Transition metal complexes of oxazolinylboranes and cyclopentadienyl-bis(oxazolinyl)borates: Catalysts for asymmetric olefin hydroamination and acceptorless alcohol decarbonylation

Abstract

The research presented and discussed in this dissertation involves the synthesis of transition metal complexes of oxazolinylboranes and cyclopentadienyl-bis(oxazolinyl)borates, and their application in catalytic enantioselective olefin hydroamination and acceptorless alcohol decarbonylation. Neutral oxazolinylboranes are excellent synthetic intermediates for preparing new borate ligands and also developing organometallic complexes. Achiral and optically active bis(oxazolinyl)phenylboranes are synthesized by reaction of 2-lithio-2-oxazolide and 0.50 equiv of dichlorophenylborane. These bis(oxazolinyl)phenylboranes are oligomeric species in solid state resulting from the coordination of an oxazoline to the boron center of another borane monomer. The treatment of chiral bis(oxazolinyl)phenylboranes with sodium cyclopentadienide provide optically active cyclopentadienyl-bis(oxazolinyl)borates H[PhB(C{sub 5}H{sub 5})(Ox{sup R}){sub 2}] [Ox{sup R} = Ox{sup 4S-iPr,Me2}, Ox{sup 4R-iPr,Me2}, Ox{sup 4S-tBu]}. These optically active proligands react with an equivalent of M(NMe{sub 2}){sub 4} (M = Ti, Zr, Hf) to afford corresponding cyclopentadienyl-bis(oxazolinyl)borato group 4 complexes {PhB(C{sub 5}H{sub 4})(Ox{sup R}){sub 2}}M(NMe{sub 2}){sub 2} in high yields. These group 4 compounds catalyze cyclization of aminoalkenes at room temperature or below, providing pyrrolidine, piperidine, and azepane with enantiomeric excesses up to 99%. Our mechanistic investigations suggest a non-insertive mechanism involving concerted C−N/C−H bond formation in the turnover limiting step of the catalytic cycle. Among cyclopentadienyl-bis(oxazolinyl)borato group 4 catalysts, the zirconium complex {PhB(C{sub 5}H{sub 4})(Ox{sup 4S-iPr,Me2}){sub 2}}Zr(NMe{sub 2}){sub 2} ({S-2}Zr(NMe{sub 2}){sub 2}) displays highest activity and enantioselectivity. Interestingly, {S-2}Zr(NMe{sub 2}){sub 2} also desymmetrizes olefin moieties of achiral non-conjugated aminodienes and aminodiynes during cyclization. The cyclization of aminodienes catalyzed by {S-2}Zr(NMe{sub 2}){sub 2} affords diastereomeric mixture of cis and trans cylic amines with high diasteromeric ratios and excellent enantiomeric excesses. Similarly, the desymmetrization of alkyne moieties in {S-2}Zr(NMe{sub 2}){sub 2}-catalyzed cyclization of aminodiynes provides corresponding cyclic imines bearing quaternary stereocenters with enantiomeric excesses up to 93%. These stereoselective desymmetrization reactions are significantly affected by concentration of the substrate, temperature, and the presence of a noncyclizable primary amine. In addition, both the diastereomeric ratios and enantiomeric excesses of the products are markedly enhanced by N-deuteration of the substrates. Notably, the cationic zirconium-monoamide complex [{S-2}Zr(NMe{sub 2})][B(C{sub 6}F{sub 5}){sub 4}] obtained from neutral {S-2}Zr(NMe{sub 2}){sub 2} cyclizes primary aminopentenes providing pyrrolidines with S-configuration; whereas {S-2}Zr(NMe{sub 2}){sub 2} provides R-configured pyrrolidines. The yttrium complex {S-2}YCH{sub 2}SiMe{sub 3} also affords S-configured pyrrolidines by cyclization of aminopentenes, however the enantiomeric excesses of products are low. An alternative optically active yttrium complex {PhB(C{sub 5}H{sub 4})(Ox{sup 4S-tBu}){sub 2}}YCH{sub 2}SiMe{sub 3} ({S-3}YCH{sub 2}SiMe{sub 3}) is synthesized, which displays highly enantioselective in the cyclization of aminoalkenes at room temperature affording S-configured cyclic amines with enantiomeric excesses up to 96%. A noninsertive mechanism involving a six-membered transition state by a concerted C−N bond formation and N−H bond cleavage is proposed for {S-3}YCH{sub 2}SiMe{sub 3} system based on the kinetic, spectroscopic, and stereochemical features. In the end, a series of bis- and tris(oxazolinyl)borato iridium and rhodium complexes are synthesized with bis(oxazolinyl)phenylborane [PhB(Ox{sup Me2}){sub 2}]{sub n}, tris(oxazolinyl)borane [B(Ox{sup Me2}){sub 3}]n, and tris(4,4-dimethyl-2-oxazolinyl)phenylborate [To{sup M}]{sup −}. All these new and other known rhodium and iridium complexes were examined in acceptorless dehydrogenative decarbonylation of primary alcohols. The catalysts survey shows that the compound To{sup M}Ir(η{sup 4}- C{sub 8}H{sub 12}) is the most active for the conversion of primary alcohols into alkane, H{sub 2}, and CO at 180 °C in toluene. Several aliphatic and aromatic primary alcohols are decarbonylated in the catalytic conditions. Furthermore, To{sup M}Ir(η{sup 4}-C{sub 8}H{sub 12}) is also able to decarbonylate polyols such as ethylene glycol and glycerol to syngas (H{sub 2} and CO) at 180 °C.