Methylenecyclopropanes dimerization

In the presence of phosphane-free Ni(0) catalysts, substituted methylenecyclopropanes dimerize at low temperatures giving formal [2 + 2] and [3 -I- 2] cycloadducts 24 and 25, respectively. The chemoselectivity of [2 -t- 2] cycloaddition depends on the substitution pattern of the substrate and is restricted to systems without further substituents at the exocyclic double bond. [3 + 2] Cycloaddition and acyclic product formation depend on further substituents at the cyclopropane moiety. In general, the product is obtained as a mixture. The combined yield resulting from cycloaddition is higher in the presence of electron-deficient alkenes, such as dimethyl ( )-but-2-enedioate. ... 

Due to their tendency to dimerize in different thermal conditions, the formal [2 + 2] cycloaddition reaction of methylenecyclopropane derivatives and their... 

Methylenecyclopropane itself undergoes the cyclodimerization at elevated temperatures (200-250°C) to give predominantly the head-to-head dimer 445 vs its head-to-tail isomer 446 (Scheme 61) [116,117], albeit in poor yield. 

Apparently, the dimerization of 4 is considerably more facile than that of MCP or BCP, resembling those of (dichloromethylene)cyclopropane (455) and radicophilic olefins with a capto-dative substitution pattern [125], some of which are known to cyclodimerize even at room temperature. Indeed, the capto-datively substituted methylenecyclopropane 85 undergoes the homodimerization at 60 °C (Scheme 65) [126]. 

Despite its high reactivity, the a-chloro ester 1-Me does not polymerize like simple acrylates. However, in close analogy to other 1,1-disubstituted methyl-enecyclopropanes [29], 1-Me slowly dimerizes in a head-to-head fashion even at room temperature to give the two diastereomeric dimethyl dispiro[2.0.2.2]oc-tanedicarboxylates (E)- and (Z)-16 (ratio 1 1) (Scheme 5), and at 120 °C the dimerization proceeds almost quantitatively (89% isolated yield) [15]. Apparently the dimerization of 1-Me occurs considerably more readily than that of methylenecyclopropane [4a,b] and of bicyclopropylidene [4 d]. Surprisingly, under high pressure (10 kbar) this dimerization did not proceed more efficiently (Scheme 5) [30], but more cleanly, which facilitated the separation of isomers. 

The methylenecyclopropane derivative 3-SPh with its capto-dative substitution pattern has demonstrated essentially the same reaction mode and underwent dimerization to afford a mixture of E) and (Z)-17 (ratio 1.3 1) upon attempted cycloaddition of 3-SPh onto bicyclopropylidene [7h, 291 (Scheme 5). The assignment of these diastereomers was secured by an X-ray crystal structure analysis of E) and (Z)-16 [11c, 30] as well as E)-17 [29]. 

Methylenecyclopropane (17) on heating to 230 °C has been reported to yield a mixture of the head-to-head dimer 18 and head-to-tail dimer 19 with the former predominating indicative of the stepwise formation via a 1,4-diradical intermediate.9... 

Thermal cyclodimcrization of methylenecyclopropane results in head-to-head cycloaddition (see Section 1.3.1.1.). By contrast the bis(cycloocta-l,5-diene)nickel(0) catalyzed reaction of methylenecyclopropane gives the head-to-tail dimerization product 6 in 9% yield in addition to a [3 + 2] dimer 7.20... 

The behavior of strained, fluorinated methylenecyclopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies AH ) for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotrifluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101,102, 103]... 

Notably, the lithium enolates have the planar methylenecyclopropane-type structure56, but give C-alkylation products49"52. X-ray structure analysis of the lithium enolate56 and bicyclobutyllithium57 TMEDA complexes revealed that both crystallize as lithium bridging dimers. 

Use of excess of reagents and prolonged reaction times lead to dialkylation, dimerization, isomerizations and cross-coupling reactions (equation 187). In the presence of methoxy groups in the side chain, alkylation is followed by migration of the double bond to form the thermodynamically more stable methylenecyclopropanes (equation 188). 

The highly strained double bond in methylenecyclopropane displays enhanced reactivity in cycloaddition reactions. In addition to normal [4+2] cycloaddition to 1,3-dienes (e.g. equation 13)32, methylenecyclopropane and its derivatives have a pronounced tendency to undergo thermal [2+2] cycloaddition reactions. For example, thermal dimerization of methylenecyclopropane in the gas phase results in formation of isomeric dispirooctanes 16 and 17 (equation 14)33. This unusual cyclization is considered to proceed via a stepwise radical mechanism involving the intermediacy of biradical 18 (equation 15)34. Equation 15 demonstrates that methylenecyclopropanes possessing substituents capable of stabilizing intermediate radicals undergo efficient [2+2] dimerization even... 

Photolysis of 3-methylenecyclobutanone in furan at 15°C afforded methylenecyclopropane in addition to tri-methylenemethane dimer [73]. If the temperature was decreased to -72°C, the amount of [73] produced was increased fivefold relative to methylenecyclopropane. 

Methylenecyclopropene (19) dimerizes to give (20) and (21) in a 92 8 ratio. Bicyclopropylidene (22) is converted in 35% yield to dimer (23) a competitive methylenecyclopropane rearrangement forming (24) is the major side reaction. 

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