Nucleophilic substitution reactions of benzyl- and diphenylmethyl-phosphonamidic chlorides with amines: competition between the usual SN2(P) mechanism and elimination–addition with an alkylideneoxophosphorane (phosphene) intermediate
作者:Martin J. P. Harger
DOI:10.1039/b007288p
日期:——
The substitution reaction of PhCH2P(O)(NMe2)Cl with Me2NH or Et2NH in CHCl3 is very sensitive to the bulk of the nucleophile (200 times slower with Et2NH), affords only the product derived from Me2NH in competition experiments, and gives largely undeuterated product with Et2ND; these features are in accord with an SN2(P) mechanism. The corresponding reaction of Ph2CHP(O)(NMe2)Cl is relatively insensitive to the bulk of the nucleophile (5 times slower with Et2NH), gives some of the product derived from Et2NH in competition experiments, and gives extensively deuterated product with Et2ND; these features point to an eliminationâaddition (EA) mechanism with an alkylideneoxophosphorane intermediate [Ph2CP(O)NMe2]. There is only a modest (13â19 fold) increase in the rate of substitution on going to ArPhCHP(O)(NMe2)Cl (Ar = 4-NO2C6H4) but with R2ND there is now very rapid H/D exchange at the α carbon atom. This suggests that the elimination stage of the EA mechanism comprises rapid reversible formation of the conjugate base followed by rate-limiting expulsion of chloride ion.
PhCH2P(O)(NMe2)Cl 与 CHCl3 中的 Me2NH 或 Et2NH 的取代反应对大量亲核试剂非常敏感(Et2NH 慢 200 倍),在竞争实验中仅产生源自 Me2NH 的产物,并且得到大部分未氘化的产物含有 Et2ND 的产品;这些特征符合SN2(P)机制。 Ph2CHP(O)(NMe2)Cl 的相应反应对大部分亲核试剂相对不敏感(与 Et2NH 反应慢 5 倍),在竞争实验中给出了一些源自 Et2NH 的产物,并与 Et2ND 给出了广泛的氘化产物;这些特征表明亚烷基氧代正膦中间体 [Ph2CP(O)NMe2] 存在消除-加成 (EA) 机制。转向 ArPhCHP(O)(NMe2)Cl (Ar = 4-NO2C6H4) 时,取代率仅略有增加(13-19 倍),但使用 R2ND 现在,H/D 交换速度非常快α碳原子。这表明 EA 机制的消除阶段包括共轭碱的快速可逆形成,随后是氯离子的限速排出。