Ring Opening of Methylenecyclopropanes

As with the chloropalladation reaction (vide supra)m the rearrangement of >/2-methyl-enecyclopropane to >/4-TMM was shown experimentally to proceed stereoselectively by disrotatory ring cleavage of the distal frontier molecular orbital considerations, which predict that the out-of-phase interaction between the metal orbital in the distal ring-opening of -methylenecyclopropane complexes can be minimized by bending the bond up away from the metal (equation 348)410 ... 

In contrast to Ni(0), Pd(0) leads exclusively to distal ring-opening of methylenecyclopropane. Thus in the presence of an appropiate diylophile, Type A cycloaddition is observed, according to Eq. 83 ... 

Ring-opening of methylenecyclopropanes in the presence of Pcl(O) or Ni(0) catalysts has served as an alternative pathway to gain access to reactive tri-methylenemethanes for [2-i-3]-cycloaddition reactions [40]. Binger obtained excellent asymmetric induction with chiral acrylamide 81 derived from Oppolzer s camphorsultam (Equation 9) [111]. Ni-catalyzed opening of cyclopropane 150 and cycloaddition afforded the desired adduct 151 in 99 1 dr. 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

Anyway, the catalysed process is not a clean one, generally also affording five-membered ring 480 and open-chain products 481, as the result of the cyclopropane ring-opening of one or both the methylenecyclopropane addends (Table 37) [116,130,2],... 

The reaction mechanism involved the cyclisation of ketyl radical anion 3 on to the methylenecyclopropane moiety in a 5-exo-trig manner. Ring opening of cyclopropane intermediate 4 gave rise to the cyclohexyl radical 5, which then cyclised in a 5-exo-dig fashion to form the second ring (Scheme 6.2).5,6... 

Path a involves unimolecular decomposition of an initial ozonide (VI). It is important to note that since other methylenecyclopropane derivatives, such as 2,3-dimethylmethylenecyclopropane, do not undergo ring opening of the cyclopropane ring, both ring strain and the presence of the carbomethoxy groups must be necessary for such an unusual decomposition of an initial ozonide. The following sequence rationalizes... 

The intermolecular addition of carbamates to 1,3-dienes (equation 147) under mild conditions has been described as well. The hydrothiolation of 1,3-dienes has also been reported. " Other related conjugate additions can be performed over methylenecyclopropanes (equation 148) with sulfonamides and the resulting product cyclizes by a second hydroamination of an olefin, finally yielding cyclic sulfonamides. This behavior is reproduced in a similar reaction for the ring opening of vinylcyclopropanes with sulfonamides. One more example in this group of reactions is the synthesis of dUiydrobenzofurans from aryl-allyl ethers. ... 

In contrast to stoichiometric conversions, phosphane-modified nickel(O) complexes catalyze the trimerization of methylenecyclopropane in 95% overall yield. Linear and cyclic trimers, all containing cyclopropane units, were formed with ratios depending on the phosphane used. According to the structure of some of the products, ring opening of the methylenecyclopropane is involved. 

The concept of ring opening of cyclopropylcarbinyl radicals has been extended to substituted methylenecyclopropanes. As shown in Scheme 4, the crucial step in the reaction sequence is the regioselective addition of a substituted thiyl radical. After opening of the cyclopropane ring, the resulting radical adds to the olefin. Subsequent cyclization and reductive regeneration of the thiyl radical with concomitant liberation of the methylenecyclopentane product complete this transformation [6]. 

Diboration of methylenecyclopropanes. Regioselective functionalization with ring opening of the substrates occurs on reaction with bis(pinacolato)diboron. The adducts can be transformed in various ways to useful substances. 

Ring scission of methylenecyclopropanes. Hydrostannylation concomitant with ring opening gives homoallylic stannanes in a stereoselective manner. When the catalyst is changed to Pd(OH)j/C, the products are distannanes due to further hydrostannylation of the vinyl group of the initial products. 

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