Radical chlorination stereochemistry

The iodination and bromination of intermediate alkylboranes proceed with clean inversion of configuration whilst the radical chlorination reaction leads to loss of stereochemistry (Figure B2.4). 

Free radical chlorination of (i)-2-fluorobutane gives a 31% yield of 2-chloro-2-fluorobutane. What is the expected stereochemistry of the product ... 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

Do j5-substituents effect the stereochemistry of the cyclopropyl radical In order to evaluate the j -substituent effect let us examine a cyclopropyl radical that can maintain its configuration, such as one with an a-fluoro as a substituent, and determine whether a P-substituent will alter its configurational stability. Inspecting the data in Table 11 one would have to conclude that j -substituents such as methyl, phenyl and ether groups have no effect on the stereochemistry of the cyclopropyl radical Also, chlorine as a )S-substituent does not have any effect on the stereochemistry. Jefford and coworkers have shown that LiAlH4 reduction of the tricyclic compound 80 (R=H, Cl) gave the same ratio (2 1) of anti-... 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

Several investigations of the redox properties of various free base hydroporphyrins and their metal derivatives have been reported. As is typical of many porphyrins and metalloporphyrins, these hydroporphyrins generally show two oxidations and one or more reductions. The reversibility of these redox reactions depends on the nature of the hydroporphyrin and its stereochemistry. For example, the cyclic voltammograms of ris-H2(OEC) and frans-H2(OEC) were superficially alike, although substantial differences existed in the stability of the cation radicals and dications of the cis and trans isomers [85]. The first oxidation of rrans-H2(OEC) was reversible whereas ds-H2(OEC) was not reversible. However, the notable features observed in the redox chemistry of hydroporphyrins is the shift of both oxidation and reduction potentials of hydroporphyrins towards more negative values compared to porphyrins, i.e., they are more easier to oxidise and difficult to reduce [78]. A significant trend was observed in the electrochemistry of free base octaethyl- [86, 87] and tetraphenyl [88,89] hydroporphyrins (Table 2). The porphyrin and chlorin of each series... 

Source C. Walling and W. Thaler, Positive Halogen Compounds. III. Allylic Chlorination with t-Butyl Hypochlorite. The Stereochemistry of Allylic Radicals, Journal of the American Chemical Society 83 3877-3884 (1961). With permission. 

The results also indicate that the chlorine and bromine abstraction steps are faster than rotational equilibration, so the stereochemistry can be explained without requiring a bridged radical. 

Dichlorination of cyclopentane under free-radical conditions is not a realistic approach to the introduction of two chlorines in a trans-1,2 relationship without contamination by isomeric dichlorides. Vicinal dichlorides are prepared by electrophilic addition of chlorine to alkenes. The stereochemistry of addition is anti. 

The stereochemistry of certain thermal reactions involving atom addition processes and resultant radical intermediates can also be studied conveniently by recoil techniques. In this type of experiment the recoil atom used to initiate the process must be thermalized before reaction, by diluting the system with a large concentration of nonreactive bath molecules. Two recently reported results on geometric isomerization serve as examples of the type of processes that can be studied (55,56). Both involve recoil chlorine atom reactions with unsaturated substrates. 

The Stereochemistry of Chlorination at C3 of (S)-2-Chloropentane 473 Anti-Markovnikov Addition of HBr 481 Radical Polymerization of Ethene (Ethylene) 484... 

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