Tetrahydridocarbonyltris(dimethylpbenylphospbine)rhenium( V) Tetrafluoroborate (2a) and LMhydrido(dihydrwen~cnrbonvltris(dimethvI-keReD3(CO)(PMe2Ph)3 was prepared similarly by treatment of ReCI,(CO)(PMe2Ph)z with LiAID4 followed by hydrolysis with D,O. The isotopomeric mixture of ReH,-,D,(CO)(PMe2Ph)3 ( x = 0-3) was prepared by treatment of ReCl3(CO)(PMe2Ph), with LiAlD4 and hydrolysis with H 2 0 / D z 0 (1:I molar ratio). Abstract: Para-substituted diphenylmethyl halides, acetates, and ethers RPh(R'Ph)CH-X (R, R' = CF3 to OCH3), upon photolysis with -250-nm light in acetonitrile solutions, undergo homolysis and heterolysis of the C-X bond to give the radicals, RPh(R'Ph)CH' (abbreviated as C), and the cations, RPh(R'Ph)CH+ ((2' ). Whereas the quantum yields for homolysis (0.2-0.4) are rather independent of the nature of the substituent on the benzene ring, those for heterolysis increase with increasing electron-donator strength from 50.07 for CF3 to 0.3 for OMe. The cationxadical ratios are also dependent on the nucleofugal properties of X. For the halides, the observed hetero1ysis:homolysis ratios correlate with the pK, values of the conjugate acids H X and not with the electron affinities of X' . In acetonitrile, heterolysis is much less endothermic than homolysis. Homolysis and heterolysis can also be effected indirectly by reaction with triplet acetophenone (produced by 308-nm photolysis). Unless stabilized by one or more MeO, the cations decay predominantly by reaction with acetonitrile to give nitrilium ions. However, since this reaction is reversible (shown for the benzhydryl cation), the nitrilium ion contributes only to an insignificant degree to the formation of the final (cation-derived) products, which result from reaction with trace water (main product, benzhydryl alcohol; minor, benzhydrylacetamide). The rate constants for addition of C+ to CH$N are in the range 3.5 X lo5 to 3.8 X IO7 s-I for the cations with R = R' = Me to R = H, R' = CF3. The rate constants for reaction of C+ with halides (ion recombination) are -2 X 1O1O M-l s-' (diffusion control). The radicals C' disappear by dimerization and disproportionation, for which a complete mass balance has been achieved by product analysis for the case of the benzhydryl system. At laser-pulse powers > IO mJ electronically excited radicals, C", are additionally formed in many cases, via absorption of a light quantum by ground-state C'.
observed percent I8O incorporation the scrambling rates were calculated by standard methodsi6.Two control experiments in each solvent were performed in order to show that the observed I8O scrambling is not the result of ( I ) chemical workup or ( 2 ) external ion return. In the first experiment 25 mg of the labeled ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of 2,6-lutidinium brosylate, and the reaction mixture was worked up immediately in the same manner. In the second experiment 50 mg of the unlabeled sulfonate ester was dissolved in the buffered solvent (25 mL) containing 1 equiv of "0 enriched (60.7%) 2,6-lutidinium brosylate and after solvolysis for 1 half-life the solution was worked up in the same manner. In the I3C NMR spectrum of the isolated ester from both experiments, no "0 was observed to be present at the a-carbon.Oxygen Scrambling Studies. 2. 4-Methyl-3-homoadamntyl Heptafluorobutyrate. A 5.3 mM solution of the ether I80 enriched (50.70%) 4-methyl-3-homoadamantyl heptafluorobutyrate in 80E (100 mg/50 mL) containing a I . 1 equiv of 2,6-lutidine was reacted at 25 OC for 9.5 1 h. The reaction flask was then placed in a 0 "C bath, and the workup was the same as that used in the I8O studies of 2s. The composition of the product mixture was analyzed by IH NMR (300 MHz) in an analogous fashion to that performed in the product studies of the tertiary ester above. In the spectrum the additional methyl doublet of the unreacted tertiary ester occurs at 0.93 ppm. The percentage I80 incorporation at the a-carbon of the two esters and of the solvolysis products was determined from the natural abundance 125-MHz "C spectrum recorded on a Bruker 500-MHz Fourier transform spectrometer with the conditions for data acquisition being similar to those in the I8O studies of 2a. No "0 incorporation was observed at the a-carbon of the alcohol and ether solvolysis products. In the unreacted tertiary ester and the rearranged secondary ester the percent present was 43.68% and 41.56% respectively, and their recorded spectra are shown in Figure 3. The proportion of unreacted tertiary ester that was equilibrated is 27.7% [(50.7 -43.68)/23.35 X 1001 while that for the secondary ester was 36.0%. The calculation of the rate of I80 equilibration (9.47 X IOd S-I) for the tertiary ester as well as the ratio of return of the originally bonded oxygen relative to the carbonyl oxygen (6.6:l) which occurs in the formation of the secondary ester from unscrambled tertiary ester is given in detail in the Supplementary Material.)$ Supplementary Material Available:T h e details of and the equations used in the Simplex calculation of Scheme I (1 2 pages). Ordering information is given on a n y current masthead page. Abstract: A kinetic method that allows the determination of reactivities of carbenium ions toward alkenes is described:Diarylmethyl chlorides (1) are completely ionized by BCI, in CH2C12 to give colored solutions of diarylcarbenium (2) tetrachloroborates, which show conductivity. Upon addition of the ...
