One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions

Abstract

The simple and efficient method was developed for the one-pot synthesis of B-substituted aryl derivatives of ortho-carborane with functional groups sensitive to organolithium and organomagnesium reagents using 9-iodo-ortho-carborane and generated in situ organozinc compounds. The method proposed was used to prepare a series of 9-aryl-ortho-carboranes, including those containing nitrile and ester groups, 9-RC6H4-1,2-C2B10H11 (R = p-Me, p-NMe2, p-OCH2OMe, o-OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, and p-COOEt). It was demonstrated that the same approach can be used for synthesis of diaryl derivatives of ortho-carborane 9,12-(RC6H4)2-1,2-C2B10H10 (R = H, p-Me). The solid-state structures of 9-RC6H4-1,2-C2B10H11 (R = p-NMe2, p-OCH2OMe, o-OMe, o-CN, p-CN, m-COOEt, and p-COOEt) and 9,12-(p-MeC6H4)2-1,2-C2B10H10 were determined by single crystal X-ray diffraction.

1,12-Diiodo-Ortho-Carborane: A Classic Textbook Example of the Dihalogen Bond

“…Despite the fact that the syntheses of the C-iodo derivatives of ortho-carborane were first reported more than 50 years ago [43,44], they were on the periphery of mainstream carborane chemistry developments, and were not even well characterized [34,45]. In this way, they radically differ from the B-iodo derivatives of ortho-carborane, which have found active use in the synthesis of B-alkyl and aryl derivatives by means of Pd-catalyzed cross-coupling reactions [46][47][48][49][50][51][52]. Therefore, the synthesis of the C-iodo derivatives of ortho-carborane is not an easy task.…”

Synthesis of 3-Aryl-ortho-carboranes with Sensitive Functional Groups

“…The organozinc compounds that are obtained in this way can be easily coupled with various aryl iodides in the presence of a catalytic amount of (Ph 3 P) 2 PdCl 2 [ 63 ]. Previously, we had successfully used this approach for the synthesis of 9-aryl- ortho -carboranes containing functional groups that were sensitive to organolithium and organomagnesium reagents [ 60 ].…”

“…The 1 H and 13 C NMR spectra of all compounds contain signals of the corresponding aryl substituents as well as the signals of the carborane cage. It should be noted that the signals of the aromatic hydrogens in the 1 H NMR spectra of 3-aryl- ortho -carboranes in CDCl 3 are noticeably shifted to the downfield region in comparison with the signals of the corresponding 9-aryl- ortho -carboranes [ 60 ], with a difference in the weighted average chemical shift of aromatic hydrogens in the range of 0.11 to 0.34 ppm. This is in agreement with the transition from the markedly electron-donating ortho -carboran-9-yl group to the slightly electron-accepting ortho -carboran-3-yl group [ 64 ].…”

“…On the other hand, the signals of the carbonyl group in the 13 C NMR spectra are also sensitive to the formation of hydrogen bonds [ 67 , 68 ]. Indeed, the signal of the carbonyl group in 3-(2′-EtOOCC 6 H 4 )-1,2-C 2 B 10 H 11 ( 8 ) (170.0 ppm) demonstrates a significant downfield shift compared to those found in 3-(3′-EtOOCC 6 H 4 )-1,2-C 2 B 10 H 11 ( 9 ) (166.5 ppm) and 3-(4′-EtOOCC 6 H 4 )-1,2-C 2 B 10 H 11 ( 10 ) (166.4 ppm), as well as in 9-(2′-EtOOCC 6 H 4 )-1,2-C 2 B 10 H 11 (161.5 ppm) [ 60 ], which is a clear confirmation of the formation of an intramolecular hydrogen bond. It is also worth noting that the mass spectrum of 8 in the negative mode, in contrast to the mass spectra of other 3-aryl- ortho -carboranes, exhibits a peak which, in addition to the loss of the carborane proton, corresponds to the abstraction of the ethanol molecule.…”

“…A series of 3-aryl derivatives of ortho -carborane were prepared by Pd-catalyzed B-H activation reactions with aryl iodides under functional group assistance; however, these reactions are currently only of academic interest rather than real synthetic methods [ 58 , 59 ]. Recently, we proposed a convenient and mild one-pot method for the synthesis of 9-aryl- and 9,12-diaryl- ortho -carboranes with sensitive functional groups, including esters and nitriles, using sequential Co- and Pd-catalyzed reactions [ 60 ].…”