Allyl carbonates rearrangement

Allylation of the 10-carborane 236 (pKa = 18-22) with diallyl carbonate is possible under neutral conditions to give 237[146], Allylation and rearrangement of the trialkylalkynylborane 238 affords the trisubstituted alkene 239 stereoselectively [ 147],... 

Various S-nucleophiles are allylated. Allylic acetates or carbonates react with thiols or trimethylsilyl sulfide (353) to give the allylic sulfide 354[222], Allyl sulfides are prepared by Pd-catalyzed allylic rearrangement of the dithio-carbonate 355 with elimination of COS under mild conditions. The benzyl alkyl sulfide 357 can be prepared from the dithiocarbonate 356 at 65 C[223,224], The allyl aryl sufide 359 is prepared by the reaction of an allylic carbonate with the aromatic thiol 358 by use of dppb under neutral condi-tions[225]. The O-allyl phosphoro- or phosphonothionate 360 undergoes the thiono thiolo allylic rearrangement (from 0-allyl to S -allyl rearrangement) to afford 361 and 362 at 130 C[226],... 

Nucleophilic substitution at an allylic carbon takes place either by an SN2 mechanism or by an Sn2 mechanism. No allylic rearrangement takes place in the former case, but it does occur in the latter case. 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

Some allyl phenyl ethers with an alkyl substituent on the end carbon of the allyl group rearrange to give the normal ortho-Claisen product together with another isomeric O-allyl phenol. The latter, formed by the rearrangement of the normal product, has been established. This is called abnormal Claisen rearrangement, is illustrated by the following example. 

The bisphosphane BPA was selected as a ligand for the Pd atom because of the high enantioselectivities which had been recorded in the allylation of sulfinate ions with racemic allylic carbonates (vide supra). Besides the variations of the carbon skeleton, the substituent on the S atom of the racemic allylic sulfinates was varied in order to see whether both aryl and alkyl sulfinates are amenable to a highly selective rearrangement. 

The sense and degree of asymmetric induction of the Pd(0)-catalyzed rearrangement of the cyclic and acyclic O-allylic thiocarbamates in the presence of BPA are the same as, or similar to, those in the Pd-catalyzed substitutions of the corresponding cyclic and acyclic racemic allylic carbonates and acetates with sulfinates and thiols. It is therefore proposed that Pd(0)/BPA reacts with the racemic O-allylic thiocarbamate with formation of a jt-allyl-Pd(II) complex, which contains as counter ion the corresponding thiocarbamate ion (Scheme 2.1.4.19) [23, 24]. Substitution of the jt-allyl-Pd(II) complex by the thiocarbamate ion gives the S-allylic thiocarbamate and the Pd catalyst. 

The Cope rearrangement of 24 gives 2,6,10-undecatrienyldimethylamine[28], Sativene (25j[29] and diquinane (26) have been synthesized by applying three different palladium-catalyzed reactions [oxidative cyclization of the 1,5-diene with Pd(OAc)2, intramolecular allylation of a /i-keto ester with allylic carbonate, and oxidation of terminal alkene to methyl ketone] using allyloctadienyl-dimethylamine (24) as a building block[30]. 

Nucleophilic Substitution at an Allylic Carbon. Allylic Rearrangements... 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

A 1,2-hydride shift has been invoked399 to account for the formation of p-methoxyphenylbutyraldehyde derivatives (337) during the treatment of />methoxy-benzyl-protected allylic alcohols (336) with zeolites. A similar C-glycosidation procedure involving Lewis acid-catalysed anomeric oxygen to carbon rearrangement of tetrahydropyranyl ether derivatives has been reported400 (see Scheme 82). It has been... 

The palladium-catalyzed decarboxylative coupling of allyl 2-(benzo[c(jthiazol-2-yl)acetates 118 provides a facile approach to 2-(but-3-enyl)benzo[c(jthiazoles 122 <07JA4138>. The reaction is initiated by nucleophilic attack of Pd(0) on the allyl ester to give Pd-7t-allyl complex 119, which undergoes nucleophilic attack at the less substituted allylic carbon from the benzothiazole nitrogen to produce 120. Decarboxylative dearomatization leads to intermediate 121, and a subsequent aza-Cope rearrangement driven by rearomatization affords the final product 122 and accounts for the unusual regioselectivity. This appears to be the first report of a tandem allylation/aza-Cope reaction driven by decarboxylative dearomatization/ rearomatization. 

The process was later improved by the use of a p-toluenesulfonyl substituent at the allylic carbon atom (equation 42). The authors claim that this modification has a powerful influence on both the selectivity and mechanism of the oxidation, exclusive oxidative rearrangement then being observed. Several other methods of achieving allylic oxidation using palladium catalysts have also been reported,although these are generally of less importance. 

The palladium(0)-catalyzed asymmetric O-allylation of phenols has been described using five-, six- and seven-membered ring allylic carbonates and acyclic allylic carbonates (eq 9). The products from these reactions were subjected to a Claisen rearrangement to provide C-alkylated phenols. A study of various ligands for the reaction of phenol with 2-cyclohexenyl-l-methyl carbonate clearly showed that the Trost ligand is superior. ... 

Substituent effects in the allyl ester rearrangements are very similar to those observed in the ester reverse ene-type eliminations. This is apparent from the relative rate comparisons of Table 8. At the a- and y-carbons, reaction rates are observed to increase in the order CF3 < H < CH3. The rate accelerations by methyl substitution for hydrogen at the a-carbons are factors of 40 and 23, and at the y-carbon are factors of 55 and 23. These effects should be compared with the rate accelerations by methyl for hydrogen substitution at the a-carbon in the ester ene reactions, i.e., from Table 2, i-PrOAc/EtOAc = 18.7 and t-BuOAc/i-PrOAc = 53. One may conclude that the positive formal charge densities at the a- and... 

Allyl Claisen rearrangements of carbonate esters have already been discussed under ester rearrangement reactions (see Table 7). 

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