Synthesis and Reactivity of [closo-1-CB₉H₉-1-N₂]: Functional Group Interconversion at the Carbon Vertex of the {closo-1-CB₉} Cluster

Abstract

The dinitrogen derivative [closo-1-CB(9)H(9)-1-N(2)] (1) was prepared from amine [closo-1-CB(9)H(9)-1-NH(3)] (2) and reacted with three types of nucleophiles: activated arenes (phenolate and aniline), divalent sulfur compounds (Me(2)S and Me(2)NCHS), and pyridine, giving products of substitution at C(cage). The reaction of 1 with pyridine gave all four isomers 11a-11d, indicating the Gomberg-Bachmann mechanism, which involves radical anion [closo-1-CB(9)H(9)](-*) (26). The radical and also closed-shell electrophilic aromatic substitution mechanisms were probed with the aid of DFT and MP2 computational methods and compared to those of phenylation of pyridine. Overall, experimental results supported by computational analysis suggest two mechanisms for the substitution of the N(2) group in 1: (i) thermal heterolytic cleavage of the C(cage)-N bond and the formation of electrophilic carbonium ylide [closo-1-CB(9)H(9)] (19) and (ii) electron-transfer-induced homolytic cleavage of the C(cage)-N bond and the formation of 26. Decomposition of 1 in MeCN is believed to proceed by the nonradical mechanism involving formation of the ylide 19 as the rate-determining step with experimental activation parameters DeltaH(double dagger) = 38.4 +/- 0.8 kcal mol(-1) and DeltaS(double dagger) = 44.5 +/- 2.5 cal mol(-1) K(-1). The electron-transfer-induced formation of 26 is consistent with the relatively high reduction potential of 1 (E(pc) = -0.54 V), which is more cathodic than that of PhN(2)(+) by 0.38 V. Transformations of the phenol 8a and the Me(2)NCHS adduct 10 were demonstrated by O-methylation of the former and hydrolysis of 10 followed by S-alkylative cyclization. Direct products and their derivatives were investigated by UV-vis spectroscopy and analyzed with the ZINDO computational method.

Synthesis and Characterization of 12-Pyridinium Derivatives of the [closo-1-CB11H12]− Anion

“…Zwitterionic derivatives I and II of monocarbaborates [closo-1-CB 9 H 10 ] − (A) and [closo-1-CB 11 H 12 ] − (B, Figure 1) possess large longitudinal dipole moments and for this reason are of interest as core elements of polar liquid crystals 1 suitable for electro-optical applications. 2 In this context, we initially investigated 1-sulfonium derivatives of type IA, 3,4 and later we developed a convenient method for access to 1-pyridinium zwitterions of anions A and B.…”

“…Functionalization of the former anion and preparation of zwitterionic materials is conveniently achieved through dinitrogen derivatives as the key intermediates. 11−13 Unfortunately, analogous dinitrogen derivatives of the [closo-1-CB 11 H 12 ] − anion (B) are generally much less stable, and their isolation appears impractical. This limitation significantly reduces access to zwitterionic derivatives of the [closo-1-CB 11 H 12 ] − anion (B).…”

“…11−13 Unfortunately, analogous dinitrogen derivatives of the [closo-1-CB 11 H 12 ] − anion (B) are generally much less stable, and their isolation appears impractical. This limitation significantly reduces access to zwitterionic derivatives of the [closo-1-CB 11 H 12 ] − anion (B). Nonetheless, we recently demonstrated that even unstable dinitrogen derivatives of monocarbaborates, such as [closo-1-CB 9 H 8 -1-COOH-6-N 2 ], 14 can be synthetically useful, when formed in situ as transient species during diazotization of the corresponding amines.…”

“…Here we report a simple and efficient method for the preparation of [closo-1-CB 11 H 11 -12-(4-C 7 H 15 OC 5 H 4 N)] (1b) and extend it to the preparation of liquid crystalline [closo-1-CB 11 H 10 -1-C 5 H 11 -12-(4-C 7 H 15 OC 5 H 4 N)] (1c) as a representative of a potentially diverse class of 12-(4-alkoxypyridinium) zwitterionic derivatives of [closo-1-CB 11 H 12 ] − anion (B). The synthetic work is accompanied by mechanistic consideration augmented with density functional theory (DFT) computational results.…”

“…Finally, for comparison purposes, 4-methoxypyridinium derivative 3b was conveniently obtained by reacting 4-methoxypyrylium triflate (11) with amine [closo-1-CB 11 H 11 -1-NH 2 ] − [NMe 4 ] + (12[NMe 4 ]) in 16% yield, according to a previously reported method (Scheme 4). 5 During recrystallization of 3b from EtOH/H 2 O, partial exchange of the methyl group for ethyl was observed.…”