Intramolecular allyl-migration

Unexpectedly, the intramolecular allylation of a,/ -unsaturated aldehydes, under identical reaction conditions, gave bicyclic product 165 as a single diastereoisomer (Scheme 13.56). This cyclization probably proceeded through a [1,2]-migration of the silyl group in the /hsilyl cationic species 163, followed by a subsequent intra-... 

Inter-ring metal migrations, dynamic NMR studies, 1, 412 Intracyclic germanium-carbon bond formation large rings, 3, 706 small rings, 3, 703 Intramolecular Alder-ene reactions with metals, 10, 576 with palladium, 10, 568 with rhodium, 10, 575 with ruthenium, 10, 572 with transition metal catalysts, 10, 568 Intramolecular allylations, in cyclizations, with indium compounds, 9, 679... 

Utilizing appropriate compounds in the fluorenyl series it was shown that the silyl groups migrate 100% intramolecularly (ii). On the other hand, allyl migration was not found to be strictly intramolecular, mtermolecularity amounting to about 28%. This means that at least the latter portion does not adhere to a strict dyotropic mechanism. 

Scheme 6.85 Enantioselective synthesis of 185 by Ir-catalyzed intramolecular allylic dearomatization and stereospeciflc migration reported by You.

You et al. reported the iridium-catalyzed enantioselective functionalization of indoles and pyrroles. In the event, indole 181 is transformed, via an intramolecular allylic alkylation, into a spirocyclic intermediate at C3, which then undergoes selective methylene migration to form 182 with no loss of enantiopurity (up to 99% ee is obtained).The indole ring can tolerate substitution with various halogens or methoxy groups, while pyrroles can be substituted with alkyl or aryl groups (13JA8169). 

Mechanistic study revealed that the reaction proceeds through intramolecular 1,3-hydrogen migration, and the chiral rhodium catalyst differentiates the enantiotopic C-l hydrogens of allylic alcohols (Scheme 29).53... 

Scheme 16.2 illustrates the catalytic mechanism proposed by Muetterties and coworkers [13]. Salient features of this mechanism are the coordination of benzene in the -fashion, to give a transient Col I( 4-C, iH, i)(PR3)2 complex, and the intramolecular hydride transfer to form the allylic intermediate Co(//3-Ctl l7) (PR3)2. Hydrogen addition would give an 4-1,3-cyclohexadiene complex that ultimately releases cyclohexane via H2 addition/hydride migration steps. Complete cis stereoselectivity of hydrogen addition was demonstrated by replacing H2 with D2. 

For Rh(I)/BINAP-catalyzed isomerizations of allylic amines, the mechanistic scheme outlined in Eq. (2) has been proposed. The available data are consistent with the notion that Rh(I)/PF-P(o-Tol)2-catalyzed isomerizations of allylic alcohols follow a related pathway [11]. For example, the only deuterium-containing product of the reaction depicted in Eq. (9) is the l,3-dideuterated aldehyde, which estabhshes that the isomerization involves a clean intramolecular 1,3-migration. The data illustrated in Eqs. (10) and (11) reveal that the catalyst selectively abstracts one of the enantiotopic hydrogens/ deuteriums alpha to the hydroxyl group. 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

In a Lewis-acid catalyzed intramolecular ene reaction,4 the alkene (ene) reacts in a pericyclic fashion with an enophile to form two new a bonds with concurrent migration of the allylic double bond. 

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