Barton reaction mechanism

There is quite some evidence for a mechanism as formulated above,especially for the six-membered transition state—the Barton reaction is observed only with starting materials of appropriate structure and geometry, while the photolysis of nitrite esters in general seldom leads to useful products formed by fragmentation, disproportionation or unselective intermolecular hydrogen abstraction. 

The solvent may serve only as the medium for the reaction, or it may in addition be a reactant, as in a solvolysis reaction. It is possible that the reaction mechanism may be changed by a change in solvent (e.g., from SnI to Sn2) or that the rate-determining step of a complex reaction may be altered. All of these phenomena can be studied by examining the solvent dependence. One goal of research on medium effects is to achieve a level of understanding that will allow us to make mechanistic interpretations from such data. Handbooks of solubility parameters are valuable (Barton, 1983). 

The mechanism of the Barton reaction may be represented by the three discrete steps shown in sequel A — D (Scheme II). First, the photochemical cleavage of the O-N bond in A to furnish the alkoxy... 

To explain the high chemical yield in the transformation A - D, coupled with the fact that the quantum yield for the photodecomposition of nitrites is less than unity, it was suggested that the Barton reaction might take place through a radical "cage mechanism.2 However, recent studies at the Institute on the mechanism of nitrite photolysis have shown this not to be so. Photolysis of an equimolecular mixture of 3/3-acetoxy-androstan-6/8-yl nitrite (102) and 3/8-acetoxy-cholestan-6/8-yf nitrite containing 98% of nitrogen as N16 (103) in iso-octane or toluene... 

Hydrogen peroxide is also the oxidant in the halogenations of alkanes under Gif conditions. In these systems, developed by Barton and his coworkers312, alkanes are selectively transformed into alkyl chlorides or bromides by polyhaloalkanes and H202 in the presence of FeCypicolinic acid catalyst in pyridine/acetic acid solvent313-315. It has clearly been established that the reaction mechanism does not involve a free-radical intermediate. 

The knowledge of the reaction mechanism and chemical kinetics is a necessary condition for the application of any branching theory. In this respect, the reader is referred to reviews in this volume by Barton, Fedtke, Lohse, Morgan, Rozenberg and some others. 

You may have noticed that hv may indicate either a nonchain or a chain mechanism. A good rule of thumb for distinguishing light-initiated nonchain and chain mechanisms is that unimolecular rearrangements or eliminations usually proceed by nonchain mechanisms, whereas addition and substitution reactions (and especially intermolecular ones) almost always proceed by chain mechanisms. However, there are a few exceptions to this rule (photochemical pinacol reaction, Barton reaction). Of course, many pericyclic reactions require light, too (Chapter 4) ... 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fragments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C-H bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

The nitrite (82), prepared from the jervine derivative (81) (Scheme 9), has been photolysed under the conditions of the Barton reaction to give, unexpectedly, the cyclic nitrone (83) the reaction may proceed by the mechanism shown in Scheme 10. 0-Acetyljervine has been photolysed and the major constituents of the complex mixture of products that was obtained have been characterized. ... 

Barton, K., and Beranek, E. (1959). Reaction mechanism of the atmospheric corrosion of metals in moist air contaminated with sulphur dioxide. Werkst. Korros., 10, 377-383 (in German). 

Fortunately, Bertrand was able to obtain nitrite esters of ethylenic alcohols as pure, characterized products. She assumed that photolysis of the nitrites would lead to the alkoxyl radical, as in the Barton reaction, with subsequent intramolecular addition. Isolation of tetrahydrofurfural oxime (62 R = H) starting from the nitrite of 4-pentene-l-ol (61 R = H) supported this hypothesis (Scheme 33). This mechanism was later confirmed by esr spin-trapping techniques and, independently by Rieke, who scavenged the cyclized radical with bromotrichloromethane. 

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