A reliable method of synthesis of 3-bromo-N-methylpyrrole by the use of N-bromosuccinimide and a catalytic amount of PBr 3 is reported. The method described opens a facile route to a variety of 3-substituted pyrroles, which are otherwise difficult to access. In addition, conditions under which pure 2-bromo-Nmethylpyrrole can be obtained have been established.In the course of our studies on spin-spin coupling constants in small ring heterocycles 1-3 we became interested in the synthesis of 2-and 3-derivatives of N-methylpyrrole as model compounds. 2-Substituted compounds are easily accessible by the use of either 2-bromopyrrole or 2-lithiopyrrole as starting materials. The latter can be prepared directly from N-methylpyrrole. 4,5 The synthesis of 3-bromosubstituted pyrroles is more difficult and special strategies have been devised in order to obtain such compounds. Among other methods, an application of sterically demanding nitrogen substituents, such as tert-butyl-, 6-8 trityl-9 or tri-iso-propylsilyl moieties, 10 has been proposed to hinder the attack of electrophilic reagents at position 2. Another approach consists of acidmediated isomerisation of the corresponding 2-isomers. The main drawback of the latter lies in the fact that usually a mixture of variously mono-and disubstituted isomers is produced. 11
An excellent linear correlation is found between a large set of experimental spin-spin carbon-carbon coupling constants, J(CC), in thiophene, pyrrole, and furan systems and the corresponding B3PW91/6-311++G(2d,p)//B3PW91/ 6-311++G(2d,p) calculated estimates. The correlation does not differ significantly from the simplest relationship possible, J(CC) exp. = J(CC) calcd. , within a small and random spread of about 1 Hz. There are 285 experimental values considered, and 202 out of these are new and come from the present work. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s, and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in pyridine and benzene ring systems. As has been shown by computations, not only large one-bond couplings but also almost all long-range ones occurring between the carbons of the heteroaromatic rings are, with a few exceptions, positive. Significant substituent effects experimentally observed in the one-bond as well as long-range couplings are very accurately reproduced by the computation. The experimental coupling magnitudes vary from ca. 1 to 98 Hz. The J(CC)'s computed for the model variously substituted trimethylsilyl and fluoro derivatives, which are not easily accessible experimentally, span a range of about 130 Hz, from ca. À2 in up to ca. +125 Hz .
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