Photolysis and thermolysis of (iodomethyl)cyclopropane: Rapid ring opening of cyclopropylcarbinyl via heavy‐atom tunneling?

Abstract

UV photolysis of (iodomethyl)cyclopropane yielded 3-buten-1-yl instead of cyclopropylcarbinyl, the expected product of photolytic C-I bond cleavage. This result suggests that the ring opening of the intermediate cyclopropylcarbinyl even at T $10 K proceeds too fast to be measured via this technique. This finding is in agreement with earlier theoretical studies which predicted the rapid decay of cyclopropylcarbinyl via heavy-atom tunneling. In an attempt to rule out the potential of secondary photochemistry enabling the ring opening, flash vacuum thermolysis experiments were also performed. However, those resulted in butadiene as sole product, in both its thermodynamically favored trans-and the higher-energy gaucheconformation. K E Y W O R D Sheavy-atom tunneling, matrix isolation, organic radicals | INTRODUCTIONQuantum mechanical tunneling (QMT) describes the penetration of energetic barriers instead of overcoming them via influx of thermal energy and was once considered a curiosity with little impact on organic reactions at noncryogenic temperatures. [1] For hydrogen transfer reactions the view has shifted drastically, such that the contribution of QMT has been recognized even in biologically important contexts. [2] In contrast to this, the consideration of the tunneling of heavier atoms like carbon has always trailed behind, although already a seminal study by Carpenter found that QMT accounts for over 97% of the automerization of cyclobutadiene at T < 0 C. [3] Recent years have seen a growing amount of computational [4,5] and experimental [6] evidence for the impact of heavy-atom tunneling on prototypical reactions under common lab conditions. [7] One such study was the joint experimental and theoretical investigation of the rapid ring opening of cyclopropylcarbinyl (1) to 3-buten-1-yl (2) by Singleton, Borden and coworkers. [8] The rate of this intramolecular rearrangement, which is frequently used as a radical clock, [9] had prior been predicted by Datta, Hrovat and Borden to contain a large contribution from QMT, [10] based on small curvature tunneling (SCT) calculations.By measuring the 12 C/ 13 C kinetic isotope effect (KIE) at natural abundance in a temperature range typical for Dedicated to Professor Barry Carpenter on the occasion of his retirement.

Quantum tunneling instability of the mythical hexazine and pentazine

“…Nonetheless, it is important to acknowledge that certain molecules, despite being predicted to be minima on the potential energy surface, may be inherently unstable due to quantum tunneling (QT) effects, 15–21 or what we called quantum tunneling instability (QTI). 22,23 Consequently, the decomposition might proceed regardless of the temperature if the reaction complies with specific conditions that enhance the QT process.…”