Chiral organometallic chemistry

New methodology in organic synthesis

The development of new methods for the construction of novel compounds plays a crucial role in the field of synthetic chemistry. The importance and prevalence of nitrogen-containing compounds in natural products and medicinal drugs has led us to develop new chemistry for their formation. 

The chemistry needs to provide high yields and high selectivities if it is to be useful. Especially important for synthesis is the discovery of new carbon–carbon bond-forming reactions. 

There are many ways to form C–C bonds and our research group is studying the use of organometallic compounds for stereoselective synthesis.

A focus of our work is on the formation, properties, reactivity and reactions of chiral α-amino-organolithium compounds:

• For their formation, an efficient and straightforward method is to simply abstract a proton with a base.

• For their properties, we are carrying out structural and kinetic studies to determine the aggregate structure and stability of the organometallic species. The rate of enantiomerization of different substituted chiral organometallic compounds is being determined. This research is providing valuable information on the role that different functional groups play on the configurational stability of chiral organometallic compounds.

• We are determining the reactivity of these compounds and their mode of reaction with electrophiles, be it with retention, inversion or racemization of configuration.

• For their reactions, we are investigating intermolecular quenches with a range of electrophiles and we have studied intramolecular carbolithiation reactions.

Lithiation then electrophilic quench

We have been studying the lithiations of saturated (and partially saturated) nitrogen heterocycles such as pyrrolidines, piperidines, and piperazines. The idea is to lithiate α- to the nitrogen atom to give a chiral organolithium intermediate. This intermediate is not isolable but can be trapped with electrophiles to give a variety of novel substituted products.

The lithiation chemistry has been aided by NMR and in situ IR spectroscopy. These methods allow us to monitor the rate of rotation of the tert-butoxycarbonyl (Boc) group attached to the nitrogen atom. The rate of this rotation is important as only one rotamer, with the carbonyl oxygen atom pointing towards the benzylic position, undergoes lithiation with n-BuLi.

For example, with N-Boc-tetrahydroisoquinoline we found that using a temperature of –50 °C was preferable to –78 °C as the slow rotation of the Boc group otherwise reduces the rate of lithiation at the 1-position. Addition of electrophiles (R-X) then provides 1-substituted tetrahydroisoquinoline products. Removal or reduction of the Boc group leads to alkaloids, such as laudanosine (Scheme 1). 

Chiral organolithium chemistry

We have developed asymmetric methodology with chiral organolithium compounds. Excellent results have been obtained for the dynamic resolution of 2-lithiopyrrolidines and 2-lithiopiperidines. The theory behind the chemistry is shown in Scheme 2.  

Addition of a chiral ligand L*, such as (–)-sparteine or diaminoalkoxide 1 to a chiral, racemic organolithium species, followed by quenching with different electrophiles gives enantiomerically enriched products.

The key issues are the configurational stability of the organolithium compound (relative to its rate of reaction) and the steric course of the reaction (retention or inversion). Asymmetric induction can typically be achieved either by the formation of an enantiomerically enriched organolithium compound that maintains its optical purity (no racemisation) or by asymmetric substitution of an organolithium compound that is complexed to a chiral ligand. In the latter case, the chiral ligand renders the organolithium complexes diastereomeric. 

One of these complexes may be present to a greater extent (thermodynamic resolution) and/or one may react with the electrophile faster than the other (kinetic resolution). If the organolithium complexes can interconvert then there is a dynamic resolution and it is possible to start with racemic organolithium species and prepare, in high yield, enantiomerically enriched products.

With N-alkyl-2-lithiopyrrolidines and N-Boc-2-lithiopiperidine, good selectivities have been achieved by a dynamic resolution under thermodynamic control (sometimes called dynamic thermodynamic resolution, DTR). The enantiomer ratio of the products is a reflection of the greater proportion of one diastereomeric complex of the organolithium species over the other. 

There is, however, some kinetic component (minor complex reacts faster) that can be observed when the reaction is quenched with less than one equivalent of the electrophile or when it is quenched slowly at temperatures in which dynamic equilibration is taking place. This chemistry has significant potential for the asymmetric synthesis of substituted chiral amines.

Kinetic resolutions with organolithium chemistry

We have developed our chiral organolithium chemistry to include chiral starting materials. This allows us to carry out kinetic resolution reactions to give enantiomerically enriched compounds. 

In this chemistry we use a chiral base [n-butyllithium complexed with the chiral diamine (–)- or (+)-sparteine]. This base prefers to remove a proton from one enantiomer of the starting material, therefore leaving behind the other enantiomer that is unreacted. 

The lithiated intermediate can be trapped with an electrophile and this allows easy separation of the trapped product from the unreacted starting material. Both compounds can be formed with high enantioselectivity. 

For example, N-Boc-2-phenyltetrahydroquinoline can be resolved with extremely high levels of selectivity, as shown in Scheme 3.

Articles

S.-H. Yeo, A. Choi, S. Greaves, A. J. H. M. Meijer, I. Proietti Silvestri, I. Coldham, Chem. Eur. J. 2023, 29, e202300815.

'Kinetic Resolution of 2-Aryldihydroquinolines using Lithiation – Synthesis of Chiral 1,2- and 1,4-Dihydroquinolines'

A. Das, A. Choi, I. Coldham, Org. Lett. 2023, 25, 987–991.

'Photocatalysis and Kinetic Resolution by Lithiation to give Enantioenriched 2-Arylpiperazines'

DOI: 10.1021/acs.orglett.3c00074

A. Choi, A. El-Tunsi, Y. Wang, A. J. H. M. Meijer, J. Li, X. Li, I. Proietti Silvestri, I. Coldham, Chem. Eur. J. 2021, 27, 11670–11675.

'Asymmetric Synthesis of 2-Arylindolines and 2,2-Disubstituted Indolines by Kinetic Resolution'