Aromatic reactivity. Part XLIV. Substituent effects of the buta-1,3-diynyl group

Abstract

The rates of cleavage of the aryl–tin bonds of X·C₆H₄·SnMe₃ compounds by acid and of cleavage of the benzyl–silicon bonds of X·C₆H₄·CH₂SiMe₃ compounds by alkali have been used to assess the substituent effects of the buta-1,3-diynyl group. For the cleavage in aqueous methanolic perchloric acid at 50·0°, rates relative to that for phenyltrimethylstannane (X = H) are: (X =)p-CCH, 0·425; m-CCH, 0·29; p-(CC)₂H, 0·26; m-(CC)₂H, 0·21; m-Br, 0·195; thus the buta-1,3-diynyl substituent deactivates both the meta- and the para-positions more strongly than does the ethynyl substituent, and the meta-position almost as strongly as does a bromo-substituent. In the cleavage by alkali in 2:5 (v/v) water–methanol at 50·1°, the compound p-H·(CC)₂·C₆H₄·CH₂·SiMe₃ is cleaved 3300 times as readily as benzyltrimethylsilane; this corresponds to a value of 0·72 for the σ^–-constant of the p-(CC)₂·H group, and indicates that the group can withdraw electrons strongly.