Bicyclo pentanes cycloaddition reactions

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cycloaddition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

In the presence of transition metal complexes, certain strained hydrocarbon systems are activated under mild thermal conditions and undergo characteristic transformations. Methylenecyclopropane (XXVI) (Noyori et al., 1970, 1972b), bicyclo[2.1.0]pentane (XXVII) (Noyori et al., 1971c, 1974a), and quadricyclane (XXVIII) (Noyori et al., 1975a) add to electron-deficient olefins with the aid of a nickel(0) catalyst such as bis(l,5-cyclooctadiene)-nickel(O) or bis(acrylonitrile)nickel(0). These cycloaddition reactions proceed... 

An example of the vinylogous reactivity is the reaction of 52 with cyclopentadiene (Tab. 14.9) [77]. Rhodium(II) acetate-catalyzed decomposition of 52 in dichloro-methane, yields a 2 1 mixture of the bicyclic system 53 derived from the [3-1-4] cycloaddition, and the bicyclo[2.2.1]heptene 54 resulting from electrophihc attack at the vinylic position followed by ring closure. When Rh2(TFA)4 is used as the catalyst, bicy-clo[2.2.1]heptene 54 becomes the dominant product, while the reactivity of the vinyl terminus is suppressed using a hydrocarbon solvent as observed in the Rh2(OOct)4-cat-alyzed reaction in pentane, which affords a 50 1 ratio of products favoring the [3-1-4] cycloadduct 53. 

Bicyclo[2.1.0]pentane (7 equation 3) readily undergoes a cycloaddition across carbon-carbon double bonds in the presence of nickel(O) complexes. The mode is formally analyzed as a thermally forbidden [2 + 2] process and is in striking contrast to that of the lower homolog, bicyclo[1.1.0]butane, which suffers cleavage of two o -bonds and affords formal allylcarbene addition products. When a solution of bi-cyclo[2.1.0]pentane (7) and [Ni(AN)2] in excess methyl acrylate is heated at 40 C for 36 h under a nitrogen atmosphere, the stereoisomeric cycloadducts exo- and c/ product methyl 3-(cyclopent-2-enyl)propionate (10a 22%). Reaction of (7) with acrylonitrile affords the corresponding adducts (8b), (9b) and (10b). 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

On reaction of spiro[bicyclo[2.1.0]pentane-5,l -cyclopropane] with 4-phenyl-4/f-l,2,4-triazole-3,5-dione the bridge was cleaved and cycloaddition to the N-N double bond took place giving 4-phenyl-2,4,6-triazaspiro[cyclopropane-l,10 -tricyclo[5.2.1.0 ]decane-3, 5 -dione] in quantitative yield. 

The title reaction exemplifies intriguing features of transition metal catalysis. In the absence of transition metal catalysts, reaction between bicyclo[2.1.0]pentane (11) and electron-poor olefins requires extremely forcing conditions and produces a very complex mixture. For example, because of the molecular orbital (MO) restrictions, tmns-l,2-dicyano-ethylene enters into the 2 + 2 cycloaddition across the central o- bond. 

An alternative pathway for reaction of 1,4-dienes is intramolecular cycloaddition giving bicyclo[2.1.0]pentanes. This pathway is usually not observed, but photolysis of l,5-diphenyl-l,4-pentadiene is an example of a compound that takes this... 

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