Carbenes reaction with bicyclobutane

Such a reaction, in which two bonds are broken in a single step, has precedent in the two-bond pluck mechanism proposed for the reactions of bicyclobutanes (and quadricyclanes) with carbenes (Scheme 7.15). " In this reaction the transition state... 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

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. 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

The sodium salt of cyclopropanecarboxaldehyde tosylhydrazone (157) behaves similarly on heating to 125 135°C. Labelling experiments using the lithium cyclopropanecarboxaldehyde-p-tosylhydrazone salt has proved the occurrence of a cyclopropylcarbene-cyclobutene C3 - C4 ring expansion instead of a simple intramolecular carbene insertion in one of the four C-H bonds and bicyclobutane ring-opening . For other examples and discussion of such a rearrangement see Ref. 189. In an apparently related reaction, when cyclopropylmethyl tosylate (158) is treated with potassium t-butoxide in dimethyl sulphoxide at room temperature for 1 h, a quantitative mixture of cyclobutene and methylenecyclopropane is produced in equal amounts (equation 110) °. 

Since in many cases the syntheses of bicyclobutane involve the intermediacy of carbenoids, it is only reasonable to assume that bicyclobutane is not highly reactive toward carbenes, at least when compared with its starting materials. The reactions usually yield a mixture of products (equation 90). Interestingly, a product of a bicyclopentane... 

Two mechanisms were advanced for the reaction of carbenes with bicyclobutane. The first one is a stepwise addition (equation 93) 5. xhjs mechanism was challenged by Mock... 

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