Stereoselective addition of acetone to the C=N bond of [4.3.0] boron heterobicycles

Abstract

The reaction of N-salicylidene-2-aminophenolate (SAP-H2) derivatives with phenyl boronic acid afforded five new boron bicyclic species in moderate yields, as confirmed by an X-ray study of the boron heterocycle derived from 2-amino-4-chlorophenol. The results also showed that the boron complexes derived from Schiff bases undergo an acetolysis reaction to give the corresponding dioxazaborocines containing all substituents on the same side also in good yields. The structure of one of the addition products was established by X-ray analysis showing that, in the solid state, it exists as a dimeric structure formed by hydrogen bonds.Key words: borocines, tridentate ligands, imines, boronic acids.

Mono- and diboronates derived from tridentate ONO ligands and arylboronic acids

“…Nonetheless, coordination of the nitrogen atom to the boron atom polarizes the C@N double bond, as evidenced by the changes in 1 H NMR chemical shifts of the azomethine group and IR bands [5,6]. This polarization is confirmed also by the facile, stereoselective addition of acetone in these compounds (Scheme 1, III) [8] but not in the free ligand. At present, there is increasing interest in this type of compounds, because some of them are useful in propylene polymerization reactions [9] via the formation of a boron cation.…”

Synthesis and characterization of boronates derived from non-symmetric amino-bis-phenols

“…In previous studies, we described the synthesis of a series of mono-, di-, tri-and tetrameric boronates derived from NO 2 tridentate ligands such as N-alkyl-diethanolamines [1], N-methyl-N-(1-methyl-2-phenyl-2-hydroxyethyl)glycines [2], 2-substituted pyridines [3], N-(2-hydroxybenzyl)-a-aminoacids [4], N-alkyliminodiacetic acids [5], N-(2-hydroxyethyl)-N-alkyl-glycines [6], N-alkyl-2-2 0 -diphenolamines [7], 2,6-pyridinedimethanol and 2-salicylideneaminoethanol [8][9][10][11][12], piperidine and piperazine alcohols [13], a-aminoacids [14,15], N-alkylaminodiacetic acids [16], ephedrine and pseudoephedrine derivatives [17], tridentate azomethine ligands [18], ethanolamine derivatives [19], aminodialcohols [20] as well as the self assembly of boronic-salicylidene Schiff bases [21] (Scheme 1). All of these ligands have one nitrogen and two oxygen donor atoms with strategic variations in the main skeleton that allowed evaluation of steric and electronic effects in the resulting chelate molecules.…”

“…Further studies revealed that it is possible to carry out the stereoselective addition of acetone to the C=N bond in a complete series of [4.3.0] boron heterobicycles derived from salicylideniminophenols (Scheme 2). In these compounds, the polarization, and therefore the selective reactivity of this bond was evidenced by acetolysis of the imine group to produce the corresponding dioxaborocines with a cis fusion and the incoming group placed also on the same side, with full diastereoselectivity [12]. The same behaviour was observed in the case of the boronates derived from N-(2-hydroxybenzyl)-a-aminoacids [4] (possessing a sp 2 carbon atom at the five membered fused ring) and 2-((2-hydroxyethylamino) methyl) phenols [25] (the five-membered fused ring has a sp 3 carbon atom) where the substituents are on the same side and only the exo disastereoisomer is formed.…”

Imino Diels–Alder reaction of boronates. Preparation and characterization of new 3,4-dihydroquinoline and 1,2,3,6-tetrahydropyridine derivatives

“…In previous studies it has been reported that the C@N bond in dioxaboracines is polarized by coordination of the nitrogen atom to boron through a N!B dative bond, as evidenced by formation of acetolysis products [34] (Scheme 3). Thus the coordination bond between the nitrogen and boron atoms polarizes the imine bond increasing the reactivity of dioxaborocines towards the imino-Diels Alder reaction by taking advantage of boron adducts.…”