Dipolar transition state reactions

Dipolar activated complexes differ considerably in charge separation or charge distribution from the initial reactants. Dipolar transition-state reactions with large solvent effects can be found amongst ionization, displacement, elimination, and fragmentation reactions such as ... 

Solvent Effects on Dipolar Transition State Reactions... 

For the mechanistically more simple transfer hydrogenation process with 2 in particular, the charge-separated nature of this transition state persuasively explains the experimentally observed increase in the initial reaction rate and the average activity as a function of reaction medium polarity by Xiao and Liu as well as Tanis. Indeed such an observation is characteristic for so-called dipolar transition state reactions,where activated complexes differ considerably in charge separation or charge distribution from the initial reactants, contrary to pericyclic reactions in... 

Reactions that take place through dipolar transition states Menschutkin reaction (10-44), electrophilic aromatic substitution. 

These reactions are among the most propitious for revealing specific microwave effects, because the polarity is evidently increased during the course of the reaction from a neutral ground state to a dipolar transition state. 

This reaction is well known but, unfortunately, using classical procedures is only possible under very harsh conditions (temperature 240 °C, sealed containers, long reaction times) and gives modest yields [70] (30%). Its difficulty constitutes a good challenge to check the effectiveness of microwave irradiation, because the mechanism involves a dipolar transition state [71] (Eq. (18) and Tab. 3.8) and this should also favor the involvement of a microwave effect. 

This study is a distinctive example of a pronounced microwave effect for a reaction occurring with a very late dipolar transition state. 

Ionic reactions of neutral substrates can show large solvent dependence, due to the differential solvent stabilization of the ionic intermediates and their associated dipolar transition states (Reichardt, 1988). This is the case for the electrophilic addition of bromine to alkenes (Ruasse, 1990, 1992 Ruasse et al., 1991) and the bromination of phenol (Tee and Bennett, 1988a), both of which have Grunwald-Winstein m values approximately equal to 1 so that the reactions are very much slower in media less polar than water. Such processes, therefore, would be expected to be retarded or even inhibited by CDs for two reasons (a) the formation of complexes with the CD lowers the free concentrations of the reactants and (b) slower reaction within the microenvironment of the less polar CD cavity (if it were sterically possible). 

Calculations at the MP2(Full)/6-31++G(d,p)//MP2(Full)/6-31+G(d) level of theory were used to investigate the SN reactions between ammonia and aziridine, aze-tidine, methylethylamine, and four fluorinated derivatives of aziridine.52 The results show that aziridine and azetidine have strain energies of 27.3 and 25.2 kcalmol-1, respectively, and that as a consequence they react 7.76 x 1023 and 2.30 x 1017 times faster with ammonia than does the methylene group of methylethylamine. However, even after subtracting the effect due to the release of ring strain, aziridine still reacts much faster than the other two substrates. This is because the electrostatic attraction of the charges in the product-like dipolar transition state are much greater for aziridine. 

High pressure is generally effective in accelerating those reactions that involve either an ionization process or a dipolar transition state [9]. 

These reactions proceed by dipolar transition states, which enables the electrophilic carbon in the heterocumulene to directly attack the X ligand of the M—X bond. The reactions are therefore facilitated if X is a good donor for example for a series of complexes Cp (NO)(R)W—X (X = amide, alkoxide or alkyl), the reactivity of isocyanates was found to decrease in the order W—N > W—O > W—C.210... 

Organic reactions can be loosely grouped into three classes depending on the character of the activated complex through which these reactions can proceed dipolar, isopolar, and free-radical transition-state reactions [15, 468]. 

For example, the rate of the Diels-Alder cycloaddition reaction between 9-(hydroxymethyl)anthracene and A-ethylmaleimide, as shown in Eq. (5-159), is only slightly altered on changing the solvent from dipolar acetonitrile to nonpolar isooctane, as expected for an isopolar transition state reaction cf. Section 5.3.3. In water, however. 

Not only Diels-Alder cycloadditions but also 1,3-dipolar cycloaddition reactions can be subject to hydrophobic rate enhancements. For example, the reaction of C,N-diphenylnitrone with di-n-butyl fumarate at 65 °C to yield an isoxazolidine is about 126 times faster in water than in ethanol, while in nonaqueous solvents there is a small 10-fold rate decrease on going from n-hexane to ethanol as solvent - in agreement with an isopolar transition-state reaction [cf. Eq. (5-44) in Section 5.3.3] [858]. Because water and ethanol have comparable polarities, the rate increase in water cannot be due to a change in solvent polarity. During the activation process, the unfavourable water contacts with the two apolar reactants are reduced, resulting in the observed rate enhancement in aqueous media. Upon addition of LiCl, NaCl, and KCl (5 m) to the aqueous reaction mixture the reaction rate increases further, whereas addition of urea (2 m) leads to a rate decrease, as expected for the structure-making and structure-breaking effects of these additives on water [858]. 

Reactions that take place through dipolar transition states. 

The reaction of diene 26 with vinyldiazomethane 24 yields predominantly the formal [4 + 1] annulation product 33864. As the cyclopropanation is considered to proceed in a nonsyn-chronous mode through a dipolar transition state 31, this could allow for the formation of dipolar intermediates if the charges are stabilized by suitable substituents, resulting in the formation of 33. This side reaction is completely suppressed by using rhodium(II) pivaloate in pentane as the solvent. 

Ene reactions. Intramolecular ene reaction of allenes with a carbonyl group or aldimine as the enophile is greatly facilitated by the presence of a silyl substituent on the allene moiety, reflecting a dipolar transition state. Tin(IV) chloride lowers the reaction temperature to 0° to 25°. 

Microwave-assisted solvent-free reactions have been used by Jenekhe [146] to synthesize quinoline derivatives. An important specific nonthermal microwave effect has been observed compared with conventional heating (Eq. 60). This MW effect is consistent with mechanistic considerations, because the rate-determining step is the internal cyclization depicted in Eq. (60) resulting from nucleophilic attack of the enamine on the carbonyl moiety occurring via a dipolar transition state. 

Ionic Liquid Effects on Reactions Proceeding through Dipolar Transition States... 

Hughes and Ingold, primarily studying substitution and elimination reactions, have extensively investigated the effect of molecular solvents on reactions passing through dipolar-transition states [2]. On the basis of a simple qualitative solvation model, which exclusively considers electrostatic interactions between ions (or dipolar molecules) and solvent molecules in initial and transition states it is generally... 

---