Lead, dehalogenation

The kinetic data for the structures in Schemes II and III lead to the following relationships of the rates of piperidino-dehalogenation (type of activation given in parentheses ind. = inductive activation, res. = resonance activation) ... 

However, cyclopentanone 3-109 with a cis orientation of the iodoalkane group and the alkyne moiety was converted into the fused cyclooctanone 3-110 in 82% yield (Scheme 3.28). In contrast, the corresponding trans-isomer only underwent 1,5-hydrogen transfer, leading to a dehalogenated starting material. 

In outer-sphere SET reductions (e.g., in electrochemical dehalogenations), hydrogen abstraction by R leads to the product RH (i.e., no step related to (ii) is required to occur). Process (ii) follows generally the activation of the substrate in the proposed hydrodehalogenation cycles, but we know also of opposite examples [77, 82, 106, 112],... 

The product-forming steps of dehalogenations by free radical pathways were discussed earlier (see Section 18.3.1.1). In non-radical mechanisms, the dehalo-genated products (RH) will be formed mostly by reductive elimination [193, 194] however, concerted processes lead directly from RX to RH (see Sections 18.3.1.2 and 18.3.1.3). 

Reaction of the l-thia-2,5-diborole 63 with potassium leads to liquid 64a [85], and dehalogenation of (Z)-2,3-bis(dichloroboryl)hexene with copper vapor or Na/K alloy results in the formation of crystalline 64b [86], both in low yields (Scheme 3.2-34). 

Dehalogenation of unsaturated organohalogenoboranes also leads to nido-Ctfii and nido-C4B4 carboranes. The nido-C5B cluster s apex boron forms very strong bonds with Lewis acids BX3. 

An alternative route to sulphones utilizes the reaction of the appropriate activated halide with sodium dithionite or sodium hydroxymethanesulphinite [6], This procedure is limited to the preparation of symmetrical dialkyl sulphones and, although as a one-step reaction from the alkyl halide it is superior to the two-step oxidative route from the dialkyl sulphides, the overall yields tend to be moderately low (the best yield of 62% for dibenzyl sulphoxide using sodium dithionite is obtained after 20 hours at 120°C). The mechanism proposed for the reaction of sodium hydroxymethanesulphinite is shown in Scheme 4.20. The reaction is promoted by the addition of base and the best yield is obtained using Aliquat in the presence of potassium carbonate. It is noteworthy, however, that a comparable yield can be obtained in the absence of the catalyst. The reaction of phenacyl halides with sodium hydroxy-methane sulphinite leads to reductive dehalogenation [7]. 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

The synthetic scheme for functionalized indolines shown in equation 83 assumes formation of a doubly metallated intermediate (335), derived from V,iV-diallyl-2,6-dibromo-p-toluidine, that may be quenched to the dehalogenated toluidine 336, or may undergo cyclization to 337. Quenching of 337 with trimethylchlorosilane in the presence of TMEDA leads to formation of indoline derivatives 338 and 339. Apparently a second cyclization of intermediate 337 to compound 340 is hard to accomplish . 

The low yields in the synthesis of electron-rich biaryls, which are the more frequently occurring natural biaryl targets, are clearly a limitation to this procedure. Further side reactions may arise from hydro-dehalogenation without biaryl coupling, and from intermolecular reactions, leading to dimers or oligomers34,35. 

Dehalogenative homocoupling of other heteroaromatic compounds also leads to the corresponding polymers 195-203 141—148... 

The term dehalogenation generally denotes any reaction in which one or more halogen atoms leave a molecule, e.g. in reductive dehalogenation. In this section, dehalogenation is used in the sense of 1,2-didehalogenation leading to the formation of a multiple bond. 

Also obscure is the nature of the reaction of dihalosilanes with active metals. Further experiments should show under what circumstances, if any, such dehalogenation reactions lead to silylenes, or to silylenoids, another area of likely future activity. In a related area, isolable silylenoids are quite new, and further development of their chemistry can be anticipated. 

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