Kinetics of the reactions of allylsilanes, allylgermanes, and allylstannanes with carbenium ions

Article abstract of DOI:10.1021/ja00013a035

Second-order rate constants for the reactions of para-substituted diarylcarbenium ions (ArAr'CH⁺ = 1) with allylsilanes 2, allylgermanes 3, and allylstannanes 4 have been determined in CH₂Cl₂ solution at -70 to -30°C. Generally, the attack of ArAr'CH⁺ at the CC double bond of the allylelement compounds 2-4 is rate-determining and leads to the formation of the β-element-stabilized carbenium ions 5, which subsequently react with the negative counterions to give the substitution products 6 or the addition products 7. For compounds H₂C = CHCH₂MPh₃, the relative reactivities are 1 (M = Si), 5,6 (M = Ge), and 1600 (M = Sn). From the relative reactivities of compounds H₂C=CHCH₂X (X = H, SiBu₃, SnBu₃), the activating effect of an allylic trialkylsilyl (5 × 10⁵) and trialkylstannyl group (3 × 10⁹) is derived. This effect is strongly reduced, when the alkyl groups at Si or Sn are replaced by inductively withdrawing substituents, and an allylic SiCl₃ group deactivates by a factor of 300 (comparison isobutene/2k). A close analogy between the reactions of alkenes and allylelement compounds with carbenium ions is manifested, and the different reaction series are connected by well-behaved linear free energy relationships. The relative reactivities of terminal alkenes and allylelement compounds are almost independent of the electrophilicities of the reference carbenium ions (constant selectivity relationship), thus allowing the construction of a general nucleophilicity scale for these compounds.