Published: March 16, 2011 r2011 American Chemical Society 3562 dx.doi.org/10.1021/jo2000463 | J. Org. Chem. 2011, 76, 3562–3565 NOTE pubs.acs.org/joc Carboranylpyrroles: A Synthetic Investigation Rashmirekha Satapathy, † Barada Prasanna Dash, † Chong Zheng, † John A. Maguire, ‡ and Narayan S. Hosmane* ,† † Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, United States ‡ Department of Chemistry, Southern Methodist University, Dallas, Texas 75275-0314, United States b S Supporting Information P yrroles are a class of nitrogen-containing heterocyclic com- pounds found in many natural products and biologically active compounds. 1 These are the key starting materials for the synthesis of a variety of macrocycles. 2 Moreover, pyrroles are useful building blocks in materials science and find applications as conducting polymers, 3 semiconductors, 4 and synthetic dyes. 5 The carborane-appended pyrroles, commonly known as the carboranylpyrroles, are important building blocks for the synth- esis of carborane-appended macrocycles and polypyrroles. 6 Carborane-appended porphyrins are useful borondrug delivery platforms for cancer treatment via boron neutron capture therapy. 7 Carboranylpyrroles have also been used for the synth- esis of functionalized conducting polymers with improved ther- mal and electrochemical properties. 8 Carboranylpyrroles are normally synthesized via multistep procedures starting from 1-allyl-2-methyl-o-carborane and 1-vi- nyl-o-carborane. 9a Bromo derivatives of pyrroles have also been used to synthesize carboranylpyrroles via the Suzuki coupling reaction, which is a single-step process. However, this procedure involves the synthesis of carboranylboronic acid, but the overall yield of the product became low. 9b All these involved only the o- carborane and its attachment at the C 3 and C 4 positions of the pyrrole ring. While the overall yields of the multistep reactions were moderate, 8ce,9 these results posed the following intriguing questions: (1) whether the carborane clusters can be linked to the unexplored C 2 position of the pyrrole ring; (2) could this reaction incorporate other icosahedral carborane cage isomers, such as meta- and para- substituted ones; (3) if these reactions go smoothly, could this methodology be made simpler and also in higher yields? These questions led us to explore the synthesis of carboranylpyrroles and the results of this investigation are reported here. Thus, we have undertaken a focused research in which the reactivity of all three aldehydes of icosahedral carboranes (o-, m- and p-carboranes) with pyrrole was explored under two different catalytic conditions, i.e., trifluoroacetic acid (TFA) and InCl 3 . In each case, the attachment of carborane clusters to the pyrrole has occurred at the C 2 position of the ring and the high yields of the so formed carboranylpyrroles were observed. The starting mono- and dialdehyde derivatives of carboranes were synthesized following a literature procedure. 10 This process involved the reactions between lithiated carboranes and excess methyl formate, followed by acidification. 10 On the other hand, the reaction between dilithiated o-carboranes with methyl for- mate leads to the formation of an undesired cyclic ether. 10a Consequently, o-carborane dialdehyde could not be produced. Treatment of carborane aldehydes with pyrroles in the presence of catalytic amounts of trifluoroacetic acid (TFA) or indium chloride (InCl 3 ) at 0 °C led to a series of carboranylpyrroles in good yields. However, unidentifiable mixtures of polymeric products were formed at elevated temperatures. Evidently, the TFA and InCl 3 -catalyzed reactions of mono aldehydes of all three types of icosahedral carboranes 1, 4, 7, and 10 with pyrrole and 1-methylpyrrole led to the formation of carboranyl monopyrro derivatives (Scheme 1). The yields obtained at both of the reaction conditions are shown in Table 1. In the cases of monopyrro derivatives, the yields obtained via InCl 3 were slightly higher. But a reverse trend was observed for the dipyrro derivatives. However, yields obtained from 1-methylpyrrole were always lower than that of pyrrole. Similarly, reactions of carborane dialdehydes of m- and p-carboranes 13 and 16 with pyrrole and 1-methylpyrrole in the presence of an acid catalyst produced a new class of carboranyl Received: February 3, 2011 ABSTRACT: The unusual reactivity of carborane mono- and dialdehydes with pyrroles in the presence of acid catalysts leads to the formation of a new class of carboranyl mono- and dipyrro derivatives.