Picosecond radical kinetics. Bond cleavage of the cubylcarbinyl radical

Nonconjugated Hydrocarbons as Rigid‐Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

“…Despite the stability of the cubane radical, different effects are observed for radicals adjacent to the caged ring system. The cubylcarbinyl radical 69 , generated by the photolysis of the corresponding N ‐hydroxy‐2‐pyridinethione ester 70 (“Barton ester”), does not show the 1,2‐shift to homocubyl radical 71 , which was found for the analogous cubylcarbinyl cation (Scheme ) . Instead a very fast ring opening occurs ( k R =2×10 10 s −1 ).…”

Oxidation of amines by flavoproteins

“…However, trans-2-phenyl(aminomethyl)cyclopropane ( 1 in Scheme 13) does not inactivate MAO or produce ring-opened products [76], even though production of the carbon radical adjacent to the cyclopropyl ring should result in rapid ring opening. To test the possibility that radical collapse is faster than opening of the cyclopropyl ring, aminomethylcubane ( 2 ) was examined as a mechanism -based inhibitor, since the rate constant for opening of the ring in the radical is much greater than that for a cyclopropyl ring [77]. No direct evidence for a ring-opened product could be obtained in this case either [76].…”

A Resin‐Linker‐Vector Approach to Radiopharmaceuticals Containing 18F: Application in the Synthesis of O‐(2‐[18F]‐Fluoroethyl)‐L‐tyrosine

“…The cascade rearrangement reactions of the cubylcarbinyl radical have attracted many research workers,1–15 primarily because of the fast reaction rate of its initial ring opening, which is ranked as one of the fastest known radical reactions 1, 2. In 1991, Eaton and Yip1 firstly obtained the cubylcarbinyl radical from cubylcarbinyl bromide and from the N ‐hydroxy‐2‐pyridinethione ester of cubylacetic acid under various conditions favoring hydrogen‐atom transfer to the radical.…”

“…The rate constant of the conversion from the cubylcarbinyl radical to the tricyclic dienes radical at 293 K was determined to be about 2 × 10 10 s −1 by Eaton and Yip 1. In 1992, Choi et al 2. measured the rate constants for the initial bond cleavage of the cubylcarbinyl radical over the temperature range from 195 to 298 K by competition kinetics using benzeneselenol (PhSeH) as a trapping agent, giving a temperature‐dependent function of log ( k · s ) = 13.16–3.72 kcal/mol/2.3 RT, from which the energy barrier of the initial conversion was obtained to be 3.7 kcal/mol and the rate constant to be 2.9 × 10 10 s −l at 298 K 2.…”

“…measured the rate constants for the initial bond cleavage of the cubylcarbinyl radical over the temperature range from 195 to 298 K by competition kinetics using benzeneselenol (PhSeH) as a trapping agent, giving a temperature‐dependent function of log ( k · s ) = 13.16–3.72 kcal/mol/2.3 RT, from which the energy barrier of the initial conversion was obtained to be 3.7 kcal/mol and the rate constant to be 2.9 × 10 10 s −l at 298 K 2. Unfortunately, the question of whether the initial conversion is a stepwise process with the methylenesecocubyl radical as a discrete intermediate or the cubylcarbinyl radical is directly converted to the tricyclic dienes radical by two synchronous bond cleavages is uncertain, because the methylenesecocubyl radical has never been intercepted experimentally,1–3 even though the ring opening of the cubylcarbinyl radical was studied at low temperatures 2. Attributed to the mechanistic complexity of the initial rearrangement of the cubylcarbinyl radical mentioned above, its usefulness as a valuable ‘free radical clock’ was detracted 4…”