Study of trans-2-tert-Butylcyclopropylcarbene by Laser Flash Photolysis and Chemical Analysis¹

Abstract

Laser flash photolysis (LFP) of trans-2-tert-butylcyclopropyldiazirine (4) produces trans-2-tert-butylcyclopropylcarbene (5). Carbene 5 can be trapped with pyridine to form ylide 6. The rate of formation of ylide 6 was resolved and found to be linearly dependent on the concentration of pyridine. The lifetime of carbene 5 was determined to be 22 ns (pentane), 18 ns (acetonitrile and CF2ClCFCl2), 15 ns (cyclohexane), and 27 ns (cyclohexane-d 12) at ambient temperature. The lifetime of carbene 5 in solution, at ambient temperature, is controlled substantially by reaction with solvent and only to a minor extent by rearrangement to cyclobutene 7. Photolysis of 4 (350 nm) in Freon-113 produces 3-tert-butylcyclobutene (7) along with products derived from attack of the carbene on solvent. Photolysis of 4 in the presence of trapping agents (tetramethylethylene, cyclopentene, propylamine, trifluoroethanol) produces adducts with little or no corresponding decrease in the yield of 7. The preferred interpretation is that cyclobutene 7 is formed by a rearrangement of the excited state of the diazirine (diazirinyl biradical), and not by a carbenic process at 0 °C and lower temperatures. This mechanism is supported by product analysis of a non-nitrogenous cyclopropylcarbene precursor. At ambient temperature, cyclobutene 7 is formed by both diazirine excited state (diazirinyl biradical) and carbene rearrangement.

Experimental and Theoretical Investigations of Ring-Expansion in 1-Methylcyclopropylcarbene

“…This analysis leads to a lifetime of 9 of only 12 ns. This lifetime compares to previously reported values for 5 , 6 , and 26 19 (cyclopropyl methyl carbene) of 18, 24, and 21 ns, respectively, which are largely controlled by reactions with solvent. Thus, the parent cyclopropyl carbene ( 6 ) undergoes rearrangement more slowly than the methyl substituted 9 , and C-migration is a significant pathway.…”

“…However, the choice of the appropriate carbene precursor is very important. , There has been substantial recent interest in the idea that conventional carbene precursors, such as diazirines and diazo compounds, can themselves give products identical to those expected from carbene intermediates . For instance, it has been shown that the ring-expanded product 3- tert -butylcyclobutene ( 2 ) and the fragmentation product tert -butylethylene ( 3 ), which are among the decomposition products of diazirine 1 in CF 2 ClCFCl 2 (Freon-113), are obtained from the precursor itself rather than the carbene 5 in solution at 0 °C . The adduct 4 , on the other hand, was formed by the trapping of 5 by 2,3-dimethyl-2-butene (TME).…”

“…Since the lifetime, τ, of 9 will be equal to 1/ k o , it is possible to estimate the lifetime of singlet 9 by using the value of k TME for (2- tert -butyl)-cyclopropylcarbene ( 5 ) in Freon-113 as determined by laser flash photolysis by Huang and Platz ( k TME = 1.02 × 10 9 M -1 s -1 ) . This analysis leads to a lifetime of 9 of only 12 ns.…”

Dynamics of 1,2-Hydrogen Migration in Carbenes and Ring Expansion in Cyclopropylcarbenes

“…Experimentally, the determination of the rate constant of 1,2-migrations in carbenes may be complicated by various concurrent reactions. For example, many investigations have obtained evidence that, at least in some cases, 1,2-atom migrations are concurrent with N 2 extrusion and occur from an excited state of diazirines, ,− , without the intermediacy of the carbene. Diazo compounds generated from diazirines have been ruled out as intermediates in the formation of olefins by 1,2-shifts because the formation of diazo compounds is unlikely at ambient temperature .…”

“…With few exceptions, , most singlet alkyl or alkylaryl carbenes undergo rapid intramolecular 1,2-hydrogen, alkyl, or aryl migrations to form alkenes − or react by other fast intermolecular proceses. − The intramolecular rearrangements have been the focus of extensive investigations, both experimental − ,,− and theoretical. ,,− …”