Transition state carbonic anhydrides

Compare the geometry of maleic anhydride+propene, the ene transition state, to those of the reactants (maleic anhydride and propene). Is bond making and breaking occurring at once In particular, is the migrating hydrogen partially bonded to two carbons (rather than being fully bonded to one carbon ) Draw a Lewis structure to represent the transition state. Use dashed lines (.. and to represent partial bonds. 

It is reasonable to relate the values of k+fk at least qualitatively to the extents of bond making and breaking in the transition state. Bond making is all important in hydrolyses of carboxylic anhydrides, diaryl carbonates and methyl arenesulfonates. Bond breaking will be important in hydrolyses of alkyl halides and sulfonates, except for methyl derivatives, and especially so in water which can effectively solvate the leaving anion. 

Consider now what happens when the two carboxyl groups react to form a small ring, for example the anhydride. The angle between the carboxyl carbons must be reduced much further, perhaps to around 90°, in the product and in the transition state leading to it. Compared with malonic acid itself, this process has less far to go in the dimethyl compound because the two alkyl groups have already forced the carboxyls part of the way towards each other. The observed diminution in bond angle caused by the introduction of the two alkyl substituents thus specifically favours the formation of the small ring. 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

The statine-like moiety in one of the first drugs, saquinovir (23-8), comprises a transition state mimic for the cleavage of phenylalanylprolyl and tyrosylprolyl sequences. Constmction starts with the protection of the amino group of phenylalanine as its phthaloyl derivative (Phth) by reaction with phthalic anhydride this is then converted to acid chloride. The chain is then extended by one carbon using a Friedel-Crafts-like reaction. The required reagent (21-2) is prepared by reaction of the enolate obtained from the /7A-silyl ether (21-3) of glyoxylic acid and lithio... 

At high temperatures with low catalyst concentration the formation of acetanilides is favored. Maleic anhydride and acetanilides may be formed directly from the mixed anhydride by an initial attack of the nitrogen on the acetate carbonyl, but this process would involve a seven membered ring transition state. Another possible route to the formation of maleic anhydride and the acetanilides is participation by neighboring carbonyl in loosening the amide carbon-nitrogen bond to the extent that the amine can be captured by acetic anhydride as shown in path D. 

There is a second type of stereoselectivity that is characteristic of the Diels-Alder reaction. The addition of a dienophile such as maleic anhydride to a cyclic diene like 1,3-cyclopentadiene could provide two products, the endo-adduct 2 and the exoadduct 3 (Eq. 12.5). However, only the e do-cycloadduct 2, in which the two boldfaced hydrogens are syn to the one-carbon bridge, is observed experimentally, and its preferential formation follows what is now commonly termed the Alder rule. The basis for this result is believed to be stabilization of the transition state 4 by secondary orbital interactions that occur through space between the p-orbitals on the internal carbons of the diene and the carbonyl carbon atoms of the dienophile, as shown by the dashed lines in 4. Analogous stabilization is not possible in transition state 5. Structure 4 is thus characterized as the one being stabilized by maximum orbital overlap. It should be noted that not all Diels-Alder reactions are as stereoselective as the one between l,3maleic anhydride mixtures of endo-and cxo-products are sometimes obtained. 

In the all carbon and hydrogen instance, the Alder-Ene reaction is considered to be concerted and is thermally allowed based on Woodward-Hoffman rules. Thus, the Alder-Ene reaction is proposed to be a six-electron process, like the Diels-Alder reaction, having transition states endo and exo) analogous to the Diels-Alder reaction. However, the Alder-Ene reaction is easily modulated by steric effects as secondary electronic stabilizing effects have yet to be clearly identified. For example, Berson reported c/5-2-butene reacted with maleic anhydride to provide about a 4 1 ratio of endo. exo adducts 5 6, while trans-l-hvAeae provided little selectivity at 43 57 ratio of 5 6. In the reaction of maleic anhydride with tra 5-2-butene, the e.xo-TS encounters a steric interaction that the endo-TS does not. Steric effects are... 

The salts of unsaturated mono- and dicarboxylic acids have received the most attention among the MCMs of ionic type. The ionic bond in a pure state is only present in salts of alkaline and alkaline earth metals. In other compounds, especially in the case of transition metal compounds, it is complicated by an admixture of covalent bonding. The general method for their synthesis, for examples 33, comes by the interaction of salts, (hydr)oxides and (hydro)carbonates of the metals or their mixtures as well as alkyl(aryl)-derivatives with unsaturated mono- and dicarboxylic acids or their anhydrides (Eq. (4-5) (see Experiment 4-1, Section 4,6). 

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