- 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
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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 bicyclohept-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 bicyclooct-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-chlorobicyclohept-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-chlorobicyclooct-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-cyanobicyclohept-5-en-2-yl acetates (14) with the same regioselectivity as 12+PhSeCl.The addition of PhSeCl, PhSeBr, NO2C6H4SCl, and (NO2)2C6H3SCl to 2-(bicyclohept-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.
- Carrupt, Pierre-Alain,Vogel, Pierre
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p. 1008 - 1028
(2007/10/02)
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- REGIOSELECTIVE ADDITIONS OF ELECTROPHILES TO OLEFINS REMOTELY PERTURBED. THE CARBONYL GROUP AS A HOMOCONJUGATED ELECTRON DONATING SUBSTITUENT.
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Electrophiles attack preferentially at the C(5) position of 2-norborn-5-enone and 2-bicyclooct-5-enone whereas the C(6) position is preferred for 2-chloronorborn-5-ene-2-carbonitriles and 2-chlorobicyclooct-5-ene-2-carbonitriles.
- Carrupt, Pierre-Alain,Vogel, Pierre
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p. 2563 - 2566
(2007/10/02)
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