Heteroatom Chemistry Volume 22, Number 3/4, 2011 Cyclobutadiene to Tetrahedrane: Valence Isomerization Induced by One-Electron Oxidation Masaaki Nakamoto, Yusuke Inagaki, Tatsumi Ochiai, Masanobu Tanaka, and Akira Sekiguchi Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received 31 August 2010; revised 10 November 2010 ABSTRACT: A new procedure for synthe- sizing tetrakis(trimethylsilyl)tetrahedrane from tetrakis(trimethylsilyl)cyclobutadiene is reported. Va- lence isomerization of cyclobutadiene to tetrahedrane induced by one-electron oxidation has been developed by the addition of tris(pentafluorophenyl)borane as an oxidant. This new method has great syn- thetic advantages for easy, quick, and high-yielding reaction to achieve gram-order-scale synthesis of tetrakis(trimethylsilyl)tetrahedrane. C  2011 Wiley Periodicals, Inc. Heteroatom Chem 22:412–416, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20699 INTRODUCTION Tetrahedrane is one of the most strained hydro- carbon molecules and also one of the valence iso- mers of cyclobutadiene (see Chart 1), which is a 4 π-electron antiaromatic molecule. A recent estima- tion of the strain in tetrahedrane is 140 kcal/mol and the strain energy of the skeleton appears to be Dedicated to Professor Kin-ya Akiba on the occasion of his 75th birthday Correspondence to: Akira Sekiguchi; e-mail: sekiguch@chem.tsukuba.ac.jp. Contract grant sponsor: Ministry of Education, Science, Sports, and Culture of Japan. Contract grant numbers: 19105001, 21750033, and 21108502. c  2011 Wiley Periodicals, Inc. much higher than the carbon-carbon bond disso- ciation energy of 81 kcal/mol [1]. Because of the strain energy, tetrahedrane possesses a higher en- thalpy of formation than cyclobutadiene [2]. For the relative energy difference between the isomers, tetrahedrane 2 is estimated to be less stable than cyclobutadiene 1 by 26.1 kcal/mol in the parent molecule [B3LYP/6-311G(2d,2p)] [3]. The isomer- ization of cyclobutadiene to tetrahedrane is a ther- mally symmetry-forbidden reaction in which the es- timated activation barrier of the parent molecule is 56.3 kcal/mol (MP2/6-311G ∗∗ ) [4b]. Although many attempts to isolate the par- ent tetrahedrane 2 have been unsuccessful [5], Maier et al. [6a] succeeded in synthesizing tetra-tert- butyltetrahedrane 4 in 1978, which was the first sta- ble tetrahedrane derivative, kinetically stabilized by four bulky tert-butyl groups. Until now, there have been only two procedures for synthesizing tetra- hedrane derivatives [6]. The first is the photore- action of cyclopentadienone, giving the tetra-tert- butyltetrahedrane 4 via CO elimination. The second method, a straightforward procedure for synthe- sizing tetrahedrane derivatives, is photoisomeriza- tion of cyclobutadiene. We and Maier et al. [4] have recently succeeded in synthesizing and iso- lating tetrakis(trimethylsilyl)tetrahedrane 6 by this method. We report herein a third method, which has the synthetic advantages of being an easy, quick, and high-yielding reaction for synthesizing tetrakis(trimethylsilyl)tetrahedrane 6 from cyclobu- tadiene 5 induced by one-electron oxidation. 412