Science of Synthesis

Zhu, H.; Fan, Q., Science of Synthesis Knowledge Updates, (2024) 3, 369.

General Introduction

This chapter provides a comprehensive supplement to the previous Science of Synthesis contribution on the preparation of ortho esters and halogenated derivatives (Section 22.7.2).[‌1‌] The first part focuses on the synthesis of 1,1-dihalogenated ethers and 1-halogenated acetals (Section ‌22.7.2.2.1‌). Protocols to 1,1-dihalogenated ethers mainly contain conversions of aliphatic formates, reactions through difluorocarbene precursors and the dichloromethyl radical, and difluoromethylations mediated by silver(I) fluoride and xenon difluoride (Sections ‌22.7.2.2.1.1‌–‌22.7.2.2.1.5‌). Dihalomethyl ethers have always been synthesized via difluorocarbene intermediates (Sections ‌22.7.2.2.1.2.1‌–‌22.7.2.2.1.2.6‌) and the dichloromethyl radical (Section ‌22.7.2.2.1.3‌). In recent years, various difluoromethylation reagents have been developed as effective replacements for chlorodifluoromethane, such as difluoromethyl trifluoromethanesulfonate, (bromodifluoromethyl)trimethylsilane, fluoroform, diethyl (bromodifluoromethyl)phosphonate, chlorodifluoromethyl phenyl ketone, and chlorodifluoromethyl phenyl sulfone. These reagents can react with substrates to afford difluoromethyl ethers under mild conditions with good substrate compatibility. Few examples have been reported about approaches to 1-halogenated acetals (Sections ‌22.7.2.2.1.6‌–‌22.7.2.2.1.8‌). Although direct fluorinations have always been alternative approaches, functional-group compatibilities are the key issues in most reactions. Desulfurative fluorination reactions are also widely used, involving hydrogen fluoride and bromine trifluoride. In addition, reactions using phosphorous pentachloride and chlorine–fluorine exchange reactions can give high yields of α,α-dihalogenated acetals.

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References