| Abstract: | Earlier work has shown that the cyclo-addition of a ketene to a conjugated diene is always (a) 2 + 2, (b) polarily directed, and (c) suprafacial with respect to the diene C?C. The adducts of ketenes and cyclopentadiene are thus always 7-substituted bicyclo 3.2.0] hept-2-ene-6-ones. New evidence is presented to show that unsymmetrically substituted ketenes add to cyclopentadiene in such a manner that the larger substituent has a greater tendency to take up the endo- position in the adduct. This is interpreted to mean that a ketene participates in such reactions antarafacially. Thus the ketene approaches cyclopentadiene (a) with its functional plane perpendicular to that of the ring, (b) with the carbonyl carbon over the middle of the ring, and (c) with the larger of the two substituents oriented preferentially away from the ring (transition state 11 ). This endo-specificity for the larger ketene substituent is demonstrated by the indicated endo/exo ratios observed in the cyclo-adducts from ketenes with the following substituent pairs: C6H5/H = >95/<5, CH3/H = 98/2, Cl/H = 97/3, CH3O/H = >95/<5, C6H5/CI = >95/<5, C6H5/CH3 = >95/<5, CH3/Cl = 80/20, CH3/CH?CH2 = ～65/35, C2H5/CH3 = ～60/40, n-C3H7/CH3 = ～60/40, CH3/Br = 56/44. These ratios enable a list to be compiled indicating the endo-specificity of the ketene substituents. The order closely parallels the space filling capacity as derived by other methods. The establishment of such ratios required reliable configurational assignments at carbon 7. These were derived by five methods based upon the following effects: (1) Both H? C7 and CH3? C7 cause nmr. signals at higher field in endo-position (compared with exo). (2) The CH3? C7 group in exo-position gives rise to a nuclear Overhauser effect with the vicinal H? C1, and in one case also with the trans-annular H? C5. (3) The nmr.-coupling constants of H? C7-exo (observed at H? C7) with H? C1 is always larger than of H? C7-endo. (4) The coupling constant of H? C7-exo with H? C6 (known to be exo-) of the LiAlH4 reduction products of the cyclo-adducts (observed at H? C6) is always larger than that of H? C7-endo. (5) The nmr. signals of most protons in the cyclo-adducts are at higher field in benzene than in chloroform solution; this “benzene shift” is larger for H? C7 or for CH3? C7 when in exo- than when in endo-position. |
