Tetrahedrane and Cyclobutadiene

Tetra-tert-butyltetrahedrane—a useless molecule? In the first part of this progress report an attempt is made to answer this provoking question in order that the reader can evaluate why it is worthwhile to track such an esoteric molecule. An account of the unusual properties of tetra-tert-butyltetrahedrane, the only tetrahedrane unequivocally identified so far, and the most highly strained saturated hydrocarbon of all, is followed by a summary of attempts to synthesize the parent compound. The emphasis in these two sections is on our own research achievements. Since any discussion of the tetrahedranes must necessarily include the corresponding cyclobutadienes, it was found appropriate to organize the article at the same time as a final account of the cyclobutadiene problem. Masamune^[8c] characterized his excellent report in 1980 in the journal Tetrahedron as a comma in the history of cyclobutadiene. Meanwhile, the significant questions have been answered. The time is now ripe to replace the comma by a full stop.     

The Cyclobutadiene Problem

Cyclobutadiene has recently become the center of reawakened interest. By use of the matrix isolation technique, cyclobutadienes can now be prepared as monomers frozen in matrices. Thus both the unsubstituted parent compound and its alkyl-substituted derivatives are amenable to study. The present progress report surveys the entire field; however, particular emphasis is placed on the author's own results from investigations concerning this classical problem of organic chemistry.     

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene

Tetrakis(trimethylsilyl)cyclobuta‐1,3‐diene ( 1 ) was subjected to a temperature‐dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene ( 2 ). From the temperature dependent EPR absorption area we derive a singlet→triplet ( 1 → 2 ) energy gap, E _ST, of 13.9 kcal mol⁻¹, in agreement with calculated values. The zero‐field splitting parameters D =0.171 cm⁻¹, E =0 cm⁻¹ are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.