Coupling of Azomethine Ylides with Nitrilium Derivatives of closo‐Decaborate Clusters: A Synthetic and Theoretical Study

Abstract

The azomethine ylides p‐R3C5H4N+CH−COC6H4R2‐p (3 a: R3=H, R2=H, X=Br; 3 b: R3=H, R2=Me, X=I; 3 c: R3=H, R2=OMe, X=I; 3 d: R3=H, R2=F, X=I; 3 e: R3=Me, R2=Me, X=Br) react with the nitrile functionality of the closo‐decaborate clusters [Bun4N][B10H9(NCR1)] (1 a: R1=Me; 1 b: R1=Et; 1 c: R1=Ph) in a CH3NO2 solution under mild conditions (20–25 °C, 2 min) to afford selectively products of the nucleophilic addition (ca. quantitative yields based on NMR analysis in [D6]DMSO, 71–87 % yield of isolated products). These products are the borylated enamino ketones as the salts bearing exclusively a tetrabutylammonium cation [Bun4N][B10H9{NCR1=C(N+C5H4R3‐p)COC6H4R2‐p}] (4 a–h,k–n) or the mixed salts [Bun4N]1‐x[p‐R3C5H4N+CH2COC6H4R2‐p]x[B10H9{NCR1=C(N+C5H4R3‐p)COC6H4R2‐p}] (4 i, 4 j, and 4 o). This reaction represents the first example of the selective nucleophilic addition of pyridinium azomethine ylides to the nitrile group and the first example of new CC bond formation in the reaction of nitrilium derivatives of boron clusters with any nucleophiles so far tested. Compounds 4 a–h,k–n were characterized by ICP‐MS, high resolution ESI‐MS, molar conductivity, and IR, 1H, 13C{1H}, and 11B{1H} NMR spectroscopy. The structures of 4 a and 4 b were determined by a single‐crystal X‐ray analysis. Theoretical calculations at the DFT level allowed the interpretation of the observed reaction selectivity, which is driven mostly by thermodynamic rather than kinetic factors, and steric repulsion between the boron cluster and the azomethine ylide molecule plays a crucial role. The activation of the nitrile group upon binding to the boron cluster is explained in terms of the electrostatic arguments. The mechanisms of the nucleophilic addition and the hypothetical cycloaddition between the azomethine ylide and nitriles were investigated in detail.

Nucleophilicity of Oximes Based upon Addition to a Nitrilium closo-Decaborate Cluster

“…7 When the closo-decaborates are functionalized with a nitrile group, they could be easily derivatized further by nucleophilic addition to the nitrile C atom. Indeed, linkage of the CN group to the closodecaborate cluster facilitates addition of HO-, 12 HN-, 13 and HC-nucleophiles 14 to the C atom of the CN moiety and also promotes, as we found, 1,3-dipolar cycloaddition of nitrones 15 and azides 16 to this bond.…”

“…Molar conductivities of (Ph 3 P-CH 2 Ph)[1−6], (Ph 3 PCH 2 Ph) [9−11], and (Ph 3 PCH 2 Ph)- [13−14] are in the range 93.0−110.8 S cm −1 mol −1 , which is somehow lower than that expected for 1:1 electrolytes (120− 160 S cm −1 mol −1 in MeCN 23 ). However, many other 1:1 electrolyte closo-decaborate species in MeCN exhibit conductivities in the range 119−127 S cm −1 mol −1 , 14 and our experimental values are well coherent with these data. Clusters In the IR spectra of (Ph 3 PCH 2 Ph)[1−6], (Ph 3 PCH 2 Ph) [9− 11], and (Ph 3 PCH 2 Ph) [13−14] (Supporting Information), medium to strong bands at 3497−3441 and 3360−3327 cm −1 , which can be attributed to the N−H stretches of the NH 2 group, and weak to medium band at 3296−3215 cm −1 , assignable to the N−H stretches of the N−H•••N moiety, were observed.…”

“…In the molecular structures of (Ph [6]•MeCN, (Ph 3 P-CH 2 Ph)[10], and (Ph 3 PCH 2 Ph) [13], the closo-decaborate cluster exhibits the usual bicapped square bipyramidal geometry (Figure 2). 7,12,[14][15][16]25 The N(1)−B(2) bond lengths (1.5291(19)−1.540(4) Å) are typical for closo-decaborate bound imines. 7,12,[14][15][16]25 The N(1)−C(1) bonds are double bonds (1.283(2)−1.2926(15) Å), 7,12,25 whereas the N(2)− C(2) and N(3)−C(2) distances are intermediate between single and double bonds (1.296(2)−1.305(4) and 1.331(2)− 1.3381(15) Å, respectively).…”

Synthesis of New Bioinorganic Systems Based on Nitrilium Derivatives of closo-Decaborate Anion and meso-Arylporphyrins with Pendant Amino Groups

“…[13][14][15][16][17][18] Previously we discovered convenient method of synthesis of disubstituted oxonium derivatives using CF 3 SO 3 H as electrophilic inductor. [19] This method is quite universal and allows to produce disubstituted derivatives containing various exo-polyhedral bonds.…”

Synthesis of Boronated Amidines by Addition of Amines to Nitrilium Derivative of Cobalt Bis(Dicarbollide)

“…Nucleophilic addition reactions to the highly polarized -N + ≡CR triple bonds in aryl-and alkylnitrilium salts [30] and nitrile complexes of transition metals [31][32][33][34][35] are widely used in organic synthesis and have large potential in the modification of polyhedral boron hydrides [36]. Thus, the nitrilium derivatives of the closo-decaborate anion react with water [37,38], alcohols [39], amines [40,41], hydrazines [42], hydrazones [42] and oximes [43,44], as well as with various carbanions [45,46] to form the corresponding addition products. Like organic nitrilium salts, they are able to participate in 1,3-dipolar cycloaddition reactions with azides and nitrones leading to the corresponding boronated tetrazoles [47] and 2,3-dihydro-1,2,4-oxadiazoles [48], respectively.…”