Barton reaction

Barton reaction
Name	Barton reaction
Other names	Barton nitrite ester photolysis
Type	Photochemical organic reaction
Named after	Sir Derek H. R. Barton
Year	1960s

Barton reaction The Barton reaction is a photochemical organic transformation that effects the replacement of a hydroxyl-derived nitrite ester with a remote radical-derived functional group, typically yielding C–H functionalization products. It features radical-mediated 1,5-hydrogen atom transfer and radical recombination steps that made it influential in Organic chemistry and Total synthesis strategies for manipulating complex molecules. The reaction played a central role in connecting photochemistry pioneered by researchers associated with Royal Society-era laboratories to practical synthetic campaigns in academic groups at institutions such as University of Oxford and Harvard University.

Introduction

The Barton reaction converts alcohols into nitrite esters which, upon photolysis, undergo homolytic cleavage to give oxygen-centered radicals that abstract hydrogen atoms at remote carbon sites, generating carbon-centered radicals that recombine to provide functionalized products. Its development arose alongside advances in photochemical methods promoted by labs at Cambridge University and California Institute of Technology. The transformation is distinctive for enabling selective C–H activation in substrates encountered in syntheses of complex natural products such as those studied by groups at Scripps Research Institute and Massachusetts Institute of Technology.

Mechanism

Photolysis of a nitrite ester initiates homolysis of the O–N bond to give an alkoxy radical and nitric oxide-derived fragment; the alkoxy radical undergoes 1,5-hydrogen atom transfer (1,5-HAT) to form a stabilized carbon radical positioned remotely from the original oxygen. That carbon radical can then recombine with nitrogen-oxygen fragments or be trapped by external reagents to install functional groups. Key mechanistic concepts appeared in the context of radical chain processes investigated at Max Planck Society and mechanistic probes influenced by researchers connected to Royal Institution. Kinetic and spectroscopic analyses performed in laboratories at University of Chicago and Columbia University supported the concerted nature of 1,5-HAT and the involvement of short-lived radical intermediates. Competing pathways include β-scission of the alkoxy radical and alternative HAT distances (1,4- or 1,6-HAT) that were explored in collaborations with research groups at ETH Zurich.

Scope and Variations

The original protocol applied to primary, secondary, and tertiary alcohols converted to nitrite esters using reagents developed in synthetic laboratories at University of Cambridge and Stanford University. Variants include intermolecular trapping of the carbon radical with oxygen, sulfur, or nitrogen nucleophiles—strategies refined in projects at Yale University and University of California, Berkeley. Modified approaches employ visible-light photocatalysis pioneered by groups at University of North Carolina at Chapel Hill and Princeton University to enable milder activation, while metal-mediated adaptations echo work from Columbia University and University of California, Los Angeles. Applications extend to steroidal substrates, terpenoids, and polyketide fragments studied in laboratories at Johns Hopkins University and University of Illinois at Urbana–Champaign.

Stereochemistry and Regioselectivity

Regioselectivity in the Barton reaction is governed by the favored 1,5-HAT geometry that mirrors transition-state preferences analyzed in computational studies from Imperial College London and University of Tokyo. Stereochemical outcomes depend on the face selectivity of radical recombination and on conformational bias present in substrates such as cyclic systems studied at University of Cambridge and University of Oxford. Diastereoselectivity improvements were pursued by teams at University of Pennsylvania and Karolinska Institute, which employed directing groups and substrate preorganization to favor specific radical approach trajectories. Computational and experimental synergy from Los Alamos National Laboratory and Argonne National Laboratory clarified the contributions of polar effects and hyperconjugation to selectivity.

Synthetic Applications

Synthetic practitioners have used the Barton reaction in the construction and late-stage modification of natural products and pharmaceuticals in programs at Scripps Research Institute, Harvard Medical School, and Pfizer. Notable applications include selective oxidation or functionalization enabling rearrangements and ring contractions in complex frameworks similar to targets pursued by labs at University of California, San Diego and Max Planck Institute for Coal Research. The methodology complements C–H activation strategies developed at Stanford University and ETH Zurich, and has been integrated into cascade sequences and convergent fragment couplings in synthetic campaigns at University of Michigan.

Experimental Conditions and Safety

Typical conditions involve conversion of an alcohol to the corresponding nitrite ester using nitrosyl donors handled under protocols standardized by safety offices at National Institutes of Health and irradiating with ultraviolet or visible light in equipment supplied by vendors associated with Royal Society of Chemistry-recommended lab standards. Due to the potential release of nitric oxide and other toxic gases, work should be conducted in well-ventilated hoods following guidelines from Occupational Safety and Health Administration and institutional environmental health and safety offices such as those at University of California, Berkeley. Photochemical setups often reflect instrumentation developed at Oxford Instruments and custom reactors described in methodology papers from American Chemical Society publications.

Historical Background and Discovery

The transformation was introduced and popularized by Sir Derek H. R. Barton while he held positions associated with Imperial College London and later influenced research at University of Oxford. The discovery emerged during an era of expanding radical and photochemical methods that included contemporaneous advances from investigators at University of Cambridge, Caltech, and ETH Zurich. The reaction’s impact contributed to Barton’s recognition, alongside other achievements that culminated in awards such as the Nobel Prize in Chemistry and shaped subsequent generations of synthetic strategy research at institutions including University of Sheffield and University of Glasgow.

Category:Organic reactions