3,4-Dichloro-1,2,3,4-tetramethylCYCLOBUTENE

In support of the prediction that cyclobutadiene is likely to have a triplet ground state, Skell and Petersen 87) have recently examined the reactions of the species obtained by gas-phase reaction of 3,4-dichloro-tetramethylcyclobutene (X) with potassium. They found that the dehalo-genated species (XI) behaved as a molecule in the triplet state would be expected to, particularly in its reaction with triplet methylene to give l,2-dimethyl-3,4-dimethylenecyclobutene (XII) and hence proposed this as the first direct evidence for the existence of a < clobutadiene as an intermediate. 

In addition, directions are given in Volume 46 for the synthesis of 3,4-dichloro-l,2,3,4-tetramethylcyclobutene (p. 34), 10-methyl-10,9-borazarophenanthrene (p. 65), perchlorofulvalene (p. 93), and a-pyrone (p. 101), molecules which by any standard deserve the designation unique. 

In addition, directions are given in Volume 46 for the synthesis of 3,4-dichloro-l,2,3,4-tetramethylcyclobutene(p. 34), 10-methyl-... 

An isomeric form of the anion that is initially formed is converted to the all-cA system rapidly at room temperature. Data on the equilibrium acidity of the parent hydrocarbon are not available, so the stability of the anion cannot be judged quantitatively. The NMR spectrum of the anion, however, is indicative of aromatic character. Several doubly charged ions are included in Scheme 8.1 some have been observed experimentally. Ionization of 3,4-dichloro-l,2,3,4-tetramethylcyclobutene in Sbp5-S02 at -75°C results in an NMR spectrum attributed to the tetramethyl derivative of the cyclobutadienyl dication. 

Similarly, treatment of but-2-yne with chlorine in the presence of boron trifluoride gives P-chlorovinylium ion. This ion undergoes [rr s + rr a] combination with but-2-yne to give the cyclobutenyl cation, whose discharge by chloride ion yields 3,4-dichloro-l,2,3,4-tetramethylcyclobutene. 

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