# Diastereoselective Reformatsky Reaction Mediated by Dichlorocyclopentadienyltitanium(III)

## TL;DR

A new titanium-based catalyst improves the selectivity of a classic chemical reaction for building carbon-carbon bonds.

## Contribution

A diastereoselective Reformatsky reaction using CpTiCl2 is introduced, achieving high syn-isomer selectivity.

## Key findings

- CpTiCl2 promotes β-hydroxy ester formation with up to 100:0 syn:anti diastereoselectivity.
- The catalyst works under mild conditions and tolerates diverse substrates like aldehydes and α-iodonitriles.
- The mechanism involves a Zimmerman–Traxler transition state favoring syn stereocontrol.

## Abstract

The Reformatsky reaction, first reported in 1887, has long been recognized as a fundamental method for carbon–carbon bond construction due to its mild conditions and functional group tolerance. Over the past few decades, this transformation has undergone a notable revival, with modern catalytic variants addressing limitations of stoichiometric protocols and expanding its role in complex molecule synthesis. Yet, despite its versatility, achieving stereoselective control remains a longstanding challenge. Herein we report the use of dichlorocyclopentadienyltitanium(III) (CpTiCl2), generated in situ from CpTiCl3 and manganese, as an efficient catalyst for Reformatsky-type couplings of aldehydes with α-haloesters and α-iodonitriles. Under mild conditions, CpTiCl2 promotes the formation of β-hydroxy esters in high yields and with significant diastereoselective preference for the syn isomer (up to 100:0 syn:anti). This behavior contrasts sharply with the poor or anti-selective outcomes previously observed with titanocene(III) chloride (Cp2TiCl). Mechanistic analysis suggests that the unique steric and electronic environment of CpTiCl2—characterized by enhanced Lewis acidity and increased coordination vacancies—favors a Zimmerman–Traxler-type transition state that enforces syn stereocontrol. The methodology tolerates a wide variety of substrates, including aliphatic and aromatic aldehydes as well as α-iodonitriles, extending the scope of titanium-mediated Reformatsky chemistry. These findings establish CpTiCl2 as a sustainable, selective, and robust organotitanium catalyst for stereoselective carbon–carbon bond formation, providing a promising alternative to the Nugent reagent and paving the way for new applications in complex molecule synthesis.