Calculations on Tunneling in the Reactions of Noradamantyl Carbenes

Abstract

Noradamantylchlorocarbene has been found experimentally to undergo ring expansion to 2-chloroadamantene at cryogenic temperatures. The rate constant, calculated with inclusion of small-curvature tunneling, is within a factor of 2 of the rate constant measured at 9 K in a nitrogen matrix. Our calculations predict that noradamantylfluorocarbene will not be found to rearrange under these conditions. The rate constant for carbon tunneling in the ring expansion of noradamantylmethylcarbene (1d) to 2-methyladamantene at T </~ 10 K is calculated to be lower by more than 8 orders of magnitude than the rate constant for formation of 3-vinylnoradamantane from 1d by hydrogen migration.

Reactions that involve tunneling by carbon and the role that calculations have played in their study

“…The rate constant for this reaction was measured by Bob Moss, Bob Sheridan, and their coworkers to be k = 2.3 × 10 −7 s −1 . The B3LYP/6‐31G(d) SCT rate constant calculated for this reaction by Kozuch, Borden, and coworkers, is 3.7 × 10 −7 s −1 , which is within a factor of two of the experimental value.…”

“…For example, we predicted that, at cryogenic temperatures, fluorocarbene 13a would be found not to undergo ring expansion to 14a , based on the B3LYP/6‐31G(d) SCT rate constant of k = 7.3 × 10 −16 , computed for tunneling below 20 K. The reason for the small rate constant that is calculated for the ring expansion of 13a to 14a is that this reaction is calculated to be much less exothermic than ring expansion of 9a to 10a . Consequently, the ring expansion of noradamantylfluorocarbene 13a to 14a is computed to have a 6 kcal/mol higher barrier than the ring expansion of 1‐methylcyclobutylfluorocarbene 9a to 1‐fluoro‐2‐methylcyclpentene ( 10a ) …”

“…Kozuch and Borden calculated the B3LYP/6‐31G(d) SCT rate constant for ring expansion of noradamantymethylcarbene ( 13d ) by tunneling at cryogenic temperatures, as well as by passage over the reaction barrier at room temperature . These calculations were performed, because, as shown in Figure , 13d can undergo a competing reaction, a 1,2‐hydrogen shift, to form vinylnoradamantane ( 17 ).…”

“…Although 1,2‐hydrogen shifts in carbenes are usually very fast, relief of ring strain is calculated to make the barrier to ring expansion of 13d to 14d slightly lower than that for the 1,2‐hydrogen shift. Consequently, at 300 K, 14d was calculated to be formed about 20 times faster than 17 from 13d …”

Cyclopropenyl Anions: Carbon Tunneling or Diradical Formation? A Contest between Jahn–Teller and Hund

“…This approach has proven to be successful in previous theoretical studies. 32,33 The rate constants were computed with Polyrate, 34 using Gaussrate 35 as the interface between Polyrate and Gaussian09. 36 Unless specified, SCT values include the CVT rate constant and the tunneling correction.…”