Cumyl derivative

The addition of two ortho-methyl groups to cumyl derivatives also results in a decrease in the Hammett reaction constant from p+ = —4.3 for cleavage of X-[8]-(4-nitrobenzoate) to p+ = —3.0 for cleavage of X-[10]-(4-methoxy-benzoate).27 This provides additional support for the conclusion that rotation about the CAr—Ca bond, which will minimize destabilizing steric interactions between methyl groups in the transition state, also reduces stabilizing resonance electron donation from the ring substituent to the benzylic carbon (Scheme 9). 

The pn- and p -values for electrophilic bromine additions to arylolefins are in the same range as those for other reactions via analogous benzylic carbocations. However, generally the comparisons are only qualitative because of significant differences in the experimental conditions and in the mechanisms. For example, as has already been mentioned, the reaction constant of t-cumyl chloride methanolysis is —4.82 (Okamoto et al., 1958), i.e. slightly higher than that for a-methylstyrene bromination in methanol, where the intermediate resembles that in the solvolysis of cumyl derivatives (Scheme 13). 

The azide ion product selectivity (A az/A s)obsd = 0.7 observed for reactions of ring-substituted cumyl derivatives [XC6H4C(Me2)Y) when X is strongly electron-withdrawing is consistent with as 0.7M for formation of an association... 

Figure 2.2. The change with changing aromatic ring substituent X in the azide (az) ion selectivities (feaz/ s)obsd ( ) determined by analysis of the products of the reactions of ring-substituted 1-phenylethyl derivatives (X-l-Y) and ring-substituted cumyl derivatives (X-2-Y) in 50/50 (v/v) water/trifluoroethanol at 25 The selectivities are plotted...

Results such as these have led to recurring discussions about the extent of stabihzation of the transition state for heterolytic cleavage at tertiary carbon by nucleophilic assistance from solvent. The difficulty lies in reconciling studies that suggest that there is a small dependence of obsd (s ) for solvolysis of tert-butyl chloride,and some cumyl derivatives, on solvent nucleophilicity with other work that shows there is no detectable stabilization of the transition state for these reactions by interaction with the strongly nucleophilic azide and hydroxide ions, and the strong neutral nucleophile propanethiol. ... 

J. P. Richard, T. L. Amyes, and T. Vontor, Absence of Nucleophibc Assistance by Solvent and Azide Ion to the Reaction of Cumyl Derivatives Mechanism of Nucleophibc Substitution at Tertiary Carbon, J. Am. Chem. Soc. 1991, 113, 5871. 

If initiation is faster or comparable to propagation and termination is negligible, kinetic plots are straight in semilogarithmic coordinates. Initiation is faster than propagation and not kinetically detectable in polymerizations of isobutene and styrene initiated by cumyl derivatives because the initiator is more easily ionized than the propagating species. However, if the initiator is less easily ionized than the propagating species as in a-methyl-styrene polymerizations initiated by cumyl derivatives, and in isobutene polymerizations initiated by /-butyl derivatives (cf., also Section III. A.5), then initiation may be incomplete and the overall polymerization rate will increase continuously. 

The rates of initiation and propagation are comparable when the covalent initiator and dormant chain ends have similar structures. Therefore, 1-phenylethyl precursors are useful initiators for styrene polymerizations, but are poor initiators for a-methylstyrene and vinyl ether polymerizations. Similarly, cumyl derivatives are good initiators for isobutene and styrene, but are poor initiators for vinyl ethers their initiation of a -methylstyrene is apparently slow [165]. 1-Alkoxyethyl derivatives are successful initiators for vinyl ethers, styrenes, and presumably isobutene polymerizations [165,192]. /-Butyl derivatives initiate polymerization of isobutene slowly [105]. This is mirrored in model studies that show that /-butyl chloride undergoes solvolysis approximately 30 times slower than 2-chloro-2,4,4-trimethylpentane [193]. This may be due to insufficient B-strain in monomeric tertiary precursors [194]. In contrast, monomeric and dimeric or polymeric structures of secondary esters and halides apparently have similar reactivity. 

To assure a sufficient initiation rate, it is recommended to use a compound that ionizes more readily than the macromolecular covalent species. This is the case for 1-alkoxyhaloethanes used as initiators for a-methylstyrene and cumyl halides for isobutene polymerizations [5,40], The latter system may, however, lead to intramolecular cyclization. Blocking either the ortho or meta position in the aromatic ring improves the efficiency of initiation with cumyl derivatives [5]. 

By application of these substrates, some of which were characterized by a distinctive odor, exclusively the terminal 4-oxysubstituted 3-methylbutyraldehydes yielded. The cumyl-derived aldehyde was converted into the primary alcohol by using NaBH,j [186]. The alcohol showed a nutty woody ozoney, fresh air cloth, and green-orange aroma profile. 

The addition of two o-methyl groups to ring-substituted cumyl derivatives to give derivatives (28) causes only a modest change (< 5-fold) in feobs for reaction in 50 vol.% CF3CH2OH-H2O 2,6-Me2/2,6-H2 rate ratios for X = MeO, Me, and H are 0.63, 1.7, and 4.4, respectively. However, for the sterically hindered cation (29, X = Me) there is a 70-fold decrease in and a 60-fold increase in fee the product rate-constant ratios for partitioning of (29) between substitution and elimination reactions promoted by solvent (feg and fee) and by azide ion and fes, respectively) have also been discussed. 

Rose, J.M., Deplace, F., Lynd, N.A. et al. (2008a) C2-Symmetric Ni(II) g-diimines featuring cumyl-derived ligands Synthesis of improved elastomeric regioblock polypropylenes. Macromolecules, 41,9548-9555. 

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