Alkyl group shifts

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

Sigmatropic migration involving alkyl group shifts can also occur ... 

If no hydrogen atom is available in the position next to the nitrogen sextet, an alkyl group shifts. In the action of acid on 2-ferf-butyloxaziranc a methyl shift occurs [Eq, (16)]. Formaldehyde... 

This cation can rearrange to a tertiary carbocation by an alkyl group shift. 

Hi) 0—0 Homolysis and alkyl group shift to give a rearranged carbonyl compound and OH... 

Comprehensive Biological Catalysis—a Mechanistic Reference Volume has recently been published. The fiiU contents list (approximate number of references in parentheses) is as follows S-adenosylmethionine-dependent methyltransferases (110) prenyl transfer and the enzymes of terpenoid and steroid biosynthesis (330) glycosyl transfer (800) mechanism of folate-requiring enzymes in one-carbon metabohsm (260) hydride and alkyl group shifts in the reactions of aldehydes and ketones (150) phosphoenolpyruvate as an electrophile carboxyvinyl transfer reactions (140) physical organic chemistry of acyl transfer reactions (220) catalytic mechanisms of the aspartic proteinases (90) the serine proteinases (135) cysteine proteinases (350) zinc proteinases (200) esterases and lipases (160) reactions of carbon at the carbon dioxide level of oxidation (390) transfer of the POj group (230) phosphate diesterases and triesterases (160) ribozymes (70) catalysis of tRNA aminoacylation by class I and class II aminoacyl-tRNA synthetases (220) thio-disulfide exchange of divalent sulfirr (150) and sulfotransferases (50). 

An enzymic counterpart of these complex base-catalysed rearrangements of sugars may be the reaction catalysed by 4-phospho-3,4-dihydroxy-2-butanone synthetase. The enzyme catalyses the formation of the eponymous intermediate in secondary metabolism from ribulose 5-phosphate. Labelling studies indicated that C1-C3 of the substrate became C1-C3 of the product, that H3 of the substrate derived from solvent and that C4 was lost as formate. X-ray crystal structures of the native enzyme and a partly active mutant in complex with the substrate are available. The active site of the enzyme from Met ha-nococcus jannaschii contains two metals, which can be any divalent cations of the approximate radius of Mg " or Mn ", the two usually observed. Their disposition is very reminiscent of those in the hydride transfer aldose-ketose isomerases, but also to ribulose-5-phosphate carboxylase, which works by an enolisation mechanism, so the enolisation route suggested by Steinbacher et al. is repeated in Figure 6.14, as is the Bilik-type alkyl group shift, for which an equivalent reverse aldol-aldol mechanism cannot be written. 

Another class of reactions that can occur by sigmatropic rearrangements involve alkyl group shifts ... 

Addition of dibromocarbene to strained carbon-carbon double bonds can lead to rearranged products. These products result from ionization of the initial 1,1-dibromo-cyclopropane adduct with ring opening to yield an allylic carbonium ion/bromide anion pair. Alkyl group shifts followed by ion pair collapse can then lead to rearranged products (see Sect. 2.6). Three examples of this phenomenon are illustrated as equations 4.2—4.4 [9, 12, 13]. For related examples, see equations 2.17—2.21. 

Carbocation rearrangements take place by hydride and alkyl group shifts. They usuaUy result in interconversion of secondary carbocations or conversion of a secondary into a tertiary carbocation. Primary alkyloxonium ions can rearrange by a concerted process consisting of loss of water and simultaneous hydride or alkyl shift to give secondary or tertiary carbocations. 

Mechanisms of the following photochemical isomerisations can be described as conversion of o-xylene into m-xylene and m-xylene into p-xylene, is accompained by 1, 2-alkyl shift. Similarly conversion of o-xylene into p-xylene and vice-versa is accompained by the 1, 3-alkyl group shift. 

Carbocations can also rearrange in the Friedel-Crafts reaction by an alkyl group shift. For example, the alkylation of benzene with l-chloro-2,2-dimethylpropane yields only (l,l-dimethylpropyl)benzene. 

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