Arene + alkene, regioselectivity

Kalena, G.P., Pradhan, P., and Banerji, A. (1999) Stereo- and regioselectivity of intramolecular 1,2-arene-alkene... 

The reversal of syn/anti selectivity in this reaction (Scheme 3.82) can be explained by a mechanism similar to that shown in Scheme 3.81. The chiral, non-racemic l,3-dioxolan-2-yl cation intermediates 194 are initially generated during enantioselective dioxyacetylation of alkene with chiral [bis(acyloxy)iodo]arene 196. Regioselective attack of a nucleophile toward the intermediate results in reversal of enantioselectivity of the dioxyacetylation [257]. 

Alkenes and Alkynes. A series of metal carbonyl cations, such as [Au(CO)n]+,287,288 [c-Pd( i-CO)2]2+,289 [Rh(CO)4]+,290 and [(Pt(CO)3)2]2+,291 was found to induce the formation of carboxylic acids from alkenes and CO in the presence of H2SO4 under mild conditions. A novel water-soluble Pd catalyst292 and Pd complexes of calixarene-based phosphine ligands293 showed high activity in the regioselective carboxylation of vinyl arenes to yield 2-arylpropionic acids or... 

The 1,2 and 3,4 regioselectivity of the ortho addition of an alkene to an arene substituted with a strong donor substituent is, according to Houk, dominated by interaction of the benzene S orbital with the alkene -tt HOMO, because the A -tt interaction would result in a 2,3 ortho adduct. For an arene substituted with a strong donor, the donor substituent, rather than the approaching alkene, will determine the symmetry of the orbitals [188],... 

Coulombic forces will determine the regioselectivity of the ortho addition [189], In the charge-transfer complexes of monosubstituted benzenes with alkenes, the charge (positive or negative) on the arene is largely located at the carbon atoms ipso and (to a lesser extent) para to the substituent. The carbon atoms of the alkene double bond will preferentially be located in the neighborhood of either the ipso carbon or (to a lesser extent) the para carbon atom of the monosubstituted benzene. This would explain the 1,2 and 3,4 selectivity in the ortho photocycloaddition. 

The regioselectivity of the addition with respect to the arene has been interpreted in terms of both meta addition of the alkene to the Sj arene via an exciplex followed by formation of the vinylcyclopropane system,and initial rearrangement of the arene to a prefulvene species which then undergoes concerted alkene addition (see below). The orientation of the substituents on the wcfa-cycloadducts from some systems requires that both mechanisms operate in competition. The reaction shows no apparent regioselectivity with simple alkenes thus 2-methylpropene gives both wc/a-cycloadduct isomers in approximatively equal amounts. ... 

A dirhodium tetracarboxylate complex coordinated by two bromocalix[4]arene macrocycles exhibited two toluene molecules coordinated to the rhodium centers and inserted in the clefts, which are formed by the vicinal -bromophenyl rings of the two calixarene units (Figure 29). This complex has been found to be an efficient catalyst for two carbene transfer reactions, alkene cyclopropanation, and intramolecular C-H insertion, in terms of stereo- and regioselectivity. 

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