Reaction regioselectivity, proximity

These sulfonylallenes undergo the Diels-Alder reaction with electron-rich dienes at the proximal C-C double bond in a regioselective manner [109],... 

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

Interestingly, in the reaction of 2-(3-chlorophenyl)-4,4-dimethyloxazoline with an alkyllithium [25], the deprotonation is chelation-controlled, but the addition of R" follows PAR. A reverse regioselectivity for alkylcuprate addition may well be explained by a complex-induced proximity effect which has been invoked to rationalize the alkyllithium additions. It is known that copper coordinates exceptionally well with amino and imino ligands. 

Lewis acid templation between an epoxide and an adjacent hydroxyl function sets the stage for an intramolecular addition process using an alkyne as a nucleophile <1995TL7145>. Cyclization reactions wherein the regioselectivity of the addition reaction is dictated by a proximal hydroxyl function is the subject of a review <2000T8779>. 

Regioselectivity. Caibon-carbon bonds are preferentially formed between the proximal sites of ene and enophile in the type I process and between the more (Mg-ene reaction) or less substituted (Pd-ene reaction) ene terminal and the proximal enophile site in the type II version. 

Hydrosilylation of 1 with trichlorosilane has been performed with Pt on charcoal (1% by weight) in quantitative yields (Scheme 1). The B-tris(trichlorosilylvinyl)borazine (2) was obtained with a high regioselectivity of proximately 80% trans hydrosUylation product [4], Pure 2 can be obtained by fiactional crystallization of the synthesized product fiom hexane. For further synthesis, both a- and P-hydrosilylation products can be used. No further hydrosilylation was observed in this case. In order to interconnect the single source precursor molecule 2 to a pre-ceramic polymer, methylamine was added to the solution of 2 in hexane, and a high viscosity, colorless oil was formed. By changing the reaction parameters (excess of CH3NH2, temperature), the viscosity of the polymer can be varied [5]. The obtained polymer (3P) is pure after evaporation of the solvent, which is checked by NMR. Other solvents like thf or toluene are also possible for the reaction, as well as for dissolution of the polymer. Furthermore, ethylamine leads to similar results in the formation of the polymer. 

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