Allylation electron-withdrawing groups

Extensive studies by Gorman and Gassman have shown that an allyl cation can be a 27r-electron component in a normal electron-demand cationic Diels-Alder reaction and, since a carbocation is a very strong electron-withdrawing group, the allyl cation is a highly reactive dienophile [19a, 21]. 

The asymmetric 1,3-dipolar cycloaddition of nitrones (515), possessing an electron-withdrawing group, to allylic alcohols was achieved by using diisopropyl (/ ,/ )-tartrate [(R,R-DIPT)] as a chiral auxiliary. The isoxazolidines (516) and... 

Using a stoichiometric amount of (i ,i )-DIPT as the chiral auxiliary, optically active 2-isoxazolines can be obtained via asymmetric 1,3-dipolar addition of achiral allylic alcohols with nitrile oxides or nitrones bearing an electron-withdrawing group (Scheme 5-53).86a Furthermore, the catalytic 1,3-dipolar cycloaddition of nitrile oxide has been achieved by adding a small amount of 1,4-dioxane (Scheme 5-53, Eq. 3).86b The presence of ethereal compounds such as 1,4-dioxane is crucial for the reproducibly higher stereoselectivity. 

The reactivity of RI is primary > secondary > tertiary. No reaction occurs with RBr or RC1. The allyl group of the stannane is introduced selectively a to the two electron-withdrawing groups. 

The regioselectivity in palladium-catalyzed alkylations has been attributed to the dynamic behavior of trihapto pentadienyl metal complexes60. For example, competing electronic and steric effects influence product formation in dienyl epoxides, but in palladium-catalyzed reactions steric factors were often found to be more important. Thus, alkylation of dienyl epoxide 76 with bulky nucleophiles such as bis(benzenesulfonyl)me-thane in the presence of (Ph3P)4Pd occurred exclusively at the terminal carbon of the dienyl system producing allyl alcohol 77 (equation 39). However, the steric factors could be overcome by electronic effects when one of the terminal vinylic protons was replaced with an electron-withdrawing group. Thus, alkylation of dienyl epoxide 78 affords homoal-lylic alcohol 79 as the major product (equation 40). 

A-(Monosubstituted amino)pyridiniums (972) are in prototropic equilibrium with A-imides (973) and dications (971). For R = H or allyl, the A-imides (973) are very strong bases and cannot usually be isolated if R is an electron-withdrawing group (e.g. acyl, sulfonyl, nitro), then the imide (973) is less basic and more stable. The cations (971) are only obtained in very strongly acid media. 

Electron withdrawing groups on the alkene considerably retard the epoxidation. For example, acrylic esters and acrylonitrile are not reactive, while allyl chloride is about one tenth as reactive as propylene towards TBHP/MoVI.240 The epoxidation of alkenes is completely stereoselective eri-alkenes are exclusively transformed into ds-epoxides and (runs-alkenes into trans-epoxides. Oxygen addition to the double bond preferentially occurs from the less shielded face of the substrate, e.g. in the selective epoxidation of terpenes by r-pentyl hydroperoxide (t-amyl hydroperoxide, TAHP) (equations 67 and 68).225... 

A final example of this class of allyl-X compounds is a vinylcyclopropane that is geminally substituted on the three-membered ring with electron withdrawing groups. Upon addition of a Pd° complex, the allylic C—C bond ruptures, with the stabilized carbanion serving as a leaving group (equation 11).96 The complex can be independently reacted with electrophiles at the carbanion as well as with nucleophiles on the allyl moiety. 

Electron-withdrawing groups close to the alcohol functionality may likewise destabilize intermediate carbocations and result in very slow oxidations. For instance, sterol 88 is oxidized with DDQ at the allylic alcohol two hundred times slower than the corresponding compound lacking the fluorine atom,94 and the treatment of hydrobenzoin (89) with DDQ results in the oxidation of a single alcohol because a second oxidation would involve a carbocation highly destabilized by the presence of a carbonyl group.95f... 

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