Dienes from bicyclobutanes

A soln. of tricyclo[4.1.0.02.7]heptane in acetonitrile treated 15 min. at room temp, with 4 mole-% rhodium dicarbonyl chloride dimer -> 3-methylenecyclohexene. Y 98%. P. G. Gassman and T. J. Atkins, Am. Soc. 93, 1042 (1971) f. e., also with iridium tricarbonyl chloride dimer, s. ibid. 95,1812 1,3-dienes from bicyclobutanes with Ag-perchlorate s. M. Sakai et al., ibid. 93, 1043. 

The behaviour of the cation radicals derived from bicyclobutane systems has been under intensive study by Gassman and his coworkers. Currently they have observed that the substituted derivative (185) photorearranges in good yield, using 1-cyanonaphthalene as the electron acceptor, to the isomer (186). Although this product can be synthesized independently by the photocyclization of the diene (187) the authors argue that this route is not in operation. The formation of the product (186) follows the one electron transfer path with subsequent rearrangement within the radical cation (188). ... 

Simpler bicyclobutanes can also be converted to dienes, but in this case the products usually result from cleavage of the central bond and one of the edge... 

Path C is surely unlikely for several reasons, chief among them is that 23 is formed even at —45 °C, at which temperature the bicyclobutane would surely be stable. The real problem is to rule out the non-carbene mechanism, path A. Brinker cleverly does this by examining the dienes formed from 24 and 25 (Scheme 7.13). The products are nicely rationalized through intermediates 22a and 22b in which the direction of the initial addition is determined by the stabilizing or... 

Although some carbenes are reported not to add to cyclopropenes207, there are several examples of inter- and intra-molecular addition leading initially to the formation of bicyclobutanes. l,2-Diphenylcyclopropene-3-carboxylates are converted to a mixture of three stereoisomeric bicyclo[1.1.0]butanes by reaction with ethoxy-carbonylcarbene generated from the thermolysis of ethyl diazoacetate an additional product is the diene (278) which is apparently formed by rearrangement of an intermediate zwitter ion208). It should be noted, however, that cyclopropenes readily undergo addition to diazo-compounds, and that subsequent transformations may then lead to bicyclobutanes (see Section 8), and that a free carbene may therefore not be involved in the above process. 

It was suggestedthat bicyclobutane formation from conjugated dienes occurs in a concerted fashion from vibrationally relaxed singlet having an allyl anion-methyl cation electronic configuration. 

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

Cholesta-3,5-diene and similar transoid dienes undergo a variety of complex photochemically induced reactions, which differ for singlet- and triplet-excited states. Singlet products are derived in part from the very strained 3,5 4,6-bis-cyclo ( bicyclobutane ) intermediates of the type (315), which open by solvent attack to give such products as (316). The singlet-excited diene may alternatively react via... 

Process (b) is typical of the photolysis of 3-alkoxycholesta-3,5-dienes (R = Me, Et). No bicyclobutane or products derived thereof were detected in this reaction. An excited polar species abstracted a proton from solvent nonstereoselectively, giving a stabilized carbenium ion, which then coordinated with solvent, without rearrangement. 

More recent results have provided additional detail on the conformational requirements for bicyclo[1.1.0]butane fonnation from conjugated dienes " . Hopf and coworkers have shown that high yields of the isomeric bicyclobutane 78 are obtained from irradiation of... 

Further evidence for the pathways described above comes from labeling studies in which deuterium replaced both hydrogens at the bicyclobutane bridgehead positions, leading to the cyclic dienes with deuteriums at carbons 2 and 3 " This study also included an unsaturated derivative of the tricyclo heptane above as well as measurements of activation parameters for all of the bridged bicyclobutane systems known at that point of time. 

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