Carbocationic center

In their reactions with suitable nucleophiles, such as tt-aromatics or heteroatom donor nucleophiles, the readily polarizable linear acylium ions shift a Tt-electron pair to oxygen, bending the ions and developing an empty p-orbital at the carbocationic center. This enables the reaction with aromatics. The acetylation of benzene can be depicted as... 

The pioneering work Forster and Hoffmann [28] on the viscosity dependence of the fluorescence quantum yield of triphenylmethane dyes (TPM) has set the foundation for several reports in these dyes (Fig. 12). It was found that both an ability to twist around the carbocationic center and the donor-acceptor properties are important [66], Specifically, a strong intramolecular quenching is observed for 34 that is virtually absent (two orders of magnitude slower quenching rate) in the bridged... 

The first mechanism was proposed by Baird et al. [189]. The carbocationic species is schematically shown in Scheme IX. The attack of monomer, well known on the carbocationic center of a metal-ion-activated olefin, proceeded in the normal manner for carbocationic polymerization. This mechanism is based on the following two evidences. Alcoholysis of the polymerization system, TiMe3Cp /B(C6F5)3, resulted in the presence of an alkoxy group at an end group, and vinyl ethers and iV-vinylcarbazole were polymerized by using the same system. 

The values of ks/kp for partitioning of carbocations are most conveniently determined as the ratio of the yields of products from the competing nucleophile addition and proton transfer reactions (equation 1 derived for Scheme 2). The determination of these product yields has been simplified in recent years by the application of high-pressure liquid chromatography (HPLC). Typically, the product peaks from an HPLC analysis are detected and quantified by UV-vis spectroscopy. In cases where the absorbance of reactants and products is small, substrates may be prepared with a chromophore placed at a sufficient distance so that its effects on the intrinsic reactivity of the carbocationic center are negligible. For example, the aliphatic substrates [1]-Y have proved to be very useful in studies of the reactions of the model tertiary carbocation [1+].21,23... 

According to the stepwise electrophilic reaction mechanism, the differences in the stereochemistries of the products from the reactions of alkenes with cyclic 49 and acyclic 51 disulfonium dications can be explained by the larger rates of the intramolecular reactions. In the case of a cyclic dication, the carbocationic center in intermediate 94, which is formed as the result of initial attack by a S-S dication on a double C=C bond reacts with nucleophile intramolecularly, thus conserving the configuration of the substituents at the double bond. On the other hand, an acyclic dication undergoes transformation to two separate particles (95 and dimethylsulfide) with a consequent loss of stereoselectivity. Additional experiments with deuteretad alkenes confirm that reaction is not stereoselective, lending further support to the stepwise mechanism (Scheme 36).106... 

It has since become increasingly clear that zwitterionic zirconates may be generated in many other reactions and may lead to unexpected and interesting chemical consequences, as suggested by the results and interpretations shown in Scheme 1.73. It should be noted that the empty orbital and electrophilicity of Zr must lead to zwitterionic species containing zirconates and carbocationic centers. Further systematic investigations in this area appear to be desirable. 

The authors succeeded in preparing (20-25) a series of long-lived carbocations bearing vinyl and related groups in the p-position to the carbocationic center (la-d, 3). Their structures were confirmed by and 13C NMR spectroscopy. These cations can be considered as a-complexes of C-centered electrophilic agents, namely vinyl and methyl-substituted vinyl cations derived from 9,10-dimethylphenanthrene and 1,2-dimethylacenaphthylene, respectively. Quite naturally, they had not been prepared by electrophilic vinyl-... 

In order to enhance the catalytic activity of a carbocationic center, the novel Lewis acid 54 was designed by Mukaiyama [149-152]. The 1-oxoisoindolium-based carbenium salt 54 [149], possessing a weak coordinating borate counter anion, proved to be a very active catalyst in the aldoUzation (Scheme 58) [150]. The Mukaiyama aldol reaction was catalyzed by 1 mol% of salt 54 and proceeded in up to 97% yield in 30 min. 

Several 1,4-disubstituted derivatives of the dication, 63, were successfully prepared. The 1,4-diphenyl, 1,4-dimethyl, 1,4-dicyclopropyl-substituted derivatives of carbodication 63 (65, 66, and 67) are exceptionally stable. The 1,4-dimethyl-1,4-cyclohexyl dication, devoid of the adjacent cyclopropyl groups, could not be prepared. The carbocationic center in all these dications are somewhat shielded as compared to their monocations. The C NMR chemical shifts of the carbocationic centers for the dications 65, 66, and 67 are 235.4, 293.4, and 260.8 ppm, respectively. The diprotonated ann -tricyclo(5.1.0.0 )octa-2,6-dione, 68, may be treated as a dicarboxonium ion, instead of dihydroxy dicarbenium ion, since the cabronyl carbon is shielded by only 25.2 ppm, much smaller than that observed for the protonated cyclohexanedione (34 ppm). The para carbons of the phenyl substituents in carbodication 65 are relatively shielded by about 5 ppm from that of the parent l,4-diphenylcyclohexane-l,4-diyl dication showing relatively less delocalization of the charge into the aromatic rings. 

Figure 7.1. Carbocationic center interacting with (a) an X substituent (b) a Z substituent ( ) a C substituent.

The filled bond orbitals of adjacent alkyl groups may donate electrons by 7r-type overlap (see Figures 3.19 and 3.20 in Chapter 3) to an adjacent carbocationic center. Thus, an alkyl group may be considered to be an X-type substituent. The highest combination of the C—H bonding orbitals of a methyl group has a n donor capability intermediate to that of the nonbonded orbitals of and F. The donor abilities of a bonds were discussed in Chapter 4. 

Figure 10.6. Activation of a C—H bond by a neighboring a) C—X bond and (b) carbocationic center.

Use orbital interaction analysis to explain stabilization of a carbocationic center by a cyclopropyl group. What kind of substituent (X , C, or Z) is cyclopropyl Explain. Predict the orientation of the planar cationic center relative to the cyclopropyl ring. 

When fluorine is directly substituted on a carbocationic center, it stabilizes those cations by resonance effects.13... 

Lambert et al.139 generated the (5-triethylsilyl-substituted carbocation 27 reacting solvated triethylsilylium ion with 1,1,-diphenylethylene [Eq. (3.26)]. (3-Silyl-substi-tuted carbocations are of high interest because of the (5-effect, that is, the ability of a silicon atom to stabilize a nonadjacent positive charge of a carbocationic center via hyperconjugation. Results have been summarized in reviews.140 142... 

Cubylcarboxonium ions have been also studied by Prakash, Olah, and co-workers.579,580 The parent cation 281 prepared under superacid conditions was stable at low temperature but decomposed to cubylacylium cation 282 as a result of further protonation and dehydration [Eq. (3.71)]. In addition to cation 281, di- and tetra-carboxonium ions and the corresponding protonated methyl esters were also observed as long-lived species stable under superacidic conditions. Experimental evidence and theoretical data indicated that the strained cubyl system effectively stabilizes the carbocationic centers through C—C bond hyperconjugation (283). On the basis of 13C data, three conformers of protonated dimethyl cubane-l,4-dicarboxylate (284-286) could be identified. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

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