Formation of a 1-bicyclo[1.1.1]pentyl anion and an experimental determination of the acidity and C-H bond dissociation energy of 3-tert-butylbicyclo[1.1.1]pentane

Dana R. Reed, Steven R. Kass, Kathleen R. Mondanaro, William P. Dailey

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Abstract

Decarboxylation of 1-bicyclo[1.1.1]pentanecarboxylate anion does not afford 1-bicyclo[1.1.1]pentyl anion as previously assumed. Instead, a ring-opening isomerization which ultimately leads to 1,4-pentadien-2-yl anion takes place. A 1-bicyclo[1.1.1]pentyl anion was prepared nevertheless via the fluoride-induced desilylation of 1-tert-butyl-3-(trimethylsilyl)bicyclo[1. 1.1]pentane. The electron affinity of 3-tert-butyl-1-bicyclo[1.1.1]pentyl radical (14.8 ± 3.2 kcal/mol) was measured by bracketing, and the acidity of 1-tert-butylbicyclo[1.1.1]pentane (408.5 ± 0.9) was determined by the DePuy kinetic method. These values are well-reproduced by G2 and G3 calculations and can be combined in a thermodynamic cycle to provide a bridgehead C-H bond dissociation energy (BDE) of 109.7 ± 3.3 kcal/mol for 1-tert-butylbicyclo[1.1.1]pentane. This bond energy is the strongest tertiary C-H bond to be measured, is much larger than the corresponding bond in isobutane (96.5 ± 0.4 kcal/mol), and is more typical of an alkene or aromatic compound. The large BDE can be explained in terms of hybridization.

Original languageEnglish (US)
Pages (from-to)2790-2795
Number of pages6
JournalJournal of the American Chemical Society
Volume124
Issue number11
DOIs
StatePublished - Mar 20 2002

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