Rearrangement, 1,2-alkyl study

The actual proportions of products obtained in many cases are not necessarily found to reflect the relative stabilities of the incipient carbocations, unrearranged and rearranged, however. This follows from the fact that their relative rates of reaction with the aromatic species almost certainly do not follow the order of their relative stabilities, and may well be diametrically opposed to it. Attack on the aromatic species by the first formed polarised complex may be faster than its rearrangement. The study of these rearrangements is also complicated by the fact that Lewis acids are found to be capable of rearranging both the original halides, and the final, alkylated end-products, e.g. ... 

Oxidations and Rearrangements. -Alkyl and -benzyl N,N-dlalkylthlocarbamates are converted to their sulfoxide derivatives Q) both Iri vivo in rats and on incubation with liver mlcrosomes and NADPH (3-5, 19-21). Studies with EPTC (25) reveal that they may also undergo hydroxylation at each alkyl carbon (designated by arrows) and that carbon hydroxylation at the -CH2 moiety gives an unstable intermediate which yields acetaldehyde on decomposition (19). 

In an enamine alkylation study, piperitone (8 X = H) gave, in addition to the expected dienamines, two rearranged dienamines via ring opening and reclosure hydrolysis yielded the o-menthenones (181Dauben s full paper (Vol. 7, p. 34) on... 

The azepinium anions from 2-methoxy-3//-azepine <90JHC107> and 2-dimethylamino-5-phenyl-3/f-azepine <84RTC225> react with electrophiles to give a number of 3-alkyl-, 3-thioalkyl-, and 3-acyl-3/f-azepines. Further alkylation studies reveal that C, is the next position of attack <85RTC166>. Under similar conditions, the anion from ethyl 2-dimethylamino-5-phenyl-3//-azepine-3-car-boxylate undergoes rearrangement with methyl iodide to give the azanorcaradiene, ethyl 2-diethylamino-7-methyl-3,4-dihydro-3,4-methano-4-phenylpyridine-7-carboxylate (36) (Equation (4)) <87H(26)685>. 

At — 65 °C in THF allylic ethers may be metalated with methyl-lithium in high yield without any Wittig rearrangement With n-hexyl iodide the metalated species (227 M = Li) underwent alkylation preferential y to oxygen. However, with cyclohexanone (227 M = Li) gave ratios the inverse of those predicted from alkylation studies. Thus when (227 R = Bu M = Li) reacted widt cyclohexanone, the product... 

The chemical behaviour of the mesoionic pyrazole (459) has been studied by Boyd et al (74JCS(P1)1028). Protonation and alkylation take place on the exocyclic nitrogen atom and a thermal rearrangement of a methyl group is observed when (459) is boiled in benzonitrile for several hours giving (460). 

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. 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

Nonfluonnated allenes also readily react with fluoroalkenes to give diverse fluonnated alkylidenecyclobutanes [727, 12S, 129, 130] (equations 55 and 56), except for tetramelhylallene, which rearranges to 2,4-dimethyl 1,3-pentadiene under the reaction conditions prior to cycloaddition (equation 57) Systematic studies of l,l-dichloro-2,2-difluoroethylene additions to alkyl-substituted allenes establish a two-step, diradical process for alkylidenecyclobutane formation [131, 132, 133]... 

The hetero ring in 4-(l -hydroxyalkylidene)-5-oxazolones is cleaved by alcoholic HCl to form alkyl a-acylaminoacylacetates, which cyclize to oxazole-4-carboxylates [Eq. (25)]. This rearrangement occurs directly in alkali, and a carbon-14 tracer study has substantiated a mechanism involving ring opening followed by the alternative ring closure. ... 

Pedersen and coworkers10 studied the El mass spectra of several alkyl 2-hydroxyphenyl sulfoxides (10) and found that, contrary to methyl phenyl sulfoxide2,11 and the corresponding sulfones10, they do not show any abundant skeletal rearrangement ions (see Section III). This is obviously due to an ortho effect as shown in structure 10. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

The carbocations generated by non-Kolbe electrolysis can rearrange by alkyl, phenyl or oxygen migration. The migratory aptitudes of different alkyl groups have been studied in the rearrangement of a-hydroxy carboxylic acids (Eq. 34) [323]. 

The nickel(II) dithiocarbamate complexes are neutral, water-insoluble, usually square-planar, species, and they have been studied extensively by a range of physical techniques. The usual methods for the synthesis of dithiocarbamate complexes have been employed in the case of Ni(II), Pd(II), and Pt(II). In addition, McCormick and co-workers (330,332) found that CS2 inserted into the Ni-N bonds of [Ni(aziri-dine)4P+, [Nilaziridinelgf, and [Ni(2-methylaziridine)4] to afford dithiocarbamate complexes. The diamagnetic products are probably planar, but they have properties typical of dithiocarbamate complexes, and IR- and electronic-spectral measurements suggested that they may be examples of N,S-, rather than S,S-, bonded dithiocarbamates. The S,S-bonded complexes are however, obtained, by a slow rearrangement in methanol. The optically active lV-alkyl-iV(a-phenethyl)dithio-carbamates of Ni(II), Pd(II), and Cu(II) (XXIV) have been synthesized, and the optical activity was found to be related to the anisotropy of the charge-transfer transitions (332). 

The initial results of an early directed evolution study are all the more significant, because no X-ray data or homology models were available then to serve as a possible guide [89]. In a model study using whole E. coU cells containing the CHMO from Adnetohacter sp. NCIM B9871,4-hydroxy-cydohexanone (3 5) was used as the substrate. The WT leads to the preferential formation of the primary product (i )-36, which spontaneously rearranges to the thermodynamically more stable lactone (R)-37. The enantiomeric excess of this desymmetrization is only 9%, and the sense of enantioselectivity (R) is opposite to the usually observed (S)-preference displayed by simple 4-alkyl-substituted cydohexanone derivatives (see Scheme 2.10) [84—87]. 

The product of the rearrangement may be stable or may react further, depending on its nature (see also pp. 1396). An ab initio study predicts that a [l,2]-alkyl shift in alkyne anions should be facile. ... 

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