Carbocations aryl shifts

This transformation has been shown (56JA2278) by 14C-labeIling to involve a 1,2-aroyl migration, rather than a 1,2-aryl shift which would necessitate a carbocation intermediate in which the charge is formally adjacent to the carbonyl group (Scheme 167). 

The rearrangement of acetals of 2-haloalkyl aryl ketones is a well-documented process yielding esters of 2-arylalkanoic acids by 1,2-aryl shift (equation 7). The mechanism of this rearrangement is reminiscent of other semipinacol rearrangements. Loss of the halogen (usually assisted by Lewis acid), yields a carbocation (4), which then undergoes a 1,2-aryl shift with carbonyl group formation. 

Many rearrangements have been invoked to explain the fragmentation pattern of molecular ions22 (It should be remembered that the fragmenting species are radical cations. Their radical functionality may open to them reaction paths which are not available to simple carbocations). Only a few illustrative examples of well-established alkyl and aryl shifts are given here. The mass spectrum of camphor displays the base peak at m/e 95, corresponding to the loss of ketene and a methyl radical. As shown... 

The C-NMR chemical shift of the trivalent carbon is a sensitive indicator of carbocation structure. Given below are the data for three carbocations with varying aryl substituents. Generally, the larger the chemical shift, the lower is the electron density at the carbon atom. 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

A long-established feature of the carbocation intermediates of reactions, such as SnI solvolysis and electrophilic aromatic alkylation, is a skeletal rearrangement involving a 1,2-shift of a hydrogen atom, or an alkyl, or aryl group. The stable ion studies revealed just how facile these rearrangements were. Systems where a more stable cation could form by a simple 1,2-shift did indeed produce only that more stable ion even at very low temperatures (see, e.g., Eq. 3). 

The 13 C NMR chemical shifts of the para carbon for various phenyl-substituted sp2-and sp-hybridized carbocations (Figure 15) indicate that the demand for rr-aryl delocalization of the positive charge for the -silyl-substituted vinyl cation 393 is lower compared with that in a-phenylethyl (78), a-methyl-a-phenylpropyl (397) and cumyl (292) cations150 153. The stabilizing effect of the /)-silyl group in 393 is comparable to that of the cyclopropyl substituent in 1-cyclopropylbenzyl cation (398). The para carbon shift in the /)-cr-silyl... 

Comparing p-silyl-substituted vinyl cations however with different a-aryl substituents shows that the stabilizing effect of a P-silyl groiq> is not constant but is dependent on the electron demand of the carbocation. Fig. 4 shows the pora-carbon C NMR chemical shift difference A8 between the p-silyl and P-unsubstituted vinyl cations (For the a-ferrocenyl substituted cation A8 C3,4 is used to probe the silyl effect). 

In the synthesis of 2//-naphtho[l,2-i]pyrans from 1-naphthols and l,l-diarylprop-2-yn-l-ols, initial protonation and loss of water from the latter generate an alkynyl carbocation, normally converted to the aryl propargyl ether. However, the concomitant formation of the highly coloured propenylidenenaphthalenones 12, a new class of merocyanine dyes, suggests attack of the allenic form of the cation at the 4-position of the naphthol followed by a 1,7-H shift <03EJ01220>. 

The epoxide 131 rearranges to afford a-hydroxyethylene-2-indanone (132) in moderate yield when heated under reflux in LiC104-toluene (Sch. 67). The proposed mechanism is lithium ion-promoted epoxide cleavage then 1,2-aryl migration to afford a relatively stable oxonium ion which cleaves to furnish a benzylic carbocation with subsequent 1,2-hydride shift affording the yS-ketoaldehyde. An attempt to bring about this transformation with BF3 OEt2 proved problematic [116]. 

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