Ligand Effects in Calcium Catalyzed Ketone Hydroboration

• Published in: European journal of inorganic chemistry 2020-05, Vol.2020 (18), p.1728-1735
• Main Authors: Brand, Steffen, Causero, Andrea, Elsen, Holger, Pahl, Jürgen, Langer, Jens, Harder, Sjoerd
• Format: Article
• Language: English
• Subjects: Aldehydes; Amidinates; Aromatic compounds; Calcium; Calcium ions; Catalysts; Cationic complexes; Chelation; Chlorobenzene; Coordination compounds; Crystal structure; Diethyl ether; Dipping; Energy dissipation; Homogeneous catalysis; Hydroboration; Inorganic chemistry; Isomers; Ketones; Lewis base; Ligands; Metal hydrides
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.202000264

The first “naked” (Lewis base‐free) cationic Ca amidinate complex [tBuAmDIPPCa(C6H6)]+[B(C6F5)4]– was prepared in 62 % yield {tBuAmDIPP = tBuC(N–DIPP)2; DIPP = 2,6‐diisopropylphenyl} by reaction of [tBuAmDIPPCaH]2 with [Ph3C]+[B(C6F5)4]– in chlorobenzene. The ether‐free complex tBuAmDIPPCaN(SiMe3)2 was obtained by removal of diethyl ether from its ether adduct. Crystal structures show that the amidinate ligand in both complexes is N,Aryl‐chelating. In this coordination mode the bulk of the amidinate ligand is comparable to that of a DIPP‐substituted β‐diketiminate ligand. Isomers with N,N‐coordinating amidinate ligands are circa 15 kcal/mol higher in energy and this coordination mode is only present in case additional ether ligands compensate for energy loss or in case of space limitation at the metal, e.g. in homoleptic (tBuAmDIPP)2Ca. A series of four Ca amidinate complexes, tBuAmDIPPCaX, were tested in the catalytic hydroboration of ketones and aldehydes by pinacolborane (HBpin). Catalytic activities increase for X– = I– < B(C6F5)4– < (Me3Si)2N– ≈ H–. For catalysts with unreactive anions, like I– or B(C6F5)4–, catalyst performance increases with the Lewis acidity of the metal and a mechanism is proposed in which HBpin and ketone coordinate to the Ca2+ ion which is followed by direct hydroboration. The more active catalysts with X– = (Me3Si)2N– or H– likely operate through a mechanism which involves intermediate metal hydride (or borate) complexes. A series of calcium amidinate complexes with various anionic ligands or counterions were shown to catalyze ketone hydroboration. The influence of ligand variation is discussed. Catalysts with inactive ligands {I– or B(C6F5)4–} follow a Lewis acid mechanism while those with reactive ligands {H– or (Me3Si)2N–} follow a hydride or borate mechanism.