Radicals, anti-Markovnikov compounds

Azido-phenylselenenylations of olefinic compounds can be effected with BAIB/PhSeSePh/NaN3 in CH2C12 (Scheme 14) [37]. Such additions proceed in anti-Markovnikov fashion and appear to be initiated by addition of the azido radical to the C,C-double bond. While cyclohexene and ds-4-octene gave 3 1 and 2 1 mixtures of diastereomeric adducts under these conditions, dihydropy-ran was converted cleanly to the trans-addition product. Regioselective azido-phenylselenenylations of dihydropyran derivatives and O-protected glycals with this reagent have also been documented [21,38,39]. 

The results obtained57 were explained by competition of ionic and radical mechanisms, which lead to Markovnikov and anti-Markovnikov adducts, respectively. At this competition the nature of the unsaturated compound play an important role in determining the preferred mechanism. Thus, the major formation of Markovnikov adducts and therefore the preference of the ionic mechanism in the series of olefins styrene > 1-methylcyclohexene > 2,3-dimethyl-1-butene > isobutene > 1-heptene correlates with the ability of substituents to stabilize the intermediate carbenium ion. 

The photoaddition of secondary phosphines onto alkenes is one of the straightforward approaches for the synthesis of organophosphorus compounds. An anti-Markovnikov phosphorus-centered free radical addition onto the olefin has been... 

Only compound XII was observed, which illustrates once more the anti-Markovnikov regioselectivity of the addition reaction. However, the divergent fate of intermediate XI must be governed by the relative rates of the two competing processes A and B (Scheme 40.2), so the radical mechanism cannot be dismissed on the basis of this experiment alone [Eq. (2) of Scheme 40.2]. It has been estimated that the rate of step B is of the order of k = 3 X 10 s" and the activation energy is only 13 kcal/mol. This means that if the mean life time of XI—before it is trapped by I—is shorter than 300 thousandths of a second, pathway B will be blocked and only compound XII will be produced. 

Radical addition to alkenes is usually difficult, except when addition occurs to conjugated carbonyl compounds (15-24). An important exception involves radicals bearing a heteroatom a to the carbon bearing the radical center. These radical are much more stable and can add to alkenes, usually with anti-Markovnikov orienta-... 

Simultaneous introduchon of both sulfur and selenium functions into carbon-carbon unsaturated compounds via a radical mechanism is also demonstrated by selenosulfonahori [149] and selenothiocarboxylatiorl [150] (Scheme 15.70). In these addihon reactions, attack of sulfur-centered radicals at the terminal position of alkenes and the subsequent Sh2 reaction on the selenium lead to the formahon of anti-Markovnikov adducts regioselechvely. The selenosulfonahon can be apphed to a variety of unsaturated compounds, for example alkynes, allenes, and vinylcyclopropanes, and combination with the selenoxide syn-elimination procedure... 

In the presence of a radical initiator, alkenes react with reactive molecules such as hydrogen bromide to give simple 1 1 adducts rather than a polymer. The initiator radical reacts rapidly with an HBr molecule to give a bromine atom (6.49), which starts the chain reaction. In the first propagation step, the bromine atom adds to the alkene 61 to give the adduct radical 62 (reaction 6.50). Since 62 abstracts a hydrogen atom from HBr by reaction (6.51) more rapidly than it would add to the alkene to form a polymer radical as in (6.43), the chain continues with reactions (6.50) and (6.51) as the propagating steps, and the product is the primary bromo compound 63. This anti-Markovniko addition is in the reverse direction to the polar addition discussed in Chapter 5. Since the radical chain reaction is faster than the polar reaction, the anti-Markovnikov product dominates if radicals are present. If the Markovnikov product is required, the reaction must be carried out in the dark, in the absence of free radical initiators, and preferably with a radical inhibitor present. 

The addition of S-H compounds to alkenes by a radical chain mechanism is a quite general and efficient reaction.The mechanism is analogous to that for hydrogen bromide addition. The energetics of both the hydrogen abstraction and addition steps are favorable. The reaction exhibits anti-Markovnikov regioselectivity. 

Synthesize the compound shown below from methylcyclopentane and 2-methylpropane using those compounds as the source of the carbon atoms and any other reagents necessary. Synthetic tools you might need include Markovnikov or anti-Markovnikov hydration, Markovnikov or anti-Markovnikov hydrobromination, radical halogenation, elimination, and nucleophilic substitution reactions. 

The free radical addition of a thiol to carbon-carbon double or triple bonds is a well-established reaction. It represents one of the most useful methods of synthesizing sulfides under mild conditions. Since its discovery [5] and its much later formulation as a free-radical chain reaction (Scheme 1) [6], the anti-Markovnikov addition of thiols to unsaturated compounds has been the subject of many reviews [8, 9]. These reactions were originally initiated by thermal decomposition of peroxides or azocompounds, by UV irradiation or by radiolysis [10]. (An example of addition of 1-thiosugar to alkenes initiated by 2,2 -azobisisobutyronitrile (AIBN) [11] is reported in equation (1)). More recently, organoboranes have been used as initiators and two examples (Et3B and 9-bora-bicyclo [3.3.1.] nonane) are reported in equations (2) and (3) [12,13]. Troyansky and co-workers [14a] achieved the synthesis of macrocycles like 12- and 13-membered sulfur-containing lactones by the double addition of thiyl radical to alkynes. An example is depicted in equation (4). The same approach has also been applied to the construction of 9- and 18-membered crown thioethers [14b]. The radical chain addition of thiyl radicals to differently substituted allenes has been considered in detail by Paste and co-workers [15], who found that preferential attack occurs at the central allenic carbon and gives rise to a resonance-stabilized ally radical. The addition of benzenethiol to allenic esters has been reported and the product formation has been similarly inferred (equation (5)) [16]. 

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