Alkylation rearrangement

Because acylation of an aromatic ring can be accomplished without rearrangement it is frequently used as the first step m a procedure for the alkylation of aromatic compounds by acylation-reduction As we saw m Section 12 6 Friedel-Crafts alkylation of ben zene with primary alkyl halides normally yields products having rearranged alkyl groups as substituents When a compound of the type ArCH2R is desired a two step sequence IS used m which the first step is a Friedel-Crafts acylation... 

Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product. 

Rearrangements. Alkyl nitrenes do not generally give either of the two preceding reactions because rearrangement is more rapid, for example,... 

Compounds 45 exhibit, in addition to sulfone-sulfinate rearrangements " ", alkyl sulfone cleavages , intramolecular Smiles-type rearrangements and extrusion of SOj , an exceptional mode of remote group interaction which leads to the loss of... 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

These results are consistent with ionization of the cyclopropylcarbinyl chloride on the zeolite, with formation of the C4H7+ cation. Attack of the chloride ion (internal return) might then occur at the three possible positions, giving the rearranged alkyl chlorides. This hypothesis was supported by the data obtained with impregnation of the NaBr on the NaY zeolite. The observation of the three alkylbromides is consistent with a mechanism involving ionization and attack of the external bromide nucleophile. 

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 addition to the migration of hydrogen atoms in sigmatropic rearrangements, alkyl shifts also take place. A large number of such reactions occur... 

Beckmann rearrangement-alkylation sequenceOxime sulfonates react with AIR, to form rearranged imines, which are reduced to amines by DIBAH (equation I). The reaction involves selective migration of the R group anti to the oxime sulfonate. 

Scheme 12.8. Mechanistic sequence for the tandem three-component nucleophilic addition/Brook rearrangement/ alkylation on acylsilanes TBS = t-butyldimethylsilyl.

Scheme 12.9. Synthesis of <5-araneosene using DMF = dimethylformamide, TMS = trimethyl-a three-component nucleophilic addition/Brook silyl, Tf = trifluoromethanesulfonyl, rearrangement/alkylation on acylsilanes, by Hu dba = dibenzylideneacetone, dppf = and Corey [30]. TBS = t-butyldimethylsilyl, TAS- 1,1 -bis(diphenylphosphino)ferrocene, DME = F = tris(dimethylamino)sulfur (trimethylsilyl)- 1,2-dimethoxyethane. difluoride, THF = tetrahydrofuran,...

Scheme 12.10. Synthesis of a-onocerin using butyldimethylsilyl, THF = tetrahydrofuran, a three-component nucleophilic addition/Brook Tf = trifluoromethanesulfonyl, DME = 1,2-rearrangement/alkylation on acylsilanes, by dimethoxyethane, TMS = trimethylsilyl, Corey and co-workers [33]. TBS = t- TBAF = n-tetrabutylammonium fluoride.

Table IV. Formation of rearranged alkyl halides (%) in Michaelis-Arbuzov reactions of neopentyl esters...

Rearrangement alkylation of cycloalkylhydroxylamine carbonates." These hy-droxylamines, which can be prepared from cycloalkyl amines or cyclic ketones, on reaction with a trialkylaluminum rearrange to an a-alkyiated nitrogen-containing heterocycic. The complete sequence is formulated for synthesis of an a-alkylatcd piperidine (equation I). 

ALLYLIC REARRANGEMENT AlkyL copper-Boron trifluoride complexes. 

Also judging from the mecK ism, we might expect Friedel-Crafts alkylation to be accompanied by the kind of rearrangement that is characteristic of carbonium ion reactions (Sec. 5.22). This expectation, too, is correct. As the following examples show, alkylbenzenes containing rearranged alkyl groups are not only formed but are sometimes the sole products. In each case, we see that the... 

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