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).3Competition 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),' cis-SnMq >1.3.10" ram-SnMq since in all these reactions 8-element-substituted carbenium ions are formed in the rate-determining step.sRecently we have developed a kinetic method to directly measure the rate of attack of carbenium ions a t the C C double bond of alkene^.^ We now report that this method can also be
Rates of hydride transfer from hydrosilanes HSiR1R2R3 with widely varying substitution to para-substituted diarylcarbenium ions have been measured in dichloromethane solution. Generally the reactions follow a second-order rate law, -d[Ar,CH+]/dt = k2[Ar2CH+] [HSiR1R2R3], and k2 is independent of the degree of ion-pairing and the nature of the counterion (exceptions are reported). The reaction rates are almost independent of solvent polarity. Kinetic isotope effects exclude an SET-type mechanism and are in accord with a polar mechanism with rate-determining formation of silicenium ions. The reactivities of para-substituted aryldimethylsilanes are linearly correlated with up ( p = -2.46), not with up+. In the series H3SiHex, H2SiHex2, HSiHex3, the relative reactivities are 1.00:155:7890, and in the corresponding phenyl series the reactivity increase is much smaller (H3SiPh:H2SiPh2:HSiPh3 = 1.00:17.2: 119). As a consequence, trihexylsilane is approximately two orders of magnitude more reactive than triphenylsilane though hexylsilane and phenylsilane show similar reactivities. Tris(trimethylsily1)silane is just slightly more reactive than trimethylsilane. Replacement of hydrogen by chlorine reduces the reactivity by one order of magnitude. Variation of the electrophilicities of the hydride abstractors does not affect the relative reactivities of the silanes, Le., constant selectivity (Ritchie-type) relationships are encountered. Correlation equations are given, which permit the calculation of hydride transfer rates from hydrosilanes to any carbenium ion on the basis of pKR+ values or the ethanolysis rate constants of the corresponding alkyl chlorides.
Kinetic investigations on t h e reactivity of allylsilanes (1-0)towards the p-methoxy substituted diphenylcarbenium ion (2) are reported.In spite of the wide use of allylsilanes in organic synthesis,l little quantitative information concerning the activation of C=C x-bonds by allylic silyl groups is available. Competition experiments have shown that the reactivity of the C=C double bond towards the diphenylmethyl cation increases by 30 700 when an allylic hydrogen of propene is replaced by a trimethylsilyl group.2 We have now determined the rate constants for the reactions of the allylsilanes (la-) towards the p-anisylphenylcarbenium ion (2) using the kinetic method previously described . 3 When allyltrimethylsilane (le) was added to a solution of (2)-BC14-in dichloromethane at -70 "C, the decrease of the carbenium ion concentration, monitored photometrically and conductimetrically, followed a second order rate law [first order with respect to each (le) and (2)].3 Addition of a tetra-alkylammonium tetrachloroborate or replacement of BC14-by BC130Me-or SnC15-did not influence the rate of the overall reaction, indicating that the p-silyl substituted carbenium ion (3) is generated in the rate determining step. Table 1 shows that the reactivity of allylsilanes strongly depends on the nature of the substituents X and Y. Replacement of methyl by larger alkyl groups (branched or unbranched) leads to a slight increase of reactivity, and exchange of the three methyl groups by phenyl reduces the reactivity by two orders of magnitude. A reduction of reactivity by three orders of magnitude is observed when one methyl group of (le) is replaced by chlorine (lb). The inductively withdrawing effect of chlorine is cumulated in allyltrichlorosilane (la). This compound, in contrast to propene, does not react with (2)-BC14-at -70°C, indicating a deactivation of the double bond by the trichlorosilyl group. \As reported for alkenes,4 an additional methyl group at the developing carbenium centre causes a strong reactivity increase, and the addition rate of (lm) is too high to be determined by our method. A quantitative value for the &-methyl effect (5950) is derived from the comparison ( l d l l ) . In contrast to (la), the trichlorosilyl derivative ( l k ) does react with (2), and the comparison (lk)/isobutene shows a deactivating effect of 360 for the trichlorosilyl group.One methyl group at the initially attacked vinylic carbon increases the reactivity to a similar degree [ ( l d l e ) = 211 as reported for alkenes (trimethylethylene/isobutene = 30 to 50),4 and the expected electronic acceleration by the second methyl group is overcompensated by steric retardation, resulting in a reduced reactivity of (lo) compared with (In).In summary, the rate effects caused by a-and P-methyl groups in allylsilanes closely resemble those reported for the corresponding reactions of normal alkenes, an indication for similar transition state structures.5 The reactivity scale in Table 1, which refers to the reference electrophile (2) will certainly...
The mechanism of the Friedel‐Crafts reaction is one of the most thoroughly studied, yet the absolute rate constants for the key step—the attack of the carbenium ion at arene—have only now been determined for the arylcarbenium ion. Since rate constants for reactions of these carbenium ions with alkenes and complexes of main group elements with the allyl group are already known, a direct comparison between the nucleophilicity or aromatic and aliphatic π systems is now possible.
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