Radicals Lewis acid complexed

The possibility of monodentate species complicates the analysis of chelation-controlled radical reactions. Monodentate complexation leads to transition states such as B (Scheme 1) that, in terms of stereoselectivity, behave similarly to un-complexed radicals. Lewis acid complexation with the ester function has the potential to lower the energy of the transition state, particularly when the incoming reagent is electrophilic (e.g. allyltrimethylsilane), thus enhancing the reactivity of such radicals relative to uncomplexed species. Because radical reactions can occur... 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

Lewis Acid Complexed Aminyl Radicals from PTOC Carbamates.. .. 29... 

Lewis acids have long been known to influence free radical polymerizations [117]. They have been particularly important in copolymerizations of hydrocarbon olefins with electron-poor monomers such as acrylates or acrylonitriles. In this way strictly alternating copolymers can be synthesized from monomer pairs which in the absence of Lewis acids would give more random copolymers. The Lewis acid complexes with the electron pair of the acceptor group of the acrylate or acrylonitrile to form the more electrophilic complexed monomer, which then copolymerizes in alternating fashion with the electron-rich hydrocarbon olefin. 

Radical reactions are also valuable strategies for the formation of quaternary carbon centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to a chiral binol ligand (103) in the allylation of -iodolactones 101 (Eq. (13.31), Table 13-6) [43]. It was established that diethyl ether as an additive in these reactions dramatically increases product enantioselectivities (compare entries 1 and 2, Table 13-6). Catalytic reactions were also demonstrated (entry 3) with no appreciable loss of selectivity. A proposed model for how diethyl ether functions to enhance selectivity in the enantioselective formation of these quaternary chiral centers is shown in 104. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

It is concluded that (62) arises by a radical pathway as already postulated, but that (63) and (145) spring from an ionic process. Lewis acid complexation of (9) by FeCls is believed to create successively the tertiary cation (150) and the alkene (151) by selective deprotonation (Scheme 22). Next, intramolecular epoxidation to (152) followed by rearrangement delivers (63). The alkene (151) is also assumed to be the precursor to the dihydropyran (149). Protonation of (151) produces the hydroperoxide (153) that evolves stepwise to (148) and (149). The antimalarial activity of (9) is ascribed to (153). 

Iodine transfer addition to allyltrimethylsilane provides a more environmentally friendly alternative to allyltributylstannane. In these allylations, which are exemplified in Scheme 6, the initially formed I-transfer product undergoes spontaneous loss of TMSI to generate the observed allylation product. Guindon has shown that allylation of the Lewis acid complexes of ) -alkoxy esters in this manner can lead to products with high anti stereoselectivity [22]. It is also believed that the presence of Lewis acids enhances the electrophilicity of the radical. Allylations of this type can also prove successful when Br-transfcr or PhSe-transfer reactions are employed. 

Cyclization of the aminyl radical 22 is slow and reversible [53], and the kinetics of related cyclizations and ff-fragmentations were measured directly by LFP [54]. Protonation of the dialkylaminyl radicals gives dialkylaminium cation radicals such as 24 that react much more rapidly [55], and Lewis acid complexes of aminyl radicals such as 23 are intermediate in reactivity [56]. Clocks such as 23 and 24 are in equilibrium with the neutral aminyl radicals, and the concentrations of the proto-nated or complexed forms are necessary if one is to use these clocks equilibrium constants for protonations and Lewis acid complexations in some solvents were determined in the initial kinetic calibration studies. 

Et2Zn instead of 136 as a radical initiator was also effective for the radical reaction. In these tin-free reactions, the mechanism of propagation is proposed to be a process in which boron or zinc Lewis acids complex to the oxime, promoting addition. Once addition of a radical to the complexed oxime occurs, fragmentation of the EtsB or Et2Zn complex generates an ethyl radical that propagates the chain. 

Scheme 3 shows the effect of Lewis acid on atom transfer reactions. Since Lewis acid complexation can increase the electron-withdrawing nature of a group involved in a complex, it should also increase the rate of addition of alkenes onto a-carbonyl radicals. Both our group [7] and Porter s group [8] have improved the scope of... 

Radical allylations have proven to be a successful route for enantioselective carbon-carbon constructions. Typically, in arrangements where the radical intermediate is complexed to a chiral Lewis acid, one can envision either monocoordinate or multidentate binding of the chiral Lewis acid complex to the substrate. This interaction is dependent on both the substrate (number of donor atoms available) and the Lewis acid s binding capabilities. Highly successful examples of both such scenarios have been realized. 

Radical reactions are also valuable strategies for the formation of quaternary carbon