Electronic effects, dimethylsulfoxide

The results (electrostatic-fit charges based on Hartree-Fock 6-3IG wavefunctions) are ambiguous. Relative to dimethylsulfide as a normal-valent standard , the sulfur in oxygen loses about half an electron, and the sulfur in dimethylsulfone loses 1.7 electrons. This would seem to suggest that dimethylsulfoxide is halfway to being a zwitterion, but that dimethylsulfone is most of the way. Charges on sulfur in sulfur tetrafluoride and sulfur hexafluoride (relative to sulfur difluoride) show more modest effects, in particular for the latter. Overall, it appears that hypervalent molecules possess significant ionic character. 

The yields of reaction products from thermal nucleophilic substitution reactions in dimethylsulfoxide of 0- and p - n itrohalobenzenes with the sodium salt of ethyl a-cyanoac-etate were found to be markedly diminished from the addition of small amounts of strong electron acceptors (nitrobenzenes). At the same time, little or no diminution effects on the yields of the reaction products were observed from the addition of radical traps such as ni-troxyls. These results are consistent with the conclusion that such reactions proceed via a nonchain radical nucleophilic substitution mechanism (Zhang et al. 1993) (Scheme 4-20). 

The benzylic protons of r-alkylarene Cr(CO)3 complexes can be completely exchanged by treating the complexes with (CH3)sCOK in dimethylsulfoxide-de 45). The benzylic protons in uncomplexed alkylbenzenes are known to be acidic consistent with the electron-withdrawing effect mentioned above, complexation with Cr(CO)3 enhances this acidity. The reaction sequence probably consists of the following steps (20) ... 

Triton X-100 sample (Fig IB). an increase in yield was seen upon addition of DPQ. There was increase in the ratio of the size of the outer peaks (arrowed) to the inner peaks. This was shown not to be as a result of solvent effects since the addition of ethanol or dimethylsulfoxide alone produced no changes. In contrast, a decrease in the triplet yield of the 0.033% Triton X-100 sample was seen in the presence of DPQ (Fig ID). The change in the outer peaks was again more marked resulting in a lineshape change. This drop in triplet yield may be due to electron transfer to the quinone. However,no evidence of electron transfer at cryogenic temperatures was detected. 

Abstract. Nucleophilic addition of amines to olefins which are activated by electron withdrawing substituents occurs readily in aqueous dimethylsulfoxide. The reaction comprises two steps (1) nucleophilic addition to form a zwitterionic complex (2) removal of the ammonio proton of the zwitterion by a base. In most cases the first step is rate limiting but in some cases proton transfer is rate limiting. The latter situation prevails either when the reverse of the nucleophilic attack step is very rapid, as in the reaction of morpholine with benzylidenemalononitrile, or when the rate of proton transfer is depressed by a steric effect, as in the reaction of morpho-line with 1,l-dinitro-2,2-diphenylethylene. The steric effects in this latter system are among the most dramatic ones reported to date. Our data also show that the kinetic barrier to nucleophilic attack is substantially higher for nitro than for cyano activated olefins. This effect seems to be related to the well known fact that proton transfers involving nitro activated carbon acids are much slower than those of cyano activated carbon acids. 

Yb(OTf)3 was effective for asymmetric induction in hetero Diels-Alder reaction of thiabutadiene and electron-deficient olefin bearing chiral auxiliary [37]. 10 mol% of catalyst effected the reaction into giving cycloadduct in excellent Tc-facial selectivity, whereas selectivity was reversed with modest degree in the absence of the catalyst (Scheme 13.15). The reaction in which a coordinative solvent such as dimethylsulfoxide (DMSO) or dimethylformamide (DMF) displayed similar selectivity came from that of noncatalyzed reaction, suggesting well-organized coordination of substrates to Yb. 

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