title: Planar-Chiral 1,1'-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene.
abstract: The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes2(Mes = 2,4,6-trimethylphenyl) and ClBCy2as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tert-butylated boryl cyclopentadienides with MCl2(M = Fe, Co) selectively leads to 1,1'-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes2groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes2rings. There is only weak, if any, direct M···B interaction in 6 and 7.
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Lerayer
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Patrice
authors.initials: P
authors.lastname: Renaut
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Brandès
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Hélène
authors.initials: H
authors.lastname: Cattey
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.
authors.forename: Paul
authors.initials: P
authors.lastname: Fleurat-Lessard
authors.affiliation: Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA), Université Paul Sabatier/CNRS UMR 5069 , 118 Route de Narbonne, 31062 Toulouse, Cedex 09, France.
authors.forename: Ghenwa
authors.initials: G
authors.lastname: Bouhadir
authors.affiliation: Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA), Université Paul Sabatier/CNRS UMR 5069 , 118 Route de Narbonne, 31062 Toulouse, Cedex 09, France.
authors.forename: Didier
authors.identifier: http://orcid.org/0000-0002-0249-1769
authors.initials: D
authors.lastname: Bourissou
authors.affiliation: Université de Bourgogne Franche-Comté , Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France.|Institut Universitaire de France (IUF) , 103 Boulevard Saint Michel, 75005 Paris Cedex, France.
authors.forename: Jean-Cyrille
authors.identifier: http://orcid.org/0000-0002-2048-647X
authors.initials: JC
authors.lastname: Hierso