Allylic transpositions

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

Allylic transposition is observed in fluoride-induced reactions of allyl(trifluoro)silanes50 and allyl(trichloro)- and allyl(trialkoxy)silanes in the presence of hydroxylic promoters, e.g., 1,2-benzenediol51,52. Pentacoordinated silicates are believed to be involved53. 

The stereoselectivity of these reactions, and the observed 1,3-allylic transposition, are consistent with the participation of chair-like, six-membered ring transition states37. 

Allylic derivatives are particularly important in the case of boranes, silanes, and stannanes. Allylic boranes effect nucleophilic addition to carbonyl groups via a cyclic TS that involves the Lewis acid character of the borane. 1,3-Allylic transposition occurs through the cyclic TS. 

As with the silanes, the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C-Sn bond and it is a weaker bond.156 The most useful reactions in terms of syntheses involve the Lewis acid-catalyzed addition of allylic stannanes to aldehydes.157 The reaction occurs with allylic transposition. 

In many allylic oxidations, the double bond is found in a position indicating that an allylic transposition occurs during the oxidation. 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

The homoallylation product 16a presumably stems from oxidative cycloaddition of a Ni(0) species across the diene and aldehyde moieties of 15, leading to an oxanickellacycle intermediate 17 (path A, Scheme 5), which undergoes 0-bond metathesis with triethylsilane giving rise to a o-allylnickel 19. On the other hand, formation of 16b may start with addition of a Ni - H species upon the diene followed by intramolecular nucleophilic allylation as described in Eqs. 4-6 (path B). Alternatively, allylic transposition of the NiH group providing 20 from 19 may be related to the formation of 16b. The different reactivity between cyclohexadiene and many other acyclic dienes is also observed for the reaction undertaken under typical homoallylation conditions (see Scheme 14). 

In the case of the related starting material 95, bearing a chlorovinyl fragment in the side chain, the expected azepinone 96 was accompanied by a new rearranged quinolizine derivative 97 that was generated from allylic transposition of the side chain to C-12b, as shown in Scheme 8 <1996TL5701>. 

Letter from Christopher Ingold to Prevost, dated 29 July 1946 and a letter from the Secretary of the Nobel Committees of the Swedish Royal Academy of Sciences to Prevost, dated 16 January 16 1954. Copies of these letters were given to me by Constantin Georgoulis, who completed a doctoral dissertation at Paris in 1960 on the kinetic study of reaction schemas for allylic transpositions, under the direction of Prevost. Another of Georgoulis s teachers was Paul Job, cousin of Andre Job. 

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

Addition of an enophile to an alkene via allylic transposition. Also known as hy-droallyl addition. 

Reaction of dicrotylzinc with 1,3-butadiene readily took place at 20 °C and addition competed between C2 and Cl, leading respectively to a primary dialkylzinc 127 or a secondary ally lie diorganozinc 128 (or any of the mixed metallotropic forms)93. Consequently, hydrolysis led to a mixture of three major hydrocarbons 129a-c but the actual regioselectivity of the addition could not be accurately evaluated. Indeed, 129c could also in principle arise from the addition of dicrotylzinc, without allylic transposition, to the C(2) atom of butadiene (equation 61). 

Interestingly, the Zr-promoted crotylzincation of 5-decyne occurred without allylic transposition (equation 75), presumably via a four-centered transition state, unlike the uncatalyzed addition of crotylzinc bromide to alkynylsilanes which proceeded with complete S 2 regioselectivity108. 

The overall reaction is best viewed as intramolecular oxidative addition of the C(l)—H bond to the Rh(I) center, causing cyclometalation (25), followed by reductive elimination of an enamine from the Rh(III) intermediate accompanied by allylic transposition. Notably, the allylamine ligand in the initial Rh(I) complex as well as the Rh(III) intermediate has an s-trans conformation with respect to the N—C(l) and C(2)—C(3) bonds, allowing the overall suprafacial 1,3-hydrogen shift to produce the is-configured enamine product. 

For reactions of allylic organometallics that proceed with allylic transposition, see page 351, Section 5. 

See page 991, Section 1, for addition to the double bond and page 231, Section 5, for oxidation with allylic transposition. 

The reactivity of the organosilanes65 and organostannanes66 towards electrophiles is dependent on the characteristics of the organic ligands. Typically, the alkylsilanes and alkylstannanes are unreactive, which is a consequence of the weakly polarized carbon-silicon and carbon-tin cr-bonds (C8-—Ms+). However, allylsilanes67 and allylstannanes are highly reactive to electrophiles because of extensive ct-tt (C—Si or C—Sn) conjugation in the ally metals and the 0-carbonium ion stabilization effect of the metal center. Consequently, electrophiles add exclusively with allylic transposition. 

Metal-catalyzed Allylic Transpositions, Oxy-Cope and Aza-Cope Reactions 563... 

Electrophilic transition metals, particularly palladium(II) salts, catalyze a number of heteroatom allylic transposition processes38 by a mechanism which almost certainly involves nucleophilic attack of a heteroatom on a metal-bound alkene (Scheme 10), often termed cyclization-induced rearrangement . 

Allylimidates underwent a clean Claisen-type rearrangement in the presence of 5 mol % palladium chloride (equation 28),40 as did allyl carbamates39 and S-allylthioimidates (equation 29).41 This S to N rearrangement has found application particularly in the synthesis of pyrimidines, systems for which thermal S to N allylic rearrangements were generally ineffective (equation 30).42 Finally, 0-allyI S-methyl dithiocarbonates cleanly underwent palladium(ll)-catalyzed O to S allylic transposition (equation 31 ).43... 

Ab Initio MO calculations of a model complex Rh(PH3)2(NH3)(CH2=CHCH2NH2) were earned out to shed light to the detailed mechanism of Rh(l)-catalyzed isomerization of allylic amines to enamines.5 This study suggests that the square-planar [RhiPHjyNHjXCf CHCHjNHj) complex is transformed to [Rh(PH3)2(NH3)(( )-CH3CH=CHNH2)]+ via intramolecular oxidative addition of the C(l)-H bond to the Rh(I) center, giving a distorted-octahedral Rh(lll) hydride intermediate, followed by reductive elimination accompanied by allylic transposition. 

C-H Bond activation [ 1 ] and C-C bond formation are two of the key issues in organic synthesis. In principle, the ene reaction is one of the simplest ways forC-C bond formation, which converts readily available olefins into more functionalized products with activation of an allylic C-H bond and allylic transposition of the C=C bond. The ene reaction encompasses a vast number of variants in terms of the enophile used [2]. 

These thioacetals can be deprotonated by K2C03 in the presence of Aliquat 336 and the anions obtained react with various electrophiles including allylic halides. In contrast to reactions with NaH as the base, reaction with an allylic bromide results in allylic transposition. 

Alkylation of allylnitroalkanes.9 Cyclic allylic nitro compounds react with stabilized carbanions or amines in the presence of Pd(0) at the allylic position without allylic transposition. The reactive intermediate is presumably an allylpalladium(Il) species. 

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