Allylation, radical

Are the carbon-carbon bond distances in allyl cation, allyl radical allyl anion all similar, or are they significantly... 

Conjugated organic radicals allyl, propargyl, benzyl and cyclopentadienyl... 

Nicolaou prepared the aryl aldehyde 84 by an efficient five-step sequence, as shown in Scheme 3.14. Beginning with 2-bromojuglone (36), radical allylation afforded the allylquinone 88 (75 %). Benzylation of the phenol group followed by... 

Allyl cation, 10 Allyl radical Allyl anion, 11... 

This radical allylation provides a route to prostaglandins (equation II). The resulting 6-methylene-PGE, derivative (3) is converted to a 6-oxo-PGE, derivative by ozonolysis (50% yield). 

Radical allylation of B-alkylcatecholboranes using easily available allyl-sulfones has been described [109-111]. By using phenylsulfones, the fragmentation produces a stable phenylsulfonyl radical that reacts with B-alkyl-... 

Scheme 56 Radical allylations using chiral lanthanide Lewis acids... 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

SCHEME 10. Catalysis of a radical allylation by the urea derivative 7171,172. -phe enhanced cis/ trans-selectivity is caused by the steric shielding in the transition stmcture... 

Free radical allylation free radical reduction (homogeneous)... 

Radical allylations with allylsilanes 71 occur under mild conditions in good to excellent yields, provided that the radical precursor and the silane have the appropriate electronic pairing [85]. The two examples in Reactions (7.75) and (7.76) show the reactivity matching of the allylating agent with the radical. These reactions offer tin-free alternatives for the transformations that are currently carried out by allyl stannanes. 

For example, radical allylic bromination of pent-2-ene must produce a mixture of three products. There are two allylic positions in the substrate, and either can suffer hydrogen abstraction. If hydrogen is abstracted from the methylene, then the two contributing resonance structures for the allylic radical are equivalent, and one product results when this captures a bromine atom. Abstraction... 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

The modification of [T-amino acids by a-selenylation and then free radical allylation leads to the formation of (A-amino acids bearing an a-allyl group (Scheme 12). Hanessian and co-workers 22 initially used this approach to produce p2 3-amino acids which were then subjected to suitable coupling procedures. Interestingly, the coupling method chosen made use of PyBOP and hence demonstrates that other peptide coupling procedures can be used in the synthesis of (3-peptides. 

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. 

In allylperoxyl radicals, allylic rearrangement leads to the 1,3-migration of the peroxyl function, with the corresponding shift of the double bond [reaction (28) Schenck et al. 1958],... 

Fig. 15. Orbital correlation for the dimerization, allyl radical + allyl radical. The process is symmetry forbidden (Hoffmann and Woodward, 1965b).

Unfortunately, substitution in the y-position, such as in crotyltin, led to poorly reactive allyltins, due to the decrease in the addition rate of the radicals to the double bond. It has been established that, generally, the competitive allylic hydrogen abstraction became predominant, destroying the crotyltin reagent548. The use of y-substituted allyltins for the photo-induced radical allylation of carbonyl compounds represents an interesting... 

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