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Regioselectivity in radical additions

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. [Pg.630]

Most of the dienes investigated experimentally show high regioselectivity in radical addition reactions. The preferred position of attack is shown in Scheme 2. [Pg.631]

Guidelines for Predicting Regioselectivities in Radical Additions to Allenes... [Pg.711]

Regioselectivity in radical addition and substitution of heterocycles 88MI7. [Pg.41]

In one of the earliest investigations of regioselectivity in radical addition reactions to polyenes, the addition of hydrogen bromide to 1,3-butadiene was observed to yield mainly the 1,4-addition product in the presence of peroxides. The preference for attack at the Cl position of 1,3-butadiene has subsequently been observed for a large number of radicalsOnly for the addition of the methyl radical has the ratio of addition to the Cl vs C2 actually been measured. A value of Cl C2 = 1.0 0.01 has been found . For all other cases, products arising from attack at C2 have not been reported. This is also true for radical addition to 2,3-dimethyl-l,3-butadiene . Additions to 1,3-pentadiene occur predominantly at the Cl position due to the steric effect exerted by the terminal methyl This is a reflection of the reduced... [Pg.631]

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... [Pg.26]

IP readily adds to C-C double bonds. Like "OH, it is a pronounced electrophilic radical (p = -0.45 Neta 1972) and thus shows a high regioselectivity in its addition reactions. With eaq , it shares a fast reaction with 02 [reaction (25) k =... [Pg.83]

Asymmetric Radical Reactions. Several reports have documented the utility of nonracemic fra/w-2,5-dimethylpyrrolidine as a chiral auxiliary in asymmetric radical reactions. For example, the addition of -hexyl, cyclohexyl, and f-butyl radicals to the chiral acrylamide of 4-oxopentenoic acid provided four diastere-omeric products resulting from a- and p-addition (eq 7). The isomers resulting from p-addition were formed with no diastereoselectivity however, the isomers resulting from a-addidon were formed in ratios of 16 1,24 1, and 49 1. Unfortunately, the application of this chemistry is limited due to the poor regioselectivity in the addition and difficulty in removal of the chiral auxiliary. [Pg.287]

The regioselectivity in diene addition reactions can also be influenced by ring strain effects in cyclization reactions. The regioselectivity is highly predictable in those cases, in which addition to the preferred diene center forms the preferred ring size. Thus, the cyclization of radical 15 proceeds readily to form the ct s-disubstituted cyclopentyhnethyl radical 16 with high selectivity. Similarly, cyclization of 17 affords exclusively bicyclic radical 18, in which the additional cyclopentane ring has been formed by addition to the terminal position of the butadiene subunit. This preference for 5-exo cyclizations onto dienes is not even dismpted by substiments at the C1 or C4 positions of the diene system, as seen for radical 19, which cyclizes to 20 (equation lO). This is in contrast to alkyl radical cyclizations to alkenes, in which major amounts of 6-endo cyclization is observed for 5-substituted systems. ... [Pg.633]

The observed non-Markovnikov regioselectivity of radical addition of HBr to an alkene is a combination of a sferic factor and an elecfronic factor. First, a bromine radical attacks the less hindered carbon of fhe double bond (the steric factor). Second, as mentioned in Section 8.5D, the relative stabilities of radicals parallel those of carbocations (Section 6.3A). [Pg.363]

Note that the addition of a radical to an alkene is often nearly thermoneutral or even strongly exothermic. Therefore, the transition state is early, and has less radical character than the extent of carbenium ion character in the transition state for the endothermic addition of a proton to an alkene. As such, electronic factors are less important in radical addition and steric factors can contribute to regioselectivity. [Pg.572]

Delocalization effects in the promoted state have been identified also in radical addition to substituted olefins, equation (21), where the spin density distribution of the triplet olefin controls the regioselectivity of the radical attack. Thus, radical attacks generally occur on the unsubstituted site, which is also the site of the higher spin density in the triplet olefin. ... [Pg.3151]

In general, the preferred regioselectivity of the addition is in a manor to give the most stable radical (thermodynamic control)... [Pg.144]

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... [Pg.70]

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... [Pg.353]

In the presence of peroxides, hydrogen bromide adds to the double bond of styrene with a regioselectivity opposite to Markovnikov s nrle. The reaction is a free-radical addition, and the regiochemistiy is governed by preferential fonnation of the more stable radical. [Pg.448]

From a synthetic point of view, the regioselectivity and stereoselectivity of the cyclization are of paramount importance. As discussed in Section 11.2.3.3 of Part A, the order of preference for cyclization of alkyl radicals is 5-exo > 6-endo 6-exo > 7-endo S-endo > 1-exo because of stereoelectronic preferences. For relatively rigid cyclic structures, proximity and alignment factors determined by the specific geometry of the ring system are of major importance. Theoretical analysis of radical addition indicates that the major interaction of the attacking radical is with the alkene LUMO.321 The preferred direction of attack is not perpendicular to the it system, but rather at an angle of about 110°. [Pg.967]

Aryl- and alkylsulfonyl radicals have been generated from the corresponding iodides and added to, e.g., propadiene (la), enantiomerically enriched (P)-(+)-propa-2,3-diene [(P)-(lc)] and (P)-(-)-cyclonona-l,2-diene [(P)-(lk)] [47]. Diaddition of sulfo-nyl radicals may compete considerably with the monoaddition [48,49]. Also, products of diiodination have been purified from likewise obtained reaction mixtures, which points to a more complex reactivity pattern of these substrates towards cumulated Jt-bonds. An analysis of regioselectivities of arylsulfonyl radical addition to allenes is in agreement with the familiar trend that a-addition occurs in propadiene (la), whereas alkyl-substitution at the cumulated Jt-bond is associated with a marked increase in formation of /3-addition products (Scheme 11.7). [Pg.708]

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. [Pg.267]

The regioselectivity in ketyl-radical-promoted ring cleavage of configurationally restricted tricyclo[3.3.0.0 ]octanone derivatives was investigated by Maiti and Lahiri, revealing new and additional information for this class of compounds in contrast to simple and unrestrained cyclopropyl ketones [9],... [Pg.191]

Many examples of the influence of steric effects on reactivity and regioselectivity in free radical additions are known. The anti-Markownikoff regioselectivity apparently is smalier than originally assumed and frequently dependent on... [Pg.26]


See other pages where Regioselectivity in radical additions is mentioned: [Pg.631]    [Pg.197]    [Pg.631]    [Pg.197]    [Pg.25]    [Pg.1108]    [Pg.1108]    [Pg.23]    [Pg.633]    [Pg.22]    [Pg.15]    [Pg.376]    [Pg.8]    [Pg.245]    [Pg.1105]    [Pg.947]    [Pg.190]    [Pg.1105]    [Pg.23]    [Pg.24]    [Pg.632]    [Pg.57]    [Pg.118]    [Pg.142]    [Pg.486]    [Pg.135]    [Pg.194]    [Pg.27]   
See also in sourсe #XX -- [ Pg.630 , Pg.631 , Pg.632 , Pg.633 ]

See also in sourсe #XX -- [ Pg.630 , Pg.631 , Pg.632 , Pg.633 ]

See also in sourсe #XX -- [ Pg.516 , Pg.520 ]




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