Abstract:We report a density functional study (B97-D2 level) of mechanism (s)

The vicarious nucleophilic substitution of potassium carbanions of the (chloromethyl)pyridines 1a and 1b, (chloromethyl)benzothiazole 1c, (chloromethyl)thiazole 1d, (chloroethyl)thiazole 1e and (chloroethyl)benzothiazole 1f with nitroarenes, leading to nitrobenzyl heterocycles 2 and 4−14 has been studied. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The vicarious nucleophilic substitution reaction of dichloromethyloxazoline 2 with nitrobenzene has been investigated. Treatment of 2 with t-BuOK followed by the addition of nitrobenzene leads to benzylic carbanions 4 or 9 depending upon the solvent used (DMSO, DMF, or THF). Subsequent treatment of 4 or 9 with aldehydes, in a Darzens-like reaction, furnishes very good yields of nitrophenyl oxazolinyloxiranes 8 and 11. 1,2-Dioxazolinyl-1,2-dinitrophenylethene 7 forms quantitatively when carbanion 4 is allowed to warm to room temperature in the absence of external electrophiles.

Using density functional theory calculations (at the B97‐D2//BP86 level) and measurements of kinetic isotope effects, we explored the mechanism of [RuH2(PPh3)3(CO)] (22) in catalytic acceptor‐less dehydrogenation of methanol to formaldehyde. 22 is found to exhibit a similar activity as the previously studied [RuH2(H2)(PPh3)3] (1 b) complex. On the computed pathway, η2→η1 slippage of Ru‐bound formaldehyde prior to decoordination is indicated to be rate‐limiting, consistent with the low kH/kD KIE of 1.3 measured for this reaction. We also explored computationally the possibility of achieving complete dehydrogenation of methanol (into CO2 and H2), through subsequent decarbonylation of formaldehyde and water‐gas shift reaction of the resulting carbonyl complex. Complete pathways of this kind are traced for 22 and for [RuH2(PPh3)2(CO)2]. An alternative mechanism, involving a gem‐diol intermediate (obtained upon attack of OH− to coordinated formaldehyde), has also been investigated. All these pathways turned out to be unfavourable kinetically, in keeping with the lack of CO2 evolution experimentally observed in this system. Our calculations show that the reactions are hampered by the low electrophilicities of the CO and HCHO ligands, making OH− uptake unfavourable. Consequently, the subsequent intermediates are too high‐lying on the reaction profiles, thus leading to high kinetic barriers and preventing full dehydrogenation of methanol to occur by this kind of mechanism.

2‐Chloroalkyloxazolines 1a and 1b react with nitroarenes in the presence of tBuOK in DMSO to give, upon acidic quenching, substituted nitroaryloxazolines 2a, 2b, 3a, 9, 10, 11a−d, 12−19, and 21−22, probably by vicarious nucleophilic substitution (VNS). The reaction of 1a is poorly regioselective, while that of 1b is completely para regioselective, thus showing that the coupling is sterically controlled. Trapping of the carbanionic intermediate B of the VNS reaction with electrophiles furnished α,α‐disubstituted (nitrobenzyl)oxazolines 4−7. Attempts to trap B with PhCHO failed: the [(p‐nitrophenyl)oxazolinyl]ethanol 8 was obtained. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)