by laser flash photolysis (LFP) methods. [7,8] In addition, it should be noted that charge transfer between oxirane and carbene occurs despite their large separation. This charge separation may be used as a guide to examine the carbene philicity.…”

“…Recently, through the elegant studies performed by Lusztyk, Warkentin, and many others, [7,8] it was found that singlet carbenes are capable of abstracting oxygen and sulfur atoms from oxiranes and thiiranes, respectively, undergoing the reaction shown above [Eq. (1)].…”

“…Six carbenes (1 ± 6), which mimic the experimental work, [7] have been chosen as model systems for heteroatom transfer in this work. These carbene species are dimethylcarbene (1), cyclobutylidene (2), cyclohexylidene (3), phenylchlorocarbene (4), methoxyphenylcarbene (5), and dimethoxycarbene (6).…”

Synthesis of some fenchyl-substituted alkenes and enol-ethers containing 3-oxyphenyl substituents by the Barton-Kellogg reaction

“…12,13 This inverse approach is required because the conventional BK reaction would require the difficult preparation of a methoxy-substituted diazomethane derivative. 19 The Bamford-Stevens method was unsuccessful to convert the corresponding tosylhydrazone to diazofenchane (4b). Thus, 4b was prepared by oxidation of fenchylhydrazone (2b) with nickel peroxide and calcium oxide in dimethoxyethane at −78 °C.…”