Diarylcarbenium ions (benzhydryl cations) are generated from diarylmethyl chlorides by 20-11s laser pulses (248 nm) in acetonitrile solution at 20 OC. The second-order rate constants for their reactions with n-and wnucleophiles (anions, alcohols, water, alkenes, allylsilanes, alkyl and silyl enol ethers) are determined by monitoring the decay of the UV-vis transients at variable nucleophile concentrations. Only reactive nucleophiles (k2 > 106-107 L mol-' s-I) can be investigated by this method because of the concomitant reactions of the carbenium ions with the solvent acetonitrile and the chloride ions roduced in the photoheterolysis. The largest observed values of k2 are -2 X 1Olo for reactions with anions and (2-4) X IO8, mol-' s-I for reactions with neutral nucleophiles. Alkoxy-substituted ethylenes are 300-1 Os times more reactive than the corresponding alkyl-substituted ethylenes. The reactivities of structurally analogous alkyl and silyl enol ethers differ by less than 1 order of magnitude. In sharp contrast to the situation previously observed for the reactions of benzhydryl cations with alkenes, the nucleophilic reactivities of the enol ethers correlate with their ionization potentials and not with the stabilities of the carbenium ions produced in the rate-determining step. The rate constants measured for the reactions of the flash photolytically generated benzhydryl cations with alkenes and allylsilanes agree well with those extrapolated from the reactivities of these nucleophiles toward less electrophilic benzhydryl cations, which have previously been determined by conventional techniques. Combination of the two sets of data yields a nucleophilicity scale with respect to the reference electrophile @-H3CC6H4)$H+.
Dedicated to Professor Christoph Riichardt on the occasion of his 65th birthdayThe investigation of substituent effects is one of the most important tools for the determination of reaction mechanisms. While effects of substituents on equilibria and rates of chemical reactions are usually attributable to differences in enthalpy, changes in the entropy of activation, particularly for reactions of reactive intermediates (fast reactions), have been recognized to be the origin of observed substituent effects." -31 For the reactions of para-substituted benzhydryl cations with n-nucleophiles, we now demonstrate that the change from enthalpic to entropic substituent effects occurs within these reaction series on going from slow to fast reactions. Consequently, the reactivity of the nucleophile determines whether changing substituents in the electrophile affects AH * or AS * .Detailed mechanistic investigations of the reactions of benzhydryl cations with a l k e n e~[~] and other n-systemsr5] have shown that generally the C-C bond formation is the rate-determining step of the reaction sequence depicted in Scheme 1, that I ir Scheme 1. The reaction of benzhydryl cations with compounds containing n-elec-is, the measured rate constant represents the attack of the carbenium ion at the n-system. Reduction of the electron-donating ability of the substituents X and Y in the carbenium ion 1 gives rise to an acceleration of the reactions, which is caused entirely by the decrease of the enthalpy of activation while the entropy of activation remains constant within the accuracy of the measurements (Fig. l). An analogous pattern has been observed forFor the reaction of the bis(p-toly1)carbenium ion (1 ; X = Y = CH,) with 2-methyl-2-butene (2) in CH,Cl,, rate constants k of (6.9 k 0.3) x lo4 Lmol-'s-', almost independent of temperature between -70 and -4O"C, have been observed, from which an activation energy E, of (0.4 0.6) kJmol-' and an enthalpy of activation AH * of (-1.4 k 0.6) kJ mol- (Fig. 1 top) can be calculated. Though no enthalpic barrier is present, the observed rate constants are five orders of magnitude below the diffusion limit as a result of the large entropy term -TAS * .How do carbenium ions, which are less stabilized by substituents than 1 (X = Y = CH,), behave? To investigate the reactions of more electrophilic benzhydryl cations (1) with the same nucleophiles (2-5), we used the laser pulse method, employing acetonitrile as a solvent because photolysis of benzhydryl chlorides in dichloromethane almost entirely causes homolytic cleavage of the C-Cl bond.[*] As variation of the solvent has only a very small influence on the rates of the reactions of carbenium ions with neutral nucleophiles,~'~ 91 the rate constant measured for the reaction of the bis(p-chloropheny1)carbenium ion (1, X = Y = Cl) with 2 ( k =7.4 x lo7 Lrnol-'s-', CH,CN, 20 "C) indicates that the replacement of X = Y = CH, by C1 in 1 causes an increase of reactivity by almost three orders of magnitude.['0] Since there was no enthalpic barrier even for...
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