Neighboring-group participation donors

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Although glycosyl triflates have been demonstrated to be intermediates with a number of armed donors, and even with disarmed donors not capable of neighboring-group participation, such as the sulfonate esters, typical disarmed donors with esters in the 2-position function in the anticipated manner through anchimeric... 

Acyl-protected fucosyl donors have also been generated very successfully (Table XXXII) (58b). Their reaction with acceptors led via neighboring-group participation to products (58b, 178). 

Neighboring group participation effects appear to play a crucial role in the nucleophilic substitution of chlorine in Michael adducts of 1-R, 2-R, 3-X. Thus, this substitution proceeds very easily in any of the adducts formed with an electron rich nitrogen, sulfur and oxygen Michael donor. For the adducts of nitrogen nucleophiles, the facile substitution of the chlorine has been suggested to occur via formation of intermediate aziridinium ions 103 [8] (Scheme 32), and this postulate was later supported by isolation of azaspiropentane derivatives under appropriate conditions in several reactions (see Sect. 3.2.2) [11b, 53,56]. It is most likely that alkylthio substituents in adducts of type 85 participate in the same way to first form spirocyclopropane-annelated thiiranium ion intermediates which are subsequently opened by attack of the incoming nucleophile. 

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