Unsymmetric 1,1-diboryl alkenes bearing one −[BPin] (BPin = pinacolatoboryl) and one −[BDan] (BDan = 1,8diaminonaphthalatoboryl) substituent each were hydrogenated in high yield and enantioselectivity using C 1 -symmetric pyridine-(diimine) (PDI) cobalt complexes. High activities and stereoselectivities were observed with an array of 2-alkyl-, 2-aryl-, and 2boryl-substituted 1,1-diboryl alkenes, giving rise to enantioenriched diborylalkane building blocks. Systematic study of substrate substituent effects identified competing steric and electronic demands in the key activating role of the boron substituents, whereby sterically unencumbered boronates such as −[BDan], −[BCat] (BCat = catecholatoboryl), and −[Beg] (Beg = ethylene glycolatoboryl) promote the hydrogenation of trisubstituted alkenes by enabling irreversible α-boron-directed insertion pathways to achieve otherwise challenging hydrogenations of trisubstituted alkenes. Deuterium-labeling studies with 1,1-diboryl alkenes support an insertion pathway generating a chiral intermediate with two different boron substituents and cobalt bound to the same carbon.

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenumcatalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo-and enantioselectivities. For quinoline derivatives, selective hydrogenation of both the carbocycle and heterocycle was observed depending on the ring substitution. Spectroscopic and mechanistic studies established molybdenum η 6 -arene complexes as the catalyst resting state and that partial hydrogenation arises from dissociation of the substrate from the coordination sphere of molybdenum prior to complete reduction. A stereochemical model is proposed based on the relative energies of the respective coordination of the prochiral faces of the arene determined by steric interactions between the substrate and the chiral ligand, rather than through precoordination by a heteroatom.

Treatment of pyridine dicarbene iron dialkyl complexes with low (≪1 atm) pressures of H2 in a benzene-d 6 solution promoted rapid hydrogen isotope exchange (HIE) of the C­(sp2)–H bonds in both electron-poor and -rich aromatic and heteroaromatic rings with benzene-d 6 as the deuterium source. The iron-catalyzed reaction proceeded with predictable regioselectivity, engaging sterically accessible C–H bonds including ortho-to-fluorine sites. The site selectivity for the catalytic HIE reaction was studied to identify the kinetic preferences for C–H activation. Structure–activity relationship studies with a series of iron precatalysts established that introduction of substituents at the 3- and 5-positions of pyridine of the pincer significantly accelerated HIE. Mechanistic studies identified N2 as an inhibitor of C–H activation, while H2 served to generate the active catalyst.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe-(CH 2 SiMe 3 ) 2 , have been developed. The first route consists of addition of 2 equiv of LiCH 2 SiMe 3 to the iron dihalide complex (CNC)FeBr 2 , while the second relies on addition of the free CNC ligand to the readily prepared (py) 2 Fe-(CH 2 SiMe 3 ) 2 (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ( Ar CNC)Fe-(CH 2 SiMe 3 ) 2 (N 2 ) ( Ar CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC chelate. In contrast, the alkyl-substituted variant ( tBu ACNC)Fe(CH 2 SiMe 3 ) 2 ( tBu ACNC = 2,6-( t Bu-imidazol-2-ylidene) 2 pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ( Ar CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ) derivatives to an H 2 atmosphere resulted in formation of the corresponding iron hydride complexes ( Ar CNC)FeH 4 . These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H 2 , generating isotopologues and isotopomers of (5-Me 2 Mes CNC)Fe-(CH 2 SiMe 3 ) 2 (N 2 ) (3,5-Me 2 Mes CNC = 2,6-(2,4,6-Me 3 -C 6 H 2 -imidazol-2-ylidene) 2 -3,5-Me 2 -pyridine) was treated successively with H 2 and then N 2 , the corresponding reduced dinitrogen complex (3,5-Me 2 Mes CNC)Fe(N 2 ) 2 was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me 2 Mes CNC)Fe(CH 2 SiMe 3 ) 2 (N 2 ).

A metal-free tandem reaction, initiated by the generation of a diazonium cation and followed by cycloetherification, was developed. Acid-promoted de-tert-butylation of N-nitroso N-tert-butylamine was used to generate a diazonium cation in situ, demonstrating a new application of nitroso chemistry. This reaction was employed in the synthesis of substituted benzofuran-3(2H)-ones and dihydrobenzo[d][1,3]oxaphosphole 3-oxides.