Reductive cleavage reactions

TABLE 2. Reductive cleavage reactions of sulphones induced by lithium in methylamine or ethylamine... 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

The mechanism of the thermal 5 rnI reaction, using 4-nitrocumyl chloride and 2-nitropropanate ion as a model has been investigated. The results provided unambiguous evidence that a decrease in driving force is able to change the mechanism of homogeneous reductive cleavage reactions from stepwise to concerted. ... 

Epoxy-l-haloalkenes undergo novel reductive cleavage reactions with samarium diiodide to give cyclopropanols or allylic alcohols depending upon the substrates and reaction conditions. Formation of the cyclopropanol is believed to proceed through a diradical intermediate <1998SL1073>. 

The following discussion will be focused on the effect of structural variations on the kinetics of fragmentation, the cleavage mode, and the stepwise or concerted nature of the process. Similar concepts have also been applied to the reductive cleavage reactions of other series of substrates, i.e. a-substituted acetophenones [246-250], perbenzoates [240, 251], peroxides [242, 252-256], sulfides [244, 257, 258], sulfonium salts [259-263], nitrocumenes [264, 265], arylmethyl aryl ethers and thioethers [266-270]. 

The stereochemical problem was solved by application of the reductive cleavage reaction. Treatment of 0,0-dimethylliensinine (LXIV) with sodium in liquid ammonia afforded (— )-0-methylarmepavine (I) and the (— )-A,0-dimethylcoclaurine IV. Accordingly (Section I), lien-sinine was assigned the RR-configuration as shown in the formula (39). 

Saa, J. M., Ballester, P., Deya, P. M., Capo, M., Garcias, X. Metal-Induced Reductive Cleavage Reactions An Experimental and Theoretical (MNDO) Study on the Stereochemical Puzzle of Birch and Vinylogous Birch Processes. J. Org. Chem. 1996, 61,1035-1046. 

The spiroalkanones formed by these reductive cleavage reactions have been exploited in sesquiterpene syntheses. 

The rates for reductive cleavage reactions are often significantly less than diffusion controlled, as might be anticipated for any reaction where a relatively strong bond is cleaved. 

Unprotected tetrono- and pentonolactone undergo an c/.-deoxygenation reaction mediated by Sml2 in THF-H20 [111] (Scheme 49). The NO reductive cleavage reaction is also accelerated by addition of water [86]. 

Figure 15 Cleavage of S-adenosyl-L-methionine by enzymes within the radicai SAM superfamiiy. The reductive cleavage reaction generates a 5 -deoxyadenosyl 5 -radical and L-methionine, a spectator in the reaction. The physiological electron donor is the flavodoxin/flavodoxin reductase reducing system, with electrons deriving ultimately from NADPH. Artificial electron donors such as sodium dithionite and 5-deazaflavin plus light can also effect reduction in in vitro activity determinations.

As discussed in Section 3.7.B, ozone adds to an alkene to generate a 1,2,3-trioxolane. Addition of ozone to 1-pentene, for example, generated 425. Rearrangement of this initially formed cycloadduct is facile, even at temperatures as low as -78°C, and results in cleavage of the carbon-carbon bond in 425, with formation of a 1,2,4-trioxolane (426). This product is usually called an ozonide and can be either oxidized or reduced to give the carbonyl compounds characteristic of the oxidative or reductive cleavage reactions. 

Metal-amine reducing agents and other dissolving metal systems can bring about a variety of reductive cleavage reactions, some of which are useful in synthesis. 

The 4,6-O-benzylidene acetal group, which is frequently used as a protecting group in aldohexoses, can be directly transformed into 4-0-benzoyl-6-bromo derivatives by a radical-mediated cleavage reaction, and 4-0-benzyl-6-hydroxy derivatives by a reductive cleavage reaction, respectively. Por example, reaction of 2-deoxy-glucopyranoside derivative 104 with JV-bromosuccinimide (NBS) smoothly afforded 6-bromo-4-0-benzoyl derivative 105 (Scheme... 

The rate of the formation of R(C=0)Co(CO)3 from R(C=0)Co(CO)4 has been determined from the kinetics of the substitution reactions with PPhs (188,207,216,217) and with (218), and from the kinetics of the reductive cleavage reactions with H2 and HCo(CO)4 (188,207) and HMn(CO)5 (215). For some examples of ki values, see Tables 15 and 18. Reliable quantitative information about the equilibrium constant has not been published yet. 

Thioaldehydes and Thioketones. (i) Synthesis of simple aliphatic thioalde-hydes and thioketones by reductive cleavage reactions of unsaturated sulphides. (ii) Photochemistry of thioketones. (iii) Synthesis of... 

Oae and co-workers have reported several reductive-cleavage reactions of sulphidimines. A series of iV-tosylimines were cleaved by DMSO, cyanide ion, and diethyl thionothiophosphate, in all cases producing the nitrogen fragment of the original imine intact (Scheme 2). The latter reaction seems an especially effective and useful reduction method. 

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