Addition reactions vinylcyclopropanes

Finally a general approach to synthesize 42-pyrrolines must be mentioned. This is tl add-catalyzed (NH4C1 or catalytic amounts of HBr) and thermally (150°C) induced rea rangement of cyclopropyl imines These educts may be obtained from commerdal cyan acetate, cyclopropyl cyanide, or benzyl cyanide derivatives by the routes outlined below. Tl rearrangement is reminiscent of the rearrangement of 1 -silyloxy-1 -vinylcyclopropanes (p. 7 83) but since it is add-catalyzed it occurs at much lower temperatures. A2-Pyrrolines constitut reactive enamines and may be used in further addition reactions such as the Robinson anei lation with methyl vinyl ketone (R.V. Stevens, 1967, 1968, 1971). 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

An alternative to dialkylcuprates in these addition reactions are copper(I) methyltrialkylborana-tes which are readily prepared from trialkylborane, methyl lithium and copper(I) halides. When these reagents were reacted with 1-benzoyl-2-vinylcyclopropane (30), 1,7-addition products were isolated in high yields. The y,i5-unsaturated ketones 31 formed were shown to be selectively trans-isomers. 

The 1,4-addition of carbenes to 1,3-dienes to give cyclopentenes is extremely rare, since 1,2-addition to give a vinylcyclopropane is much more favorable. Unfortunately, vinylcyclopropanes can be converted into cyclopentenes on heating at higher temperature. In special conditions direct 1,4-addition reactions are observed, but only in poor yield (Scheme 5.40). 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

The low yields obtained with extremely strong bases, LDA and NaH, suggested the formation of another reactive intermediate. Therefore, the reaction was carried out in the presence of methyl acrylate as a Michael acceptor and afforded a new adduct 35 in addition to the expected vinylcyclopropane derivative 33 (see Equation 8). 

The palladium-catalyzed reaction of iodobenzene and an allenyl malonate provided vinylcyclopropane in a highly regioselective manner (Scheme 16.7) [11, 12]. A jT-allylpalladium complex, generated by the addition of PhPdl to a 2-allenyl malonate, can be trapped by an internal malonate anion to afford a vinylcyclopropyl derivative. The site selectivity in this cyclization is dependent on the nature of the entering RX groups, catalytic systems involving phosphine ligands, solvents and bases. 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

Franzen34 photolyzed CH2N2-butadiene mixtures in the pressure range 31-335 mm., with butadiene in excess by a factor of 2-15. Franzen also observed cyclopentene as a product, the ratio of cyclopentene to vinyl cyclopropane decreasing from 0.25 at 35 mm. to 0.095 at 335 mm. Franzen proposed that some of the cyclopentene resulted from 1,4 addition of methylene to butadiene, on the grounds that all excited vinyl-cyclopropane should be collisionally deactivated at pressures as high as 335 mm. However, the ratio of cyclopentene to vinylcyclopropane obtained by Franzen at 335 mm. is close to that predicted by the ratio of rate constants for reactions (63) and (64) calculated by Frey.44... 

Feldman239,240 and Oshima and Utimoto241 have introduced a very sophisticated sequence of reactions that effects a radical annulation starting from a vinylcyclopropane. Scheme 59 provides two examples and a likely mechanism for this sequence, which involves fragmentation, addition and cyclization. The... 

Epimerization of vinylcyclopropanes by Grubbs I-type ruthenium catalysts (28) has been explored.33 The reaction can also be effected by the Grubbs-Hoveyda catalyst (29) provided that an additional phosphine is added. Mechanistic studies (experimental and theoretical) suggest that the epimerization goes through a ruthenacyclopentene intermediate (30). 

As a related reaction, the bicyclo[5.3.0]decane derivative 64 was obtained at 30 °C by the Rh-catalysed intramolecular [5+2] cycloaddition of the alkyne with the vinylcyclopropane moiety in 61. The latter behaves as a pseudo-1,3-diene in oxidative addition, and generates 62. This is followed by rearrangement to 63, whose reductive elimination gives 64 [20]. [Rh(CO)2Cl]2 is a better catalyst than RhCUPl+P. The reaction can be extended to alkenes [20a],... 

As a combined reaction of (3-cleavage of a cyclopropylcarbinyl radical and intermolecular addition, treatment of vinylcyclopropane (266) and activated alkyne in the presence of PhSSPh and AIBN forms a cyclopentene skeleton (267), through the initial addition of a thiyl radical to the vinyl group, P-cleavage of the cyclopropylcarbinyl radical, addition of the carbon-centered radical to the alkyne, ring closure of a vinyl radical via 5-exo-trig manner, and finally subsequent P-elimination of the thiyl radical, as shown in eq. 3.107 [272-276]. Here, PhSSPh acts as a catalyst, since the thiyl radical is regenerated. Aliphatic disulfides such as... 

The highly reactive semibullvalene structure undergoes facile acid-catalyzed 1,4-addition of protic solvents to one of the vinylcyclopropane units.201 The reaction... 

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