Abstract
The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH3]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy3)2HCl(H2)] (Cy: cyclohexyl) were studied, resulting in bis(σ)‐borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X‐ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho‐substitution of the aryl groups is necessary for the formation of terminal borylene complexes.
Highlights
The dehydrogenative coupling of element hydrides for the formation of element–element bonds has attracted significant attention in recent years, and is becoming increasingly valuable in the synthesis of main-group molecules and polymers.[1]
Given the possibility of constructing boron–boron bonds through dehydrocoupling, we were interested in investigating whether the dehydrogenation process during the borylene formation from bis(s-BÀH) ruthenium complexes[14a,b] suffers from limitations related to the substitution pattern of the boronbound aryl group of dihydroboranes, and if a chloride ligand at the metal center is necessary or not
We synthesized a series of dihydroboranes and metal-organic borohydrides, allowing us to embark on a systematic investigation of the steric and electronic factors required for borylene complex formation on ruthenium(II)
Summary
Later inspired us to develop a more atom-efficient route to the synthesis of diboron(4) reagents of relevance to organic chemistry,[6] leading to the establishment of the first synthetically viable dehydrogenative coupling of pinacolborane (HBPin, Pin = 1,2-O2C2Me4) and catecholborane (HBCat, Cat = 1,2-O2C6H4) to the corresponding diboranes(4) using either homo- or heterogeneous catalysts in 2011.[7] Prior to this, Marder and co-workers had observed the formation of small amounts of B2Pin from HBPin as a byproduct during the catalytic borylation of CÀH bonds with HBPin.[8] These results added another entry to the handful of methods for the selective construction of electron-precise BÀB bonds.[6,9]. Given the possibility of constructing boron–boron bonds through dehydrocoupling, we were interested in investigating whether the dehydrogenation process during the borylene formation from bis(s-BÀH) ruthenium complexes[14a,b] suffers from limitations related to the substitution pattern of the boronbound aryl group of dihydroboranes, and if a chloride ligand at the metal center is necessary or not. We synthesized a series of dihydroboranes and metal-organic borohydrides, allowing us to embark on a systematic investigation of the steric and electronic factors required for borylene complex formation on ruthenium(II)
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