Cyclobutadiene attempted synthesis

The smallest member, cyclobutadiene, was the objective of attempted synthesis for many years. The first success was aehieved when cyclobutadiene released from a stable iron complex was trapped with various reagents. ... 

Let us close this account of simple prismanes with a look at hexaprismane. The synthesis of cubane was reported in 1964 pentaprismane was said (1974) to show extraordinary synthetic inaccessibility [16], but was made in 1981 [13] and hexaprismane has so far (early 2007) resisted all attempts at synthesis [5]. As Hopf points out [5], hexaprismane can in principle be assembled from two benzene rings or from cyclobutadiene and cyclooctatetraene ... 

The first attempt at the synthesis of cyclobutadiene (I), also by Willstatter 94) in 1905 was not successful. Since then numerous workers have attempted to prepare cyclobutadiene and its derivatives. Although no stable simple derivatives have been obtained, evidence has accumulated which appears to point to the intermediacy of cyclobutadienes in some reactions (7,8, 92a). 

Sergheraert s group also attempted the synthesis of an enkephalin derivative with the organometaUic amino acid cyclobutadiene iron tricarbonyl alanine 26 [73]. This compoimd, however, did not withstand the conditions of solid phase peptide synthesis and could not be isolated in pure form. 

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