Damian M. Grainger

N to C aryl migration in lithiated carbamates: α-arylation of benzylic alcohols

We report a new mode of reactivity displayed by lithiated O-benzyl carbamates carrying an N-aryl substituent: upon lithiation, the N-aryl group is transferred cleanly from N to C. An arylation of the carbamate results, providing a route to alpha,alpha-arylated secondary or tertiary alcohols. We also report density functional theory calculations supporting the proposal that arylation proceeds through a dearomatizing attack on the aromatic ring, a significantly lower energy pathway than the 1,2-acyl transfer observed with related N-alkyl carbamates.

Biocatalytic Dynamic Kinetic Resolution for the Synthesis of Atropisomeric Biaryl N-Oxide Lewis Base Catalysts

Atropisomeric biaryl pyridine and isoquinoline N-oxides were synthesized enantioselectively by dynamic kinetic resolution (DKR) of rapidly racemizing precursors exhibiting free bond rotation. The DKR was achieved by ketoreductase (KRED) catalyzed reduction of an aldehyde to form a configurationally stable atropisomeric alcohol, with the substantial increase in rotational barrier arising from the loss of a bonding interaction between the N-oxide and the aldehyde. Use of different KREDs allowed either the M or P enantiomer to be synthesized in excellent enantiopurity. The enantioenriched biaryl N-oxide compounds catalyze the asymmetric allylation of benzaldehyde derivatives with allyltrichlorosilane.

Enzymatic desymmetrising redox reactions for the asymmetric synthesis of biaryl atropisomers.

Atropisomeric biaryls carrying ortho-hydroxymethyl and formyl groups were made enantioselectively by desymmetrisation of dialdehyde or diol substrates. The oxidation of the symmetrical diol substrates was achieved using a variant of galactose oxidase (GOase), and the reduction of the dialdehydes using a panel of ketoreductases. Either M or P enantiomers of the products could be formed, with absolute configurations assigned by time-dependent DFT calculations of circular dichroism spectra. The differing selectivities observed with different biaryl structures offer an insight into the detailed structure of the active site of the GOase enzyme.

Development of a Stepwise Reductive Deoxygenation Process by Ru‐Catalysed Homogeneous Ketone Reduction and Pd‐Catalysed Hydrogenolysis in the Presence of Cu Salts

A stepwise catalytic reduction of ketone 1 to alcohol 2 and subsequently to aryl(imidazo[1,2‐b]pyridazinyl)methane 3 is described, which provides synthetically useful chemoselectivity at acceptably low catalyst loadings. Undesired reactive sites include an aryl chloride, heteroarylchloride and benzylic amine group. The presence of these functional groups presents a significant challenge to chemoselectivity for both reduction steps. For selective CO reduction of highly functionalised 1, high chemoselectivity was observed at low catalyst loading by using Wills’ tethered Ru transfer‐hydrogenation catalyst 13. The selective hydrogenolysis of 2 was then accomplished under acidic hydrogenation conditions by using a Pd/C catalyst in the presence of Cu salts. This procedure has been demonstrated on a multi‐gram scale, which makes this approach a viable method to use a combination of homogeneous and heterogeneous catalysis.

Hydrogenation and Reductive Amination of Aldehydes using Triphos Ruthenium Catalysts

An air‐stable and readily accessible ruthenium dihydride complex catalyses aldehyde hydrogenation under neutral conditions. A high activity has been shown in a number of examples, and solvent‐free conditions are also applicable, which favours industrial‐scale applications. The catalyst has also been demonstrated to be active at low catalyst loadings for the reductive amination of aldehydes under mildly acidic conditions. A number of examples of chemoselectivity challenges are also presented in which the catalyst does not reduce carbon−halogen groups, alkene or ketone functionality. The advantage of using the pre‐formed complex, Triphos‐Ru(CO)H2 (1), over in situ formed catalysts from Triphos and Ru(acac)3 (acac=acetylacetonate) is also shown in terms of both chemoselectivity and activity, in particular this can be seen if low reaction temperatures are used.