Substituted benzhydryl

A very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

Picosecond absorption spectroscopy studies of the contact ion pairs formed in the photo-initiated, S N 1 reaction of three substituted benzhydryl acetates (18) provided the rate constants for the k and k2 steps of the reaction (Scheme 10), in acetonitrile and DMSO.83 The activation parameters for the k and k2 steps were obtained from the temperature dependence of these steps and the transition state energies were calculated from the rate constants. This allowed the energy surfaces for three substituted substrates to be calculated in each solvent. The effect of solvent reorganization on the reactions of the unsubstituted and methyl-substituted benzhydryl contact ion pairs (CIP) was significant, causing a breakdown of transition state theory for these reactions. The results indicated that it will be very difficult to develop a simple theory of nucleophilicity in, S N1 reactions and that Marcus theory cannot be applied to SnI processes. 

The reaction of cyanide ion with substituted benzhydryl carbenium ions to form nitriles and isocyanides is controlled by the rates of reaction at carbon and nitrogen.124 In slow reactions, far from the diffusion limit, the attack is completely at the cyanide carbon. Very fast reactions, with little or no reaction barrier reacting at the diffusion-controlled limit, occur at both the N and the C of the cyanide ion. XN2 reactions occur almost exclusively at carbon regardless of the substrate or source of the cyanide ion. The HSAB principle cannot predict the products of these reactions. 

The same line of arguments rationalizes why the reactivity ratio of two electrophiles is growing with increasing difference of their standard ionization free enthalpies. Under conditions of almost complete ionization, the p-phenoxy-substituted benzhydryl cation is 5400 times more reactive than the bis(p-methoxy)-substituted analog, whereas in presence of catalytic amounts of Lewis acid, the relative reactivity of the two compounds is 0.016 (Fig. 18). 

The dimethoxy-substituted benzhydryl chloride is also fully ionized in a SnCl4/EtOAc/CH2Cl2 solution [60], and the reactivity toward 2-methyl-l-pentene is identical with that in the BCI3/CH2Q2 solution (Fig. 19). AnPhCHCl is not fully ionized under these conditions, however, and... 

More reliable estimates of monomer reactivity are available from reactions of model compounds (Chapter 2). For example, the rate constants of addition of the same standard benzhydryl carbenium ion to various substituted styrenes correlate very well to Hammett s + = 4.9 [193]. Addition of various p-substituted benzhydryl cations to the same standard alkene yielded p(Acr = — 5.1 [193]. These results demonstrate that carbocationic polymerizations are extremely sensitive to even small changes in the monomer structure. They also demonstrate that the reactivity of carbenium ions scales nearly perfectly to the... 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

Table 6. Oxygen scrambling and racemization results for p-substituted benzhydryl p-nitro-benzoates (54 )76)...

The experimental electrofugality in the 5 1 reactions of substituted benzhydryl phenyl sulflnates is directly related to the theoretical electrofugality calculated using Koopman s theorem and nucleophilic Fukui functions.It is suggested that the... 

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. ... 

This example illustrates that the relative electrophilicities of two alkylating agents can be influenced by varying the amount of Lewis acid. Figure 5 shows that the reactivity differences become quite remarkable, when the difference of stabilization of the carbenium ions increases. The phenoxy substituted benzhydryl compound, for example, is 5400 times more reactive than the dimethoxy substituted compound, when an excess of BCI3 is used, whereas a reactivity ratio of 0.016 is observed with catalytic amounts of BCI3. 

The weak Lewis acid SbCl3 does not even fully ionize the p,p -dimethoxy substituted benzhydryl chloride, and only one branch of the SbCl3/CH2Cl2 graph can be seen in Figure 6. 

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