Boron-Catalyzed, Diastereo- and Enantioselective Allylation of Ketones with Allenes

Borane-catalysed C2-selective indole reductive functionalisation

Indolines are common motifs within pharamceuticals and natural products. Boron catalysis enables the chemoselective allylation of indoles to give allylic indolines in excellent diastereoselectivity. Mechanistic studies revealed in situ formation of the allylic borane, allylation of the imine tautomer of the indole and B-N/B-H transborylation for catalytic turnover.

Recent Advances in Asymmetric Catalysis Using p-Block Elements

The development of new methods for enantioselective reactions that generate stereogenic centres within molecules are a cornerstone of organic synthesis. Typically, metal catalysts bearing chiral ligands as well as chiral organocatalysts have been employed for the enantioselective synthesis of organic compounds. In this review, we highlight the recent advances in main group catalysis for enantioselective reactions using the p-block elements (boron, aluminium, phosphorus, bismuth) as a complementary and sustainable approach to generate chiral molecules. Several of these catalysts benefit in terms of high abundance, low toxicity, high selectivity, and excellent reactivity. This minireview summarises the utilisation of chiral p-block element catalysts for asymmetric reactions to generate value-added compounds.

Copper-Catalyzed Enantioselective and Regiodivergent Allylation of Ketones with Allenylsilanes

We report herein a regiodivergent and enantioselective allyl addition to ketones with allenylsilanes through copper catalysis. With the combination of CuOAc, a Josiphos-type bidentate phosphine ligand and PhSiH3 , allyl addition to a variety of ketones furnishes branched products in excellent enantioselectivities. The regioselectivity is completely reversed by employing the P-stereogenic ligand BenzP*, affording the linear products with excellent enantioselectivities and good Z-selectivities. The linear Z-product could be converted to E-product via a catalytic geometric isomerization of the Z-alkene group. The silyl group in the products could provide a handle for downstream elaboration.

Group 13 exchange and transborylation in catalysis

Catalysis is dominated by the use of rare and potentially toxic transition metals. The main group offers a potentially sustainable alternative for catalysis, due to the generally higher abundance and lower toxicity of these elements. Group 13 elements have a rich catalogue of stoichiometric addition reactions to unsaturated bonds but cannot undergo the redox chemistry which underpins transition-metal catalysis. Group 13 exchange reactions transfer one or more groups from one group 13 element to another, through σ-bond metathesis; where boron is both of the group 13 elements, this is termed transborylation. These redox-neutral processes are increasingly being used to render traditionally stoichiometric group 13-mediated processes catalytic and develop new catalytic processes, examples of which are the focus of this review.

Transborylation-Enabled Boron Catalysis

This review highlights transborylation (controlled boron-boron exchange) and its applications as a turnover strategy in boron-catalysed methodologies. Catalytic applications of B–C, B–O, B–N, B–F, B–S, and B–Se transborylations are discussed in the context of transborylation-enabled catalysis, across a wide range of organic transformations including hydroboration, C–C bond formation, C–H borylation, chemoselective reduction, and asymmetric reduction.

1 Introduction

2 B–C Transborylation

3 B–O Transborylation

4 B–N Transborylation

5 B–F Transborylation

6 B–S Transborylation

7 Conclusion