摘要:
The stereodirecting effects of substrate methoxy, hydroxy, methylthio, and methyl substituents were examined in the addition of 1:CH2 to the double bonds of substrates 1a-d. The carbene, generated by photolysis of CH2N2, inserted into the C-H bonds of solvent and substrate, added to the substrate double bond to give products 2a-d, and attacked the oxygen or sulfur atom of substrates 1a-c to produce ylide intermediates which underwent 2,3-sigmatropic rearrangement to give products 3a-c. A preference for addition syn to the methoxy group of substrate 1a was observed when the reaction was run in pentane solution (syn-2a/anti-2a, 1.14 +/- 0.02), while a preference for formation of anti-2a was observed in diethyl ether solution (syn-2a/anti-2a, 0.92 +/- 0.03). A preference for 1:CH2 addition anti to the substrate substituent was observed for substrates 1b-d in both pentane and ether solution. The effect of the methoxy substituent was also examined in the addition of 1:CH2 to syn-7-methoxynorbornene (5b). Explanations for the substituent effects are offered based on both steric hindrance and interaction between 1:CH2 and the substituent, including formation and subsequent reaction of the ylide intermediates.