Cyclobutadienes matrix trapping

Allenes can also be synthesized from diazocyclopropanes by chemical or photochemical [l,2,l]-elimination of nitrogen. In the thermolysis of 28 to give 30 the carbene-intermediate 29 could be trapped 18), and in the low-temperature photolysis of 31 the triplet carbene 32 could be detected by EPR-spectroscopy 17). 32 is longlived in a polycrystalline matrix and rearranges to 33 (28 %) at a temperature of —154 °C 19>. Numerous applications are included in Ref. 20). Especially noteworthy are the syntheses of stable cyclobutadienes by Masamune (90%)21) and Regitz (67%)22). 

Although several derivatives of cyclobutadiene are known and are discussed shortly, cyclobutadiene itself has been observed only as a matrix-isolated species, that is trapped at very low temperature in a frozen inert gas. The first successful synthesis of cyclobutadiene was achieved by release from a stable iron complex." " Various trapping agents react with cyclobutadiene to give Diels-Alder adducts, indicating that it is reactive as both a diene and a dienophile." Dehalogenation of trans-3, 4-dibromocyclobutene gave the same reaction products." ... 

Later, the experimental evidence for square cyclobutadiene was called into question. Krantz reported the photolysis of bicyclopyranone in which the carbon atom eliminated as CO2 was labeled with C. One important infrared band that had been assigned to a vibration of square planar cyclobutadiene in earlier studies was altered by the isotopic change, suggesting that this band was due to CO2 trapped with the cyclobutadiene in the rigid rare gas matrix. Thus, the experimental data did not answer the question of the structure of cyclobutadiene. Later work on the theoretical determination of the infrared spectrum of cyclobutadiene ° and further matrix isolation spectroscopy experiments, including the use of polarized IR spectroscopy... 

Many photochemical reactions are carried out at low temperatures as low as 4 K to slow down the reaction rate for the study of the lifetimes of the reactive intermediates. The most useful matrix materials are solid argon, solid neon and solid nitrogen. The initial photoproduct is trapped within a rigid matrix that inhibits the decay of the reactive species in diffusion process. For example, 5-hydroxy-a, P, y, 5-unsaturated valerolactone 1 on photochemical decomposition gives cyclobutadiene 2 and carbon dioxide. The intermediate and the products of this reaction are characterized in low-temperature matrix isolation process. 

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