Polar solvents uncatalyzed

While the mechanism of the ammonium salt catalyzed alkylation is unclear, in polar solvents the enantioselectivity of the addition of dialkylzincs to aldehydes generally drops considerably, probably due to uncatalyzed product formation or complexation of the zinc reagent with the polar solvent rather than with the chiral auxiliary. 

The effects of solvent on the thermal chemiluminescent decomposition of 1 described above are summarized as follows. First, SPD proceeded in an aprotic polar solvent to afford bright light with AniaxSPD = 493-498 nm similar to BID by a CT-induced decomposition mechanism. Second, 1 underwent uncatalyzed TD to give excited 5 which emitted yellow light with A ,axTD = 536 nm due to ESIPT in nonpolarp-xylene. 

Uncatalyzed amidations of acids have been realized under solvent-free conditions and with a very important microwave effect [67 a]. The best results were obtained by use of a slight excess of either amine or acid (1.5 equiv.). The reaction involves thermolysis of the previously formed ammonium salt (acid-base equilibrium) and is promoted by nucleophilic attack of the amine on the carbonyl moiety of the acid and removal of water at high temperature. The large difference in yields (MW > A) might be a consequence of interaction of the polar TS with the electric field (Eq. (15 a) and Tab. 3.6). 

Speculation exists in the literature that many IMDA reactions of simple l,7,9-decatrien-3-ones, which are often generated by oxidation reactions in acidic media or are liberated in protic solvents, may be protic or Lewis acid catalyzed owing to the mildness of the cyclization conditions and the excellent stereoselectivity of their cyclizations (refs. 5h, 17, 18). The study by Gajewski cited in ref. 17c, however, establishes that the IMDA cyclization of (11) is an uncatalyzed, solvent polarity independent reaction. 

KSA react with aldehydes smoothly in the presence of a catalytic amount of a phosphine [79] or a lithium amide (Scheme 10.24) [80]. In addition, uncatalyzed aldol reactions of KSA proceed efficiently in polar aprotic solvents such as DMSO, DMF, DME [81], and MeCN [82]. Lubineau et al. reported an uncatalyzed aldol reaction of SEE in H2O in 1986 [83]. Recently, Loh et al. have found a similar reaction of KSA although KSA are much more sensitive to hydrolysis than SEE [84]. 

The reaction pathway can lead either to the expected Diels-Alder cycloadducts A or the monoadduct B or bisadduct C resulting from a Michael-type addition (Scheme 10.22), In the case of catalysis, with the exception of LPDE and Znl2, the acidic character of Yb(OTf)3 or BiCh diverts the reaction along both pathways or favors the exlusive formation of Michael-type products. Such chemical behavior is not uncommon in catalyzed furan reactions [106]. At variance with this is the uncatalyzed high pressure cycloaddition and the reaction carried out in solvophobic media at atmospheric pressure which are particularly selective and afford the Diels-Alder cycloadduct A in nearly similar yields. Interestingly, the reaction also proceeds chemoselectively in water-like solvents at ambient pressure but not in hydrocarbon solvents and methanol. In water-like solvents the reactivity cannot be ascribed to polarity effects only, since methanol and glycol have similar values. Solvophobic interactions are very probably mostly responsible for the enhanced reactivity. This is supported by the similar values of the endoiexo ratio. 

The p constant in Table I is always very high in comparison to values found with reactions in solution. Also, because the uncatalyzed reaction shows such a steep dependence, we may assume that this phenomenon is not connected with the presence of the solid catalyst but very probably with the highly polar nature of the reaction and with the absence of any solvent. In general, the sign and values of the p constant in Table I support the assumed polar character of the olefinforming eliminations (cf., for example, Maccoll, 19 Rase and Kirk, 24). 

The reaction is rather complex it follows uncatalyzed and/or one or two catalyzed pathways. Furthermore the reaction order with respect to amine varies with solvent. For instance in the reaction of 4 -nitrophenyl 3-nitro cinnamate at 20 °C the amine has the order 1 in acetonitrile, 1-1-2 in 1,2-dichloroethane, 2-1-3 in benzene and 1H-2-1- 3 in 1-methylnaphthalene. In general the number of amine molecules required for the reaction increases with decreasing solvent polarity. The Hammett q constant diminishes in the same trend in polar medium q is high, in less polar but complexing (TT-donor) solvent it is lower. 

Like the [1,5] shifts we looked at first, the Cope rearrangement is intramolecular, uncatalyzed, and shows no strong dependence on solvent polarity. The activation energy for the Cope rearrangement is about 34 kcal/mol, a value far below what one would estimate for the simple bond breaking into two separated allyl... 

The Ugi reaction is exothermic in nature and usually completed within minutes of adding the isocyanide. High concentration (0.5 M-2.0 M) of reactants gave the highest yields. Polar aprotic solvents, like DMF work well however, methanol and ethanol have also been used successfully. This uncatalyzed reaction has an inherent high atom economy as only a molecule of water is lost and chemical yield are high in general. 

The uncatalyzed heterolytic cleavage of the H-H bond (H2->H + H ) is also difficult. The strength of the H-H bond and its lack of polarity contribute to the poor kinetic and thermodynamic acidity of H2. The pfC values for a series of mono-protic acids , dissolved in either tetrahydrofuran (THF) or acetonitrile solvent, are given in Table 4.2. Dihydrogen with an estimated pfCa of 49 in THF solvent is among the weakest acids. 

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