112. The Carbonyl Group as Homoconjugated Electron-Releasing Substituent. Regioselective Electrophilic Additions at Bicyclohept-5-en-2-one, Bicyclooct-5-en-2-one, and Derivatives

Article abstract of DOI:10.1002/hlca.19890720519

In CHCl3, CH3CN, or AcOH, benzeneselenenyl chloride (PhSeCl), bromide (PhSeBr), and acetate (PhSeOAc), 2-nitrobenzenesulfenyl chloride (NO2C6H4SCl), and 2,4-dinitrobenzenesulfenyl chloride ((NO2)2C6H3SCl) added to bicyclo<2.2.1>hept-5-en-2-one (5) in an anti fashion with complete stereo- and regioselectivity, giving adducts 20-24 in which the chloride, bromide, or acetoxy substituent (X) occupies the endo position at C(6) and the Se- or S-substituent (E) the exo position at C(5).The addition 5+(NO2)2C6H3SCl->24 was accompanied by the formation of (1RS,2RS)-2-(2,4-dinitrophenylthio)cyclopent-3-ene-1-acetic acid (25).The latter was the major product in AcOH containing LiClO4.The addition of PhSeCl and PhSeBr to bicyclo<2.2.2>oct-5-en-2-one (6) were less stereoselective (proportion of exo vs. endo mode of electrophilic attack was ca. 3:1) but highly regioselective giving adducts 27/28 and 29/30, respectively, the regioselectivity being the same as that of the electrophilic addition of 5.The reaction of PhSeCl with a 4:1 mixture of 2-exo-chloro- and 2-endo-chlorobicyclo<2.2.1>hept-5-ene-2-carbonitriles (12) was slower then addition 5+PhSeCl; it gave adducts 31/32 (4:1) in which the PhSe moiety occupies the exo position at C(6) and the Cl atom the endo position at C(5).The addition of PhSeCl to 2-chlorobicyclo<2.2.2>oct-5-ene-2-carbonitriles (13) was very slow and gave adducts with the same regioselectivity as 12+PhSeCl, but opposite with that of reactions of the corresponding enones 5 and 6.PhSeX (X=Cl, Br, OAc) added to 2-cyanobicyclo<2.2.1>hept-5-en-2-yl acetates (14) with the same regioselectivity as 12+PhSeCl.The addition of PhSeCl, PhSeBr, NO2C6H4SCl, and (NO2)2C6H3SCl to 2-(bicyclo<2.2.1>hept-5-en-2-ylidene)propanedinitrile (49) were not regioselective, showing that a dicyanomethylidene function is not like a carbonyl function when homoconjugated with a ? system.The results are in agreement with predictions based on MO calculations suggesting that a carbonyl group homoconjugated with an electron-deficient centre can behave as an electron-donating, remote substituent because of favourable n(CO)<->?C(1),C(2)<->p(C(6)) hyperconjugative interaction.