Diarylmethyl chlorides

Ionisation and Dissociation of Diarylmethyl Chlorides in BC13/CH2C12 Solution Spectroscopic Evidence for Carbenium Ion Pairs, R. Schneider, H. Mayr, and P.H. Plesch, Berichte der Bunsengesellschaft fur Physikalische Chemie, 1987, 91, 1369-1374. 

The relative chloride affinities of the benzhydryl cations from Scheme 5 are now graphically displayed in Scheme 7 (bottom, right). The correlation between the chloride affinities of diarylcarbenium ions and the ethan-olysis rate constants of the corresponding diarylmethyl chlorides [39] allows the chloride affinity scale of the diarylmethyl cations to extend to the less stabilized systems (Scheme 7, bottom left). In the AG° scale shown in Scheme 7, 10 kJ mol-1 corresponds to K = 374. 

If the chloride transfer from diarylmethyl chlorides to metal chlorides MCI followed Eq. (13), one might derive relative chloride affinities of MCI and Aryl2CH + from the same type of NMR experiments, which led to the relative chloride affinities of various Aryl2CH + species. In this way one would also obtain a chloride affinity scale of various metal halides. The situation is more complex, however ... 

Figure 1 Conductance of 1.1 x 10-3 M diarylmethyl chloride solutions in the presence of variable amounts of BCI3 (CH2C12, —70° C). (From Ref. 41, reprinted with permission of VCH Verlagsgesellschaft.) Substituents X, Y are indicated beside the graphs.

Stereochemical investigations of intermolecular additions showed that the syn/anti-adduct ratio was dependent on the nature of the Lewis acid. In reactions of diarylmethyl chlorides to (Z)- and (E)-2-butene the formation of the antiadduct was generally favored (Scheme 18) [89]. Because the different product mixtures have been obtained from the stereo-isomeric 2-butenes, the intermediacy of long-lived, freely rotating sec-alkyl cations has been excluded. 

Figure 14 Correlation between AG for the reactions of diarylcarbenium ions with alkenes and A G,° for the ionization of the corresponding diarylmethyl chlorides with BCI3. (From Ref. 69, reprinted with permission of VCH Verlags-gesellschaft.)...

Figure 15 Free-energy profiles for the boron trichloride-induced additions of diarylmethyl chlorides to 2-methyl-l-pentene. (From Ref. 69, reprinted wiih permission of VCH Verlagsgesellschaft.)...

Figure 16 Relative reactivities of two diarylmethyl chlorides toward 2-methyl-1-pentene in CH2O2 at -70° C as a function of the Lewis acid concentration. (From Ref. 183, reprinted by permission of Kluwer Academic Publishers.)...

The solvolysis of diarylmethyl chlorides in ethanol correlates with ct+ and shows a p value of -4.2, indicating that ERGs strongly facilitate the reaction.This p value is consistent with a mechanism involving carbocation formation as the ratedetermining step. ERGs facilitate the ionization by a stabilizing interaction with the electron-deficient carbocation that develops as ionization proceeds. 

Carbonium ion stabilities may be linked to the SnI mechanism by comparing values with solvolysis rates. A linear relationship has been found to exist between the relative rates of solvolysis of diarylchloromethanes in methanol, ethanol, and 2-propanol and the pATr of the corresponding diarylcarbinol. The most reasonable explanation for this correlation is that the free energy of activation for solvolysis of the diarylmethyl chlorides corresponds to that required for ionization of the carbon-chlorine bond to a carbonium ion-chloride ion pair. 

The ionization rate constants (fcj) for the 5 1 reactions of various diarylmethyl chlorides whose Ef values are known have been measured in the presence of piperidine, pyridine, or PPhj, which suppresses ion-pair return, in several dipolar aprotic solvents. The Hammett p values range from -4.0 to -4.3 in the different dipolar aprotic solvents, proving that the reactions occur by an 5 1 mechanism. The nucleofu-gality parameters Nf and Sf from the log k = Sf Ef + Nf) equation) for the diarylmethyl... 

MeO , OH , or EtO and methyl fluoride, anisole, and 4-fluoroanisole on the gas-phase 5 2 reactions between dimethylmethylphosphonate and methylformate and HOO versus HO , MeO , or EtO in an attempt to discover the origin of the O -effect on the Sf 2 reaction of carbanions with 4-substituted benzyl chlorides in liquid ammonia " on the solvolysis reaction and the 2 reaction between phenoxide ions, and both neutral and negatively charged amines and 4-substituted benzyl chlorides in liquid ammonia on the ionization rates (the step) of the 5" 1 reactions of many substituted trityl halides and carboxylates in aqueous acetone and in aqueous and pure acetonitrile in the presence of piperidine on the ionization rates ( i) of the 5 reactions of various diarylmethyl chlorides in the presence of piperidine, pyridine, or PPh3 in several dipolar aprotic solvents on the solvolyses of X,Y-substituted benzhydryl acetates in various aqueous MeOH and EtOH solutions and on the dispersions observed in Grunwald-Winstein correlations of 5 solvolyses of substrates with substituents containing adjacent tt-electrons. ... 

Initiation rate constants (ie, the rate constant of addition of the initiating carbocations to the monomer as shown in the set of equilibria in eq. 7) were determined for a large number of monomers using carbocations derived from the reaction of diaryl and substituted diarylmethyl chlorides with various Lewis acids (21). As indicated by the reaction scheme shown in equation 7, initiation can occur with the participation of solvent-separated ion pairs or tree ions. According to MayRs results, paired and free carbenium ions have identical reactivity and the reaction rates are independent of the nature of the counteranion. Rate constants were found to increase only slightly with increasing solvent polarity... 

Lewis acid initiated reactions of diarylmethyl chlorides with alkenes are suited for model studies, since many of these reactions give 1 1 adducts in quantitative yield. Kinetic as well as thermochemical investigations of these reactions have been reported. Competition experiments (Figure 3) have now been used to determine the influence of Lewis acids on the relative electrophilicities of diarylmethyl derivatives. When a small amount of an alkene is added to a mixture of two diarylmethyl chlorides in presence of BCI3, the relative reactivity of the two competitors can be derived from the ratio of the two 1 1 products. 

Figure 3 Relative Reactivity of two Diarylmethyl Chlorides towards 2-Methyl-1-pentene in CH2CI2 at -70 °C as a Function of the Lewis Acid Concentration. ...

Figure 6. Relative Reactivities of Diarylmethyl Chlorides in Presence of an Excess of Lewis Acid. ...

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