Dienes/methylenecyclopropanes

Diboration of Unsaturated Hydrocarbons. B2pin2 adds to unsaturated hydrocarbons in the presence of a catalytic amount of a Pt complex to afford diborated products in high yield with excellent regio- and stereoselectivity. The reaction is recognized to proceed through a catalytic cycle, which involves (a) oxidative addition of the B-B bond to Pt , (b) insertion of the unsaturated hydrocarbon into the B-Pt bond, and (c) reductive elimination of the product to regenerate Pt (eq 1). The diboration of alkynes, allenes, conjugated dienes, methylenecyclopropanes, anda, -unsaturated carbonyl compounds is efficiently catal) ed by phosphine-based Pt complexes (eqs 2-6), whereas phosphine-free Pt complexes are favorable for the reaction of simple alkenes because of the low coordination ability of the alkene over phos-... 

While 2-arylsubstituted methylenecyclopropanes reacted with tetracyano-ethylene (131, TCNE) to give [3 + 2] adducts, i.e. methylenecyclopentanes [37], via cleavage of the cyclopropyl CC bond, benzylidenecyclopropanes 156 and 157 behaved as dienes toward TCNE (Scheme 23). 

Finally, both difluoromethylenecyclopropane (456) (Table 41, entry 5) and its isomer 2,2-difluoro-methylenecyclopropane (5) (Scheme 73) gave a [2 + 2] cycloaddition with l,l-dichloro-2,2-difluoroethylene (520), but the latter required more drastic conditions. Compound 5 reacted with dienes similarly to perfluoromethylenecyclopropane, affording exclusively [4 + 2] cycloadducts (see Sect. 2.1.1) [9],... 

Vinylbuta-1,3-diene produces the 1,2- and 3,4-mono-insertion adducts with dichlorocarbene in a 4 1 ratio [37]. A similar preference in reactivity is observed with 3-methylene-cyclohexenes [90]. 1,2-Dienes react with two equivalents of dichlorocarbene to form spiropentanes [21, 90] (Scheme 7.6). Spiropentanes (50-95%) are also obtained from methylenecyclopropanes [36,106] and by the reaction of electron-deficient alkenes with an excess of chloroform [31] (see Scheme 7.12)... 

Pentadiene 36 and isoprene (37) gave with 34a exclusively the regioisomers 39 and 40, respectively. Bicyclic methylenecyclopropanes 34a and 34b gave stereospecifically the isomers 38a and 38b deriving from the attack on the convex face of the methylenenorcarane 34a (or 34b). The reaction yields were generally good, but obtained with an excess of the diene. Only 34c gave 38c in 85% yield in the presence of only one molar equivalent of diene 35 [14]. 

Methyl 2-bromo-2-cyclopropylideneacetate (11a) has never been tested in these reactions, but has been used as a starting material for the stepwise construction of 1,6-heptadienes with methylenecyclopropane units for intramolecular Heck reactions. Thus, bromo ester 11a, after reduction, subsequent conversion of the resulting alcohol to the bromide and coupling with enolates of substituted malonates, was transformed into dienes of the type 254 (Scheme 73) - versatile synthetic blocks for the preparation of functionally substituted spirocyclopropanated bicyclo[4.3.0]nonenes 255a-d by a domino Heck-Diels-Alder reaction [122a]. 

The strain associated with small-ring compounds often engenders chemical properties not encountered in larger ring systems. Methylenecyclopropanes and cyclobuta-1,3-dienes are known to undergo thermal cyclodimerization. 

Metal-catalyzed cyclodimerization has been restricted to buta-1,3-dienes, norbornadienes (see Houben-Weyl, Vol. 4/4, pp 295-299) and more recently to strained alkenes such as cyclopropenes and methylenecyclopropanes (see Sections, 3., l.B.2.2.2. and 2.B.2.4.2.). 

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... 

At 150 °C, the analogous 2-methylenetricyclo[4.1.0.01,3]heptane (34) equilibrated with 35 and 36 as products of a methylenecyclopropane rearrangement and a subsequent retro-Diels Alder reaction, respectively, until at 180 °C an irreversible rearrangement to bicyclo[4.2.0]octa-l,5-diene and 3,4-dimethylenehexa-l,5-diene (39) via diradical 37 took place.223,224 Increasing pressure favored the formation of the bicyclic system 38 (50% yield at lOOTorr).223... 

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... 

Pd(PPh3)4-catalysed isomerization of methylenecyclopropanes in acetic acid proceeds smoothly at 1-substituted or 1,1-disubstituted dienes (Scheme 90). A plausible mechanism is hydropalladation and /3-carbon-Pd elimination followed by /3-hydride elimination, established from a deuterium labelling experiment.133... 

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. ... 

The Pd -catalyzed [3 + 2] cycloaddition reactions of (97) exhibit very different selectivities from those of the corresponding methylenecyclopropane codimerizations. One major distinction is the chemoselec-tivity only electron-deficient alkenes will react to form methylenecyclopentane. The nucleophilic nature of this TMM synthon is indicated in the exclusive annulation of the electron-poor double bond of 2,3-di-methoxycarbonylnorbomadiene (equation 109). - No such differentiation of alkenes is evident in the methylenecyclopropane codimerization with the same diene (equation 65). 

Asymmetric induction is possible when chiral additives are used. With the Simmons-Smith procedure, as with free carbenes, conjugated dienes give 1,2-addition, and allenes give methylenecyclopropanes or spiropentanes. ... 

Substituted methylenecyclopropanes react with diiron nonacarbonyl to give trimethylenemethane and 1,3-diene iron tricarbonyl complexes . Theoretical analysis of the former... 

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