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 FBMes (Mes = 2,4,6-trimethylphenyl) and ClBCy as 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 MCl (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 BMes groups 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 CpBMes rings. There is only weak, if any, direct M···B interaction in 6 and 7.

The Taming of Redox‐Labile Phosphidotitanocene Cations

“…[39] The orbital transitions of selected excited states were characterized by using the natural transition orbital (NTO) method. [40] Following the benchmark calculations of Hadt et al [41] and our previous work, [42] EPR g parameters were computed by using the GGA BPW91 functional with the aug-cc-pVTZ-J basis set for the Ti atom, [43] and the IGLO-III basis set for other atoms. [44] The A parameters were computed by using the hybrid B3LYP functional with the same basis sets.…”

Highly Functionalized Ferrocenes

“…Consistently, the singly occupied molecular orbital is a combination of a Co‐centered 3d orbital with π(BC) orbitals on each CpBMes 2 rings. There is no significant direct M ··· B interaction in 76–78 …”

“…Borylcyclopentadienyllithium salts X–Y were obtained from sequential lithiation/borylation of dimethylfulvene and then reacted with iron dichloride to form complexes 76–77 in 61–93 % (Figure ) . In contrast to the Cp assembly synthesis of diphosphino ferrocenes previously described (Figure and Figure ), this synthesis was surprisingly fully diastereoselective and only led to the formation of the rac diastereoisomer.…”

“…Thus, the post‐functionalization of the tert ‐butylated ferrocene platform constitutes a powerful strategy for the diastereoselective synthesis of symmetrically substituted 1,1'‐diphosphines. Accordingly, the analogous compounds holding i‐ Pr groups, 50 and 51 , and trimethylsilyl (TMS), 52 and 53 , have also been synthesized from ferrocene post‐functionalization (Figure ) , . Compounds 50 and 51 were synthesized from precursor 48a in two steps, and 52 and 53 obtained in a one‐pot protocol from 49a , without isolating the intermediate 49b (35 % and 33 % overall yields, respectively).…”

“…In studies conducted in cooperation with G. Bouhadir and D. Bourissou, bis‐Lewis acidic diborylferrocenes were synthesized via the assembling at metal salt of preformed Cp rings …”