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

Abstract

AbsbPct Second-order rate constants for the reactions of para-substituted diarylmrbenium ions (ArAr‘CH’ = 1) with allylsilanes 2, allylgermanes 3, and allylstannanes 4 have been determined in CH2C12 solution at -70 to -30 OC. 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 j3-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 X IOs) and trialkylstannyl group (3 X lo9) is derived. This effect is strongly reduced, when the alkyl groups at Si or Sn are replaced by inductively withdrawing substituents, and an allylic SiCI, 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. Allylsilanesl and allylstannanes2 have extensively been used as allyl anion equivalents during the last two decades. Their regioselective reactions with electrophiles have been explained by the intermediate formation of carbenium ions, which are hyperconjugatively stabilized by the carbonsilicon or carbon-tin bond in the 8-position (Scheme I).3 Competition experiments have shown that allyltrimethylsilane is 5 orders of magnitude more reactive toward in situ generated diarylcarbenium ions than propene; Le., the allylic trimethylsilyl group reduces the activation energy for electrophilic attack at the CC double bond by 18 kJ m01-I.~ Analogous investigations on the effect of allylic germy1 or stannyl groups on the nucleophilicity of CC double bonds have, to our best knowledge, not been performed. Indirect evidence for the magnitude of these effects can be obtained from the solvolysis rates of 8-element-substituted alkyl halidesS or trifluoroacetates (Chart 1): rates of acid-catalyzed R3MOH eliminations from 8-metal-substituted alcohols,6 or the rates of hydride abstraction in alkyl-substituted silanes, germane, and stannanes (Scheme II),’