This manuscript describes the synthesis and characterization of new derivatives of di-N-heterocyclic carbene (diNHC) ligands derived from trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diylmethanediyl (DEAM) and trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diyl (DEA). Synthesized as the diazolium salts, the new ligands are employed, in conjunction with known derivatives, to examine specific properties than influence the chiral induction in rhodium-catalyzed arylboronic acid addition to cyclohex-2-enones. Three properties of the diNHC ligands are modified and include: 1) the size and composition of the N-heterocyclic substituent (Me, iPr, Bn, CHPh2, o-MeBn, and R-CHMePh), 2) the type of N-heterocycle (benzimidazole versus imidazole), and 3) the size of the chiral pocket (DEAM versus DEA). Results from the catalytic studies indicate the DEAM ligand, which contains an extra CH2 group compared to DEA, is too flexible to induce enantiomeric excess. The DEA derivatives containing an imidazole or benzimidazole provide distinctly different results despite relatively small differences between the heterocyclic carbene donors. An unexpected source of chiral induction is rationalized using results from catalytic data (% e.e.) and complemented by X-ray structural data and DFT calculations.

A new C-2-symmetric chelating di-N-heterocyclic carbene (NHC) ligand is reported. The stable free di-carbene (+/-)[DEAM-BY] (3) forms upon treating the imidazolium salt(+/-)[DEAM-BI][OTf](2) (2) with potassium bis-trimethylsilylamide (where DEAM-BY=trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzyl)imidaz-2-ylidene and DEAM-BI = trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-benzyl)imidazolium). Metalation reactions of 2 with [Rh(COD)Cl](2) and [Ir(COD)Cl](2) are carried out under mild conditions to produce either mono- or bimetallic complexes. Each compound is characterized by NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography. Published by Elsevier Ltd.

A new C-2-symmetric di-N-heterocyclic carbene (di-NHC) ligand is synthesized, and its coordination behavior with Rh(I) salts is examined. A comprehensive list is presented describing the synthesis of all chiral di-NHC ligands reported to date, including pertinent catalytic and structural features. The historical perspective indicates few structural archetypes have been investigated and more structural alternatives are needed. Accordingly, the synthesis of the diimidazolium salts [DEA-MI](I)(2) (4) and [DEA-MBI](I)(2) (8) (where DEA = 9,10-dihydro-11,12-ethanoanthracene; MI = methylimidazolium, and MBI = methylbenzimidazolium) are presented. When 8 is treated with 2 equiv of KN(SiMe3)(2), the strained enetetramine [DEA-MBY] (9) is obtained and is characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The highly strained mononuclear complex [(DEA-MBY)Rh(COD)]I (10-COD) is obtained when 8 is treated with KN(SiMe3)(2) followed by half an equivalent of [Rh(COD)CI](2). A H-1 NMR spectrum of 10-COD revealed an unusual downfield signal at 9.26 ppm, assigned to an aliphatic bridge proton, and provides a clue as to the relative orientation of the anthracene ligand to the metal center. Ultimately, a single-crystal X-ray diffraction experiment assisted in determining the absolute solid-state structure of 10-COD. Although an X-ray crystal structure was not obtained for the imidazole derivative [DEA-MY][Rh(COD)]I (11-COD), a similar structure is implied by the occurrence of a downfield signal for the bridge proton at 8.24 ppm. In addition, the dinuclear complexes [mu-DEA-MY][Rh(NBD)I](2) (12-NBD) and [mu-DEA-MBY][Rh(COD)CI](2) (13-COD) are obtained. A structural comparison of the ligand precursors 2 and 7 center dot (HCl)(2) and the dinuclear species 12-NBD and 13-COD is presented.