Strong deshielding in aromatic isoxazolines

Very strong proton deshielding was found in di/tri-aromatic isoxazoline regioisomers prepared from acridin-4-yl dipolarophiles and stable benzonitrile oxides (BNO). Three alkenes, (acridin-4-yl)-CH=CH-R (R = COOCH3, Ph, and CONH2), reacted with three BNO dipoles (2,4,6-trimethoxy, 2,4,6-trimethyl, 2,6-dichloro) to give pairs of target isoxazolines with acridine bound to C-4 or C-5 carbon of the isoxazoline (denoted as 4-Acr or 5-Acr). Regioselectivity was dependent on both the dipolarophile and dipole character. The ester and amide dipolarophile displayed variable regioselectivity in cycloadditions whereas the styrene one afforded prevailing 4-Acr regioisomers. 2,4,6-Trimethoxy-BNO was most prone to form 5-Acr isoxazolines while mesitonitrile oxide gave major 4-Acr isoxazolines. Basic hydrolysis of the amide cycloadduct led to an unexpected isoxazolone product. The structure of the target compounds was studied by NMR, MS, and X-ray crystallography.

Prediction by (13)C NMR of regioselectivity in 1,3-dipolar cycloadditions of acridin-9-yl dipolarophiles

Strong correlation was found between (13)C NMR chemical shifts of dipolarophilic CH=CH carbons and regioselectivity in 1,3-dipolar cycloadditions of new acridin-9-yl dipolarophiles with stable benzonitrile oxides (BNO). Accordingly, two starting dipolarophiles, (acridin-9-yl)-CH=CH-R (R = COOCH3 or Ph), reacted with three BNOs (2,4,6-trimethoxy, 2,4,6-trimethyl, and 2,6-dichloro) to give a mixture of two target isoxazoline regioisomers in which the acridine was bound either to isoxazoline C-4 carbon (4-Acr) or C-5 one (5-Acr). Methyl 3-(acridin-9-yl)propenoate afforded major 4-(acridin-9-yl)-isoxazoline-5-carboxylates (4-Acr) and minor 5-(acridin-9-yl)-4-carboxylates (5-Acr). 9-(2-Styryl)acridine regiospecifically afforded only 4-Acr cycloadducts. The ratios of regioisomers were compared with analogous reactions of acridin-4-yl dipolarophiles. Regioselectivity was dependent on a polarity of the CH=CH bond, donor effects in BNO, and stabilization by stacking of aromatic substituents in the products.

Theoretical Study of Chemo-, Regio-, and Stereoselectivity in 1,3-Dipolar Cycloadditions of Nitrones and Nitrile Oxides to Free and Pt-Bound Bifunctional Dipolarophiles

1,3-Dipolar cycloadditions of nitrones RCH=N(CH3)O and the nitrile oxide CH3CNO to the bifunctional cyanoalkynes NC-CCR' and cyanoalkenes E-NCCH=CHR' (R=H, Ph; R'=H, Ph)--both free and ligated to PtII and PtIV--were investigated by theoretical methods at B3LYP and, for some reactions, CCSD(T) and CBS-Q levels of theory. Chemo-, regio-, and stereoselectivity of the processes and factors, which affect the reactivity and selectivity, were analyzed, and verified trends are discussed in details. Coordination of dipolarophiles to PtII and, particularly, to PtIV facilitates the CN relative to the CC additions of nitrones due to higher activation of the CN group in comparison to the CC group. The bonding of the ligands to platinum also favors the meta versus ortho pathways and endo versus exo pathways that sometimes lead to a switch of the reaction direction. Introduction of Ph groups into the reactant(s) molecules also leads to the promotion of the CN versus CC routes, and this effect is especially strong when both reactants are Ph-substituted. The substituent effect is accounted for by steric repulsions imposed by the Ph groups in transition states (TSs) and by the loss of a conjugation in phenylnitrone and phenylcyanoalkene molecules upon the TS formation. Solvation inhibits the CN and meta-CC additions and, hence, generally favors the CC versus CN pathway, the ortho versus meta pathway, and the exo versus endo pathway. All reactions except one proceed concertedly via a nearly synchronous mechanism for the CN and meta-CC additions to free ligands and asynchronous mechanism for the other processes. For the reaction CH2=N(CH3)O + PtIV-1, a stepwise route is realized.