Dipolar transition states

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. 

The silyl moiety was deemed essential terminally unsubstituted allenes and allenes bearing a methyl substituent on the terminus failed to cyclize under either thermal or Lewis-acidic conditions. Weinreb explains this observation by proposing dipolar transition state 134 (Scheme 27), rationalizing that the /5-silyl group provides necessary stabilization for the partial positive charge developing on the central carbon of the allene. 

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). 

Comparison of C=C barriers in Table 10 with those in Tables 3 and 4 shows that the cyclopentadiene ring has a moderate capacity for stabilizing a dipolar transition state, being slightly less efficient than two COzR groups or one CN and one C02R group. 

The action of added ion-pairs may also be visualized as the establishment of an ion-pair atmosphere about a dipolar transition state. Simple thermodynamic treatment predicts linearity of log k3 in cs, but it has been shown that contributions of ion-pair-ion-pair repulsion, higher aggregation, and the effect of salt on the dielectric constant introduce curvature in the log k3-cs plot that is in the direction of a k3-cs dependence. 

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]. 

Dye-sensitized photo-oxygenation of the norbornenes (189 X = Me) and (190 X = H), which have been synthesized cleanly from the acid (191), occurs in a one-step cyclic process via a dipolar transition state with no evidence for a perepoxide intermediate (189 X = Me) gives the alcohol (190 X = OH) from the corresponding allylically rearranged hydroperoxide along with some exo-alcohol and the ketone... 

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... 

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... 

The rate enhancement observed with polar solvents corresponds well with a dipolar transition state in the Claisen rearrangement see Section 7.2.2.12. 

Reactions that take place through dipolar transition states. 

Equation (13) has been applied to rate data (Evans and Parker, 1966 Evans and Parker, unpublished work) for the 8 2 decomposition of trimethylsulphonium bromide (14). The dipolar transition state has °y( rCHsSMe,)+ = 35 7 y CHs8Mea)+ = 0 73 and ymrOHsSMej)+ = I O at 25°C in the solvents ethanol (E), dimethylaoetamide and nitromethane (N) respectively, relative to dimethylformamide. In other words, the polar transition state for (14) is more solvated by ca. 2 kcal mole in dipolar aprotic solvents than it is in the protic solvent ethanol. The value of the activity coefficient of the polar transition state, °yBrOH8SMe2t = 35-7, is Comparable with those of polar molecules in protic solvents, relative to DMF (cf. Table 3). 

WL Adam, A. K. Smerz, Solvent effects in the regio- aird diastereoselective epoxidations of acyclic allylic alcohols by dimethyldioxirane Hydrogen bonding as evidence for a dipolar transition state, J. Org. Chem. 61 (1996) 3506. 

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