Dehydrogenative Coupling of Aldehydes with Alcohols Catalyzed by a Nickel Hydride Complex

Abstract

A nickel hydride complex, {2,6-( i Pr2PO)2C6H3}­NiH, has been shown to catalyze the coupling of RCHO and R′OH to yield RCO2R′ and RCH2OH, where the aldehyde also acts as a hydrogen acceptor and the alcohol also serves as the solvent. Functional groups tolerated by this catalytic system include CF3, NO2, Cl, Br, NHCOMe, and NMe2, whereas phenol-containing compounds are not viable substrates or solvents. The dehydrogenative coupling reaction can alternatively be catalyzed by an air-stable nickel chloride complex, {2,6-( i Pr2PO)2C6H3}­NiCl, in conjunction with NaOMe. Acids in unpurified aldehydes react with the hydride to form nickel carboxylate complexes, which are catalytically inactive. Water, if present in a significant quantity, decreases the catalytic efficiency by forming {2,6-( i Pr2PO)2C6H3}­NiOH, which causes catalyst degradation. On the other hand, in the presence of a drying agent, {2,6-( i Pr2PO)2C6H3}­NiOH generated in situ from {2,6-( i Pr2PO)2C6H3}­NiCl and NaOH can be converted to an alkoxide species, becoming catalytically competent. The proposed catalytic mechanism features aldehyde insertion into the nickel hydride as well as into a nickel alkoxide intermediate, both of which have been experimentally observed. Several mechanistically relevant nickel species including {2,6-( i Pr2PO)2C6H3}­NiOC­(O)­Ph, {2,6-( i Pr2PO)2C6H3}­NiOPh, and {2,6-( i Pr2PO)2C6H3}­NiOPh·HOPh have been independently synthesized, crystallographically characterized, and tested for the catalytic reaction. While phenol-containing molecules cannot be used as substrates or solvents, both {2,6-( i Pr2PO)2C6H3}­NiOPh and {2,6-( i Pr2PO)2C6H3}­NiOPh·HOPh are efficient in catalyzing the dehydrogenative coupling of PhCHO with EtOH.

Silane Activation with Cobalt on the POCOP Pincer Ligand Platform

“…The former was shown to convert PhCHO to Ph 2 SiH­(OCH 2 Ph) as the sole product, whereas the use of PhSiH 3 led to a 37:59:4 mixture of PhSiH 2 (OCH 2 Ph), PhSiH­(OCH 2 Ph) 2 , and PhSi­(OCH 2 Ph) 3 . , By contrast, Ph 3 SiH, PhMe 2 SiH, and Et 3 SiH are not viable silanes under the catalytic conditions, showing no conversion of PhCHO and no hydrosilylation products (entries 5–7). In our experience with catalytic hydrosilylation of aldehydes, acid impurities can sometimes shut down the reaction, as demonstrated in the catalytic system based on {κ P ,κ C ,κ P -2,6-( i Pr 2 PO) 2 C 6 H 3 }­NiH . This is not a concern for the cobalt system; using an undistilled sample of PhCHO containing ∼3 mol % PhCO 2 H had a minimum impact on the efficiency and yield (entry 8).…”

Nucleophilic Nickel and Palladium Pincer Hydroxides: A Study of Their Reactions with Dimethyl Carbonate and Other Non‐Alkylating Organic Electrophiles

“…Therefore, we removed conformational uncertainty by replacing i ‐propyl substituents by methyl groups ( i. e ., Me PCP ligand instead of iPr PCP). In our own experience, [14b,15a] and also for others, [26,28b] this simplification does not have excessive impact on the main conclusions. As shown below, the agreement between experimental and computed data proves satisfactory in general.…”

Nickel and Palladium Complexes with Reactive σ‐Metal‐Oxygen Covalent Bonds

“…Because nucleophilicity is the dominant feature of hydroxides, alkoxides and other compounds with reactive M−O bonds, they tend to react more frequently through the second pathway. Therefore, small molecules with marked electrophilic character, like heterocumulenes of various sort (e. g. CS 2 , RNCO o RNCS), aldehydes, as well as acid oxides like SO 2 do insert readily into M−O bonds, whereas migratory insertions, so prevailing in the reactivity of M−C bonds are much less common for alkoxides. Only substrates with exceptional binding capacity like CO can override the natural tendency of the alkoxide to fill up any coordination vacancy by electron pair donation.…”