4- Bromobenzoic anhydride

A. Gas-Solid Reactions in Polar Crystals Ammonia-p-Bromobenzoic Anhydride... 

Figure 37. (a) Packing arrangement of p-bromobenzoic anhydride, viewed along the c axis. (b) Crystals of p-bromobenzoic anhydride after amisotropic reaction with ammonia. (Reproduced by kind permission of I. C. Paul and D. Y. Curtin.)... 

Recently, Curtin and Paul (114) have used the pyroelectric effect for the direct assignment of the absolute structure of crystals of p-bromobenzoic anhydride. 

The presence of a polar axis confers anisotropic activity to organic crystals (Curtin Paul, 1981 Desiraju, 1984b). Common polar (noncentrosymmetric) space groups adopted by organic crystals areP2i2i2i and P2i- While chiral crystals must have polar directions, polar crystals need not be chiral. Anisotropic reactivity is seen for instance in the reaction of ammonia with p-bromobenzoic anhydride, which crystallizes with a polar axis the polar axis directs the reaction, p-bromobenzoic anhydride is chiral as well as polar. Chirality of the crystalline anhydride has been exploited to resolve a racemic gaseous amine the chiral crystal preferentially reacts with one of the enantiomers of the amine. Thus when p-bromobenzoic anhydride crystals are exposed to vapours of racemic phenylethylamine, the resulting amide contains one of the enantiomers in excess. 

The reactions with ammonia can also be extended to benzoic acid anhydrides(VIII) in this case the products are a mixture of the ammonium salt (VI) and the benzamide (IX) [6, 9]. Arguments similar to those above can be used to explain the observed reaction anisotropy. Of particular interest, however, is the case of p-bromobenzoic anhydride ((VIII), X=Br). p-Bromobenzoic anhydride crystallizes in the space group... 

Fig. 8. The packing of / -bromobenzoic anhydride (VIII) in the crystal. It can be seen that the carbonyl groups of the anhydride molecules all point in the same direction along the polar axis.

Fig. 9. A crystal of /i-bromobenzoic anhydride reacting with ammonia gas. The polar axis runs vertically in the plane of the paper. There is clear differentiation of the progress of the reaction in the two directions of the polar axis. Times are top, from left to right 0 hr, 4 hr, 7 hr bottom, from left to right 13 hr, 25 hr. (From Science 187 (1975), in which this figure appeared as Figure 9 on page 25.

The reactions of amidines or guanidines with a-functionalized carbonyl compounds continue to be utilized for the synthesis of imidazoles. Thus, the mixed anhydride of acetic and chloroacetic acids reacts with symmetrical diarylguanidines to give l-aryl-2-arylaminoimidazolin-4-ones, and there is competitive formation of imidazoles and pyrimidines in the reaction of benzamidine with 3-bromobenzo-4-pyrones (18). Imidazoles are minor products, but are favored in nonpolar solvents. The use of a-dicarbonyl compounds with guanidine gives 2-amino-4-hydroxy-4-methyl-4//-imid-azoles, which give excellent yields of 2-aminoimidazoles on catalytic hydrogenation. " ... 

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