Chloropalladation of Alkyl-Substituted Methylenecyclopropanes

Article abstract of DOI:10.1021/ja00384a021

The chloropalladation reactions of methylenecyclopropanes bearing alkyl substituents at the three-membered ring are shown to involve 1,3 addition of the elements of Pd-Cl to the organic molecule, with cleavage of the 2,3 ? bond of the ring.Isopropylidenecyclopropane is inert toward chloropalladation, in contrast to 2,2-dimethylmethylenecyclopropane, which gives a 9:1 mixture of 13 and 14.The cis and trans isomers of 2,3-dimethylmethylenecyclopropane give an identical chloropalladation product, 16, which exists as a mixture of two diastereoisomeric pairs of enantiomers in a solvent-dependent ratio.The apparent lack of the selectivity in the latter reaction is shown to be due to rapid η³ to η¹ to η³ transformations of the organic ligand after the chloropalladation step.The true sterechemistry of 1,3 chloropalladation is revealed in the reactions of cis-9-methylenebicyclo<6.1.0>nonane (19) to give only 20 and of trans-9-methylenebicyclo<6.1.0>nonane (21) to give a 4:1 mixture of 22 and 23.Formation of these products is rationalized in terms of suprafacial addition of elements of Pd-Cl to a ring that is opening stereospecifically in a disrotatory mode, with the breaking bond bending away from the metal. cis-7-Methylenebicyclo<4.1.0>heptane (17) affords 18 in a reaction that involves suprafacial addition of Pd-Cl while the ring opens disrotatory with the breaking bond bending toward the metal, followed by a rapid η³ to η¹ to η³ isomerization of the kinetic product 25.The absence of η³ to η¹ to η³ interconversions of 20, 22, and 23 is rationalized in terms of steric blocking by the nine-membered rings; no such impediment exists for 18.Monomeric acetylacetonato (acac) derivatives of 16, 18, 20, and 22 are described.Theoretical calculations at the extended Hueckel level indicate that the two disrotatory modes of ring cleavage require similar activation energies for a model methylenecyclopropane-PdCl2(NCH) complex.The disrotatory motion of the carbon-carbon bond breaking away from the metal is very slightly favored on electronic grounds.On the other hand, the conrotatory route has a much higher activation energy associated with it.The reaction is, technically, symmetry allowed.The HOMO-LUMO crossing is only weakly avoided, and a small HOMO-LUMO gap is obtained at the transition state.Therefore, the conrotatory path exhibits behavior akin to that in symmetry-forbidden reactions.At some stage in the ring opening, the Cl transfer from Pd to the organic ligand begins.There is a strong interaction between the filled Cl lone-pair orbital and the LUMO of the complex that is concentrated on the two noncoordinated methylene carbons.The Cl transfer should require little, if any, additional activation energy.