Three types of oxime species, i.e.,
4-morpholylcarbamidoxime
(hydroxyguanidine), phenylacetamidoxime and benzamidoxime
(amidoximes), and cyclohexanone oxime and benzophenone
oxime (ketoximes), react at room temperature with the 2-nitrilium closo-decaborate clusters, leading to 2-iminium closo-decaborates (14 examples; 57–94%). These species
were characterized by ICPMS-based boron elemental analysis, HRESI–-MS, molar conductivity, IR, 1H{11B}, and 11B{1H} NMR spectroscopies, and additionally
by single-crystal X-ray diffraction (for six compounds). On the basis
of kinetic data, ΔH
⧧, ΔS
⧧, and ΔG
⧧ of the additions were determined, showing a 4 order-of-magnitude
decrease in reactivity from the hydroxyguanidine to the aromatic
ketoxime as entering nucleophiles. The results of DFT calculations
indicate that the mechanism for these reactions is stepwise and is
realized through the formation of the orientation complex of the nitrone
form, R2R3CN+(H)O–, of oximes with [B10H9NCEt]−, giving further an acyclic intermediate (the rate-determining
step), followed by proton migration, leading to the addition product.
The calculated overall activation barrier for these transformations
is consistent with the experimental kinetic observations. This work
provides, for the first time, a broad nucleophilicity series of oximes,
which is useful to control various nucleophilic additions of oxime
species.
Three types of N(H)-nucleophiles were used to study the nucleophilic addition to the CN group of the 2-propanenitrilium closo-decaborate cluster giving N-closo-decaborato amidrazones.
The reactions of salts of the anion [2 B 10 H 9 (N≡CMe)] -with aliphatic alcohols ROH (R = C n H 2n+1 (n = 1-6) CH 2 CH 2 (OEt), Pr i , Bu i , Bu t , i C 5 H 11 ) are studied. These reactions result in hydrolytically stable imidates [2 B 10 H 9 {NH=C(OR)Me}] -. Their structures were con firmed by the data from mass spectrometry, IR, 1 H, 11 B, and 13 C NMR spectroscopy. The molecular geometry of [2(Z) B 10 H 9 {NH=C(OBu)Me}] -, which formed in nucleophilic addi tion reaction of n butyl alcohol to [2 B 10 H 9 (N≡CMe)] -, was established by X ray diffrac tion analysis.
The Z-configured nitrones − O + N(Me)C(H)C 6 H 4 R 2p (R 2 = OMe (2a), Me (2b), NO 2 (2c)) react with the nitrile functionality of the closo-decaborate clusters [Bu n4 N][B 10 H 9 (NCR 1 )] (R 1 = Me (1a), Et (1b), Bu t (1c), Ph (1d)) in CHCl 3 solution under mild conditions (20−25 °C, 16−18 h) to afford the products of cycloaddition: viz., the borylated 2,3-dihydro-1,2,4-oxadiazoles. This reaction represents the first example of boron-mediated 1,3dipolar cycloaddition of allyl anion type dipoles, i.e. nitrones, to the nitrile group. Alteration of the lipophilic [Bu n4 N] + counterion with the hydrophilic Na + via the metathetical reaction with NaBPh 4 in 3a,b,e,f allows the modification of their hydrophilic−lipophilic properties and, consequently, solubility. Compounds 3a−j and 5a−d were characterized by high-resolution ESI-MS, IR, and 1 H, 13 C{ 1 H}, and 11 B{ 1 H} NMR spectroscopy. The structures of 3a,e,f were determined by single-crystal X-ray diffraction.
The novel members of the 1,2-diboraoxazoles family have been obtained. In the present work, we have carried out the intramolecular ring-closure reaction of borylated iminols of general type [B10H9N=C(OH)R]− (R = Me, Et, nPr, iPr, tBu, Ph, 4-Cl-Ph). This process is conducted in mild conditions with 83–87% yields. The solid-state structures of two salts of 1,2-diboraoxazoles were additionally investigated by X-ray crystallography. In addition, the phenomena of bonding interactions in the 1,2-diboraoxazole cycles have been theoretically studied by the Quantum Theory of Atoms in Molecules analysis. Several local and integral topological properties of the electron density involved in these interactions have been computed.
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