Abstraction halogen

The design of an efficient chain reaction to facilitate the reduction of secondary alkyl iodides adjacent to electron-withdrawing groups has been accomplished by reaction 

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TABLE 4. Absolute kinetic data for halogen abstraction by some radicals from sulfonyl halides... 

The presence of an OR or SiR3 substituent P to the carbon bearing the radical accelerates the rate of halogen abstraction. ... 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

It is also worth emphasizing that the initiation and termination steps are not included in the central chain process. For instance, in metal hydride-promoted domino reactions the initial halogen abstraction (or SePh displacement, etc.) and the final hydrogen abstraction from R MH are not classified as part of the domino sequence. More precisely, only the propagation steps within the mechanism of this process will be considered as a strict integral part of the domino reaction. 

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

Type II may deactivate simply by reverting back to a tr-alkyl (I), followed by termination via a Ziegler route, namely halogen abstraction from a metal, or rearrangement to a 7r-olefin complex according to Eq. (15) ... 

Halogen abstraction from hypervalent sulfur halides has also been reported for the imidazol-2-ylidenes (IV) (Scheme 8.22). This reaction gives a nice example of the synthetic utihty of V-heterocyclic carbenes. Indeed, this adduct is the first structurally characterized derivative featuring the chlorosulfite ion (SO2CI ). ... 

Solvato complexes of platinum(II) of the type fraws-[PtY(solvent)L2]+ (Y = hydride, alkyl, or aryl solvent = alcohol or ketone L = tertiary phosphine or arsine) have been known since 1961.1 They are obtained by halogen abstraction from the corresponding halo complexes tran.s-[PtXYL2] in the presence of the desired solvent.2 The methanol complex is also rapidly and quantitatively formed when trans-[PtH(N03)(PEt3)2] is dissolved in this solvent.2... 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

The Co(II) compounds react with alkyl halides according to Equation 1. The initial and rate-determining step is halogen abstraction (14). For L = phosphines... 

Similarly, NO+ is also capable of halogen abstraction from alkyl halides.513 514 In the presence of a suitable oxygen donor such as dimethylsulfoxide, nitrosonium ion can act as a nitrating agent493,494 [Eq. (5.194)]. 

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