Zimmerman rearrangement

Conversion of 1,4-dienes to vinylcyclopropanes under photolysis. Also known as the Di-tc-methane rearrangement. 

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-319-03979-4 298, Springer International Publishing Switzerland 2014 

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Dienes can be converted photochemically to vinyl cyclopropanes. This reaction, also named the Zimmerman rearrangement 322), is common to molecules in which two n moieties are substituents on a methane carbon 323). The skeletal changes follow the mechanistic sequence in (3.14). 

The di-TT-methane rearrangement was discovered and conceptually developed by H. E. Zimmerman (University of Wisconsin), one of the earliest examples being the photoisomerization of barrelene (1) into semibullvalene (2) (equation 2). Through intensive subsequent work, this photoreaction was shown to be quite general for 1,4-dienes. In view of the major contributions by the original author, the rearrangement is also referred to as the Zimmerman rearrangement. 

In order to study this unusual dimerization in detail 49 [40] was synthesized and subjected to the same reaction conditions. The cumyl group inhibits the Grovenstein-Zimmerman rearrangement as in this case the formation of an anion at a tertiary center would be encountered, so only cleavage to the vinyllithium compound 18 is possible. Indeed, after reacting 49 with lithium metal for 4 h at room temperature and subsequent work-up with dimethyl sulfate the dimer 51 is isolated in 82 % yield, with t-butylbenzene (53) as the other product in 84 % yield (Scheme 11). 

The light-induced conversion of acyclic divinylmethane reactants 1 to vinylcyclopropanes 2 (cf. Houben-Weyl, Vol. 4/5a, pp 413-433), also termed the Zimmerman rearrangement, usually occurs from the excited singlet state of the diene.The resulting vinylcyclopropane is often prone to undergo consecutive photoreactions and therefore these transformations are usually run to about 50% conversion. Under these conditions the material balance (1 - -2) is still > 95%. 

One of the most interesting and general photochemical reactions is the di-ir-methane rearrangement. It has been extensively investigated, most notably by Zimmerman and coworkers, leading some to refer to it as the Zimmerman rearrangement. 

The most common photochemical reaction of 1,4- diene is the di-ic methane rearrangement or Zimmerman rearrangement, in which a 1, 4- diene carrying alkyl or aryl substituent at carbon-3, can be photochemically converted into vinyl cyclopropane. 

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