Highly sterically hindered carbon acids: the intrinsic reactivity of 5,5′,5″-trimethyl- and 3,3′,3″,5,5′,5″-hexamethyl-2,2′,2″,4,4′,4″-hexanitrotriphenylmethanes
作者:François Terrier、Lan Xiao、Patrick G. Farrell、Danielle Moskowitz
DOI:10.1039/p29920001259
日期:——
important steric effects are operating in the ionization of 2 and 3. These arise from the accumulation of ortho-nitro groups in the triphenylmethane system which makes the approach of the base reagents from the exocyclic carbon of 2 and 3 very difficult. The finding of extremely low intrinsic reactivities for 2 and 3 and the observation of a much greater catalytic efficiency of primary amines than of
5,5',5“-三甲基-和3,3',3”,5,5',5“-六甲基-2,2'的可逆去质子化的速率常数(k p B,k – p BH)在H 2 O–Me 2 SO(20:80 )中测定了伯脂肪族胺,哌啶和吗啉以及苯氧根阴离子和氢氧根阴离子产生的2,2'',4,4',4''-六硝基三苯基甲烷(2和3))在25°C下。将所得结果与2,2',2'',4,4',4''-六硝基三苯基甲烷(1a)的结果进行比较,结果表明,在与硝基相邻的位置引入甲基会显着降低环外的热力学酸性CH基团:ΔP ķa1a 2 = 1.68; ΔP ķ A1A 3 = 6.48。提示这些减少很可能是硝基基团从其连接的芳族平面上扭曲出来的反射,因此降低了共轭碳负离子C-2和C-3的共振稳定性。其他重要的空间效应在2和3的电离中起作用。这些来自三苯基甲烷系统中邻硝基的积累,这使得从2和3的环外碳接近碱性试剂非常困难。发现极低的内在反应