Atom-transfer reagents

Dimesityldioxirane, a crystalline derivative, has been isolated by Sander and colleagues and subjected to X-ray analysis. The microwave and X-ray data both suggest that dioxiranes have an atypically long 0—0 bond in excess of 1.5 A. Those factors that determine the stability of dioxiranes are not yet completely understood but what is known today will be addressed in this review. A series of achiral, and more recently chiral oxygen atom transfer reagents, have been adapted to very selective applications in the preparation of complex epoxides and related products of oxidation. A detailed history and survey of the rather remarkable evolution of dioxirane chemistry and their numerous synthetic applications is presented in Chapter 14 of this volume by Adam and Cong-Gui Zhao. Our objective in this part of the review is to first provide a detailed theoretical description of the electronic nature of dioxiranes and then to describe the nuances of the mechanism of oxygen atom transfer to a variety of nucleophilic substrates. 

Dioxirane chemistry is well documented . Extensive kinetic, stereochemical and 0-labeling data suggested that dimethyldioxirane is the oxygen-atom transfer reagent in the Oxone-acetone system . Murray and Jeyaraman have shown that dialkyl dioxiranes can be isolated by low-temperature distillation from the reaction mixture of oxone and ketone . 

Nitrous Oxide (N20). In contrast to nitric oxide, N20 is not readily reduced by hydrogen atoms. However, it is a good O-atom transfer reagent and spontaneously reacts with metal surfaces at elevated temperatures. Under alkaline conditions with a freshly formed platinum surface (Pt ), N20 induces a vol-tammetric reduction peak (Figure 11.8) 10... 

H-atom transfer reagents [Bu3SnH, (Me3Si)3SiH see Section 1.10] and... 

The most useful reaction in the literature for this is the pyrolysis of a suitable per-ester in the presence of a hydrogen atom transfer reagent, but the yields are often unsatisfactory.2 8 Conversion of an acid to the corresponding aldehyde and subsequent rhodium-based decarbonylation involves two steps, but is more reliable.4 The Borodin-Hunsdiecker reaction converts the acid to a nor-halide, which can be reduced by radical methods. However, this works well only with primary acids, is incompatible with many sensitive functions,5 and is expensive since the Ag salt of the acid is usually used. 

This type of chemistry is well suited for the manipulation of amino acids and peptides which tend to undergo racemization at the a carbon under ionic conditions. First examined was the decarboxylation of A-protected amino acids by using a thiol as hydrogen atom transfer reagent, Scheme 25.71 The power of... 

The N-hydroxypyridine-2-thione esters of Boc-protected amino acids undergo decarboxylation on photolysis in the presence of a hydrogen-atom-transfer reagent such as t-butyl mercaptan in 80-95% yield. Decarboxylation of the side-chain carboxy groups of suitably protected aspartic and glutamic acids can also be effected, but yields are lower (50-80%). ... 

Second, the iodine in I O should tend to attack the softer atom, I, rather than the harder atom, O. Third, because both HOIO and OI2 have not been synthesized and are probably equally unstable, most likely the formation of the thermodynamically more stable OH" (AGr° = -157.3 kj/mol) is favored over I (AGr° = -51.59 kj/mol) in the intermediate step. The hydrolysis of OI2 should be similar to that for I2 H20 and OH lead to the formation of I02. I02 is also unstable in solution and should react quickly to form I03 with oxidants (e.g., 03 and H202) that are effective oxygen-atom-transfer reagents or with HOI in a reaction sequence similar to equation 9a. 

Ozone is also a better-oxygen-atom-transfer reagent in the atmosphere because UV light results in the formation of 02 and O atoms, which react with halides. The role of metals in the catalysis of iodide oxidation is also more likely in the atmosphere because of the enrichment of iodide relative to the other halides in rainwater and aerosols (13). 

Du Bois J, Tomooka CS, Hong J, Carreira EM (1997) Nitridomanganese(V) complexes design, preparation, and use as nitrogen atom-transfer reagents. Acc Chem Res 30 364-372... 

The ability of bromomalononitrile to transfer Br to a stabilized allylic radical further illustrates the high reactivity of malononitrile-based atom transfer reagents. Giese has demonstrated successful Br-transfer additions to enol ethers with bromomalonates in the presence of BujSnH. The a-bromo ethers thus generated underwent spontaneous loss of HBr to regenerate enol ether functionality (Scheme 12) [38]. 

Figure 22 Proposed mechanism for O2 evolution from [(H20)(terpy)Mn(0)2Mn(terpy)(0H2)]. The terpy ligands have been omitted for clarity. In this mechanism, [(H20)(terpy)Mn(0)2Mn(terpy)(0H2)] (39) reacts with an O-atom transfer reagent (denoted XO, such as HSOs") to form an adduct (40). Scission of the X-O bond forms a terminal-oxo species (41) that can react with either XO or H2O to form O2. Reaction of (41) with H2O would be expected to form the (II/III) complex (42), which would be rapidly oxidized by XO back to (39). ...

Oxathiocins obviously remain exceedingly scarce. Cyclization routes provide access, in modest yields, to the ring system, but may be limited by the inaccessibility of appropriate precursors. The use of sulfur atom transfer reagents to effect the cyclization of diols may represent a more general entry to the 1,2-oxathiocin ring system, but the synthetic potential of this route clearly has scarcely been examined. 

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