Reductive Dehalogenations

To a stirred solution of the iodolactone (296 mg, 0.74 mmol) in dry benzene (6 mL) was added n-tributyltin hydride (0.40 mL, 1.5 mmol) and a catalytic amount of AIBN. The mixture was heated at reflux for 30 min. It was then cooled to room temperature and concentrated to approximately 2 mL under reduced pressure. Flash chromatography (50 g of silica gel, 2 1 ethenpetroleum ether then 4 1 CH2Cl2 ether of the crude product) provided 180 mg (89%) of the lactone as a colorless solid. 

A solution of the dichlorolactone (1.5 g, 2.15 mmol) and freshly prepared tributyltin hydride (1.3 mL, 4.9 mmol) in benzene (20 mL) was heated at 78 °C for 15 min. in the presence of a catalytic amount of Et3B (0.7 mL of a 1 M solution in THF, 0.7 mmol). After evaporation of the solvent in vacuo, the residue was dissolved in ether (25 mL). The ether solution was treated with a saturated aqueous solution of potassium fluoride (25 mL). The precipitated tri-n-butyltin fluoride was filtered and washed thoroughly with ether. The combined ether layers were dried over magnesium sulfate and evaporated in vacuo. The residue was chromatographed on silica gel using a gradient of eluents (hexanes EtOAc, 20 1 to 4 1) giving 1.27 g (92%) of the lactone as a clear oil. 

Reviews (a) Chatgilialoglu, C. Organosilanes in Radical Chemistry Principles, Methods, and Applications Wiley Sons, Ltd West Sussex, U.K., 2004. (b) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188-194. 

The unsaturated [3.3.1] bicycle (360 mg, 0.78 mmol), 10% Pd/C (130 mg, 0.12 mmol), and EtOAc (8 mL) were combined, and the reaction vessel was evacuated and back-filled with hydrogen (1 atm). The reaction mixture was stirred under hydrogen for 30 min and 

A suspension of Lindlar s catalyst [Pd (5% on CaC03), 213 mg, 0.10 mmol], quinolone (0.81 mL, 6.8 mmol), and the alkyne (587 mg, 2.19 mmol) in EtOAc (22 mL) was stirred for 30 min. The flask was charged with hydrogen (1 atm), and the reaction mixture was stirred until JH NMR analysis indicated complete conversion (14 h). The reaction mixture was filtered through Celite and concentrated in vacuo. The crude product was purified by flash chromatography on silica gel (elution with EtOAc/CH2Cl2, 1 5 then EtOAc) to give 487 mg (83%) of the a.v-olcfm as a white solid. 

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Nevertheless, an anaerobic system may be the method of choice under certain conditions (/) contamination with compounds that degrade only or better under anaerobic conditions, (2) low yield aquifers that make pump and treat methods or oxygen and nutrient distribution impractical, (J) mixed waste contamination where oxidizable compounds drive reductive dehalogenation of chlorinated compounds, or (4) deep aquifers that make oxygen and nutrient distribution mote difficult and cosdy. 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

The classic method for controlling stereochemistry is to perform reactions on cyclic substrates. A rather lengthy but nonetheless efficient example in the prostaglandin field uses bicyclic structures for this purpose. Bisacetic acid derivative S is available in five steps from Diels-Alder reaction of trans-piperylene and maleic anhydride followed by side-chain homologation. Bromolactonization locks the molecule as bicyclic intermediate Esterification, reductive dehalogen-... 

Two important examples of reductive metabolism of xenobiotics are the reductive dehalogenation of organohalogen compounds, and the reduction of nitroaromatic compounds. Examples of each are shown in Figure 2.13. Both types of reaction can take place in hepatic microsomal preparations at low oxygen tensions. Cytochrome P450 can catalyze both types of reduction. If a substrate is bound to P450 in the... 

Degradation of trichloroethylene by anaerobes via reductive dehalogenation can be problematic because a common product is vinyl chloride, a known carcinogen (Ensley 1991). In an anaerobic colunm operated under methanogenic conditions, 100% transformation of injected tetrachloroethylene and trichloroethylene to... 

There has been considerable interest in the abiotic dechlorination of chlorinated ethenes at contaminated sites. Reductive dehalogenation has therefore been examined using a range of reductants, many of them involving reduced complexes of porphyrins or corrins. 

Krone UE, K Laufer, RH Thauer, HPC Hogenkamp (1989) Coenzyme F43Q as a possible catalyst for the reductive dehalogenation of chlorinated C-1 hydrocarbons in methanogenic bacteria. Biochemistry 28 10061-10065. 

Liu Z, RG Arnold, EA Betterton, E Smotkin (2001) Reductive dehalogenation of gas-phase chlorinated solvents using a modified fuel cell. Environ Sci Technol 35 4320-4326. 

Totten LA, U Jans, AL Roberts (2001) Alkyl bromides as mechanistic probes of reductive dehalogenation reactions of vicinal stereoisomers with zerovalent metals. Environ Sci Technol 35 2268-2274. 

McGrath JE, CG Harfoot (1997) Reductive dehalogenation of halocarboxylic acids by the phototrophic genera Rhodospirillum and Rhodopseudomonas. Appl Environ Microbiol 63 333-335. 

The reductive dehalogenation of polyhalogenated methanes (Castro et al. 1985) and polyhalogenated ethanes (Li and Wackett 1993) by Pseudomonas putida strain PpG786. 

Li S, LP Wackett (1993) Reductive dehalogenation by cytochrome P450(,j jy[ substrate binding and catalysis. Biochemistry 32 9355-9361. 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

In a wider context, extracellular redox mediators have been implicated in a number of reductions. The specific role of reductive dehalogenation by porphyrins and corrins has been discussed in Chapter 1. 

Curtis GP, M Reinhard (1994) Reductive dehalogenation of hexachloroethane, carbon tetrachloride, and bromoform by anthrahydroquinone disulfonate and humic acids. Environ Sci Technol 28 2393-2401. 

Dennie D, I Gladu, E Lepine, R Villemur, J-G Bisaillon, R Beaudet (1998) Spectrum of the reductive dehalogenation activity of Desulfitobacteriumfrappieri PCP-1. Appl Environ Microbiol 64 4603-4606. 

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