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

Abstract

Diazotization of [closo-1-CB11H10-1-R-12-NH2](-)[NMe4](+) (4[NMe4]) in neat 4-methoxypyridine leads to 12-(4-methoxypyridinium) zwitterions [closo-1-CB11H10-1-R-12-(4-MeOC5H4N)] (2) in ∼50% yield. Demethylation of 2 with LiCl in dimethylformamide provides access to 12-pyridones 5[NMe4], which can be O-alkylated with alkyl triflates giving 12-(4-alkoxypyridinium) zwitterions, such as 1. This three-step process is more efficient than direct diazotization of amine 4[NMe4] in neat higher 4-alkoxypyridine. The new method was demonstrated for the synthesis of [closo-1-CB11H10-1-C5H11-12-(4-C7H15OC5H4N)] (1c), which exhibits a smectic A phase. Molecular and electronic structures of 4-methoxypyridinium zwitterion 2b and its C(1) isomer [closo-1-CB11H11-1-(4-MeOC5H4N)] (3b) were investigated by single-crystal X-ray diffraction and spectroscopic methods, respectively, and the experimental results were compared to those obtained with density functional theory methods. Lastly, the mechanism for formation of zwitterions 2 was investigated computationally revealing low energy for dediazoniation of the [closo-1-CB11H10-1-R-12-N2] (14) intermediate (ΔG298 ≈ 25 kcal/mol) to form boronium ylide 15, with weak dependence on substituent R. Dinitrogen derivative 14c was observed by (11)B NMR spectroscopy.

Difunctionalized {closo‐1‐CB11} Clusters: 1‐ and 2‐Amino‐12‐ethynylcarba‐closo‐dodecaborates

“…The trends observed for the ethynyl functionalized boron clusters and especially for 1 b and [12‐HCC‐ closo ‐1‐CB 11 H 11 ] − indicate that resonance‐type effects are less important in { closo ‐1‐CB 11 } clusters than inductive effects for the transfer of electronic changes compared to benzenes that exhibit an analogous substitution pattern. This finding is in accordance with an experimental and theoretical study on {1‐HO 2 C‐12‐R‐ closo ‐1‐CB 11 } derivatives that comes to the same conclusion 34. Furthermore, a theoretical study on a series of closo ‐boron clusters and benzene derivatives as well as selected other hydrocarbons with triple bonded substituents that included alkynes provided similar evidence for a smaller electronic interaction between π substituents with the twelve vertex clusters { closo ‐1,12‐C 2 B 10 } and { closo ‐B 12 }, which are closely related to the { closo ‐1‐CB 11 } cluster, compared to benzene derivatives 50…”

“…The most versatile, selective functionalization at the antipodal boron atom and at a single boron atom of the lower B 5 belt is electrophilic iodination18, 21, 26–30 followed by a cross‐coupling reaction 13. 15, 22, 23, 27–35 So far, different substituents that are bonded via a carbon atom13, 15, 23, 27–33 or a nitrogen atom22, 34 have been attached to the B12 atom of the cluster using a cross‐coupling protocol.…”

“…Clusters with two or more functional groups that enable a further derivatization are promising building blocks for applications where { closo ‐1‐CB 11 } clusters act as bridging groups or spacers. However, only a limited number of { closo ‐1‐CB 11 } clusters of this type have been reported, so far,1, 28 for example, 1‐Me 2 N‐2‐Me 2 S‐ closo ‐1‐CB 11 H 10 ,36 [1‐HO 2 C‐12‐H 2 N‐ closo ‐1‐CB 11 H 10 ] − ,34 [1‐HO 2 C‐12‐I‐ closo ‐1‐CB 11 H 10 ] − ,18 [1,2‐(H 2 N) 2 ‐ closo ‐1‐CB 11 H 10 ] − ,19 [7,12‐(HCC) 2 ‐ closo ‐1‐CB 11 H 10 ] − ,24 the triazole [1,2‐(PhN 3 )‐ closo ‐1‐CB 11 Cl 10 ] − ,37 and [1‐(4‐R‐C 6 H 4 )‐12‐I‐ closo ‐1‐CB 11 H 10 ] − (R=Br, MeO 2 C) 13. 30…”

Thermal and Photophysical Properties of Highly Quadrupolar Liquid‐Crystalline Derivatives of the [closo‐B12H12]2− Anion

“…An alternative synthesis of compounds 2B[ n ]‐ p , especially those derivatives for which the 4‐alkoxypyridines were not readily available, was demonstrated by using the 4‐methoxypyridinium derivative 2B[1]‐ p . Following a procedure developed for 4‐alkoxypyridinium derivatives of the [ closo ‐1‐CB 11 H 11 ] − anion, the methoxy group in 2B[1]‐ p was demethylated with LiCl in DMF, and the resulting 4‐pyridone derivative 8‐ p was O‐alkylated with ( S )‐3,7‐dimethyloctyl triflate ( 9 ) to yield optically active 2B[10*]‐ p in an overall yield of 55 % (Scheme ).…”

Polar Liquid Crystals Derived from Sulfonium Zwitterions of the [closo‐1‐CB11H12]– Anion

“…The preparation of 12‐sulfonium zwitterions 1[ n ] took advantage of our recently developed method for functionalization of closo ‐borates . Thus, the parent anion B , prepared by a modified literature procedure, was C ‐alkylated according to the previously published procedure, and the resulting C ‐alkyl products 4[ n ] were treated with PhI(OAc) 2 (Scheme ). The desired 12‐phenyliodonium derivatives 5[ n ] were easily separated from small amounts of the 7‐iodonium isomers by chromatography .…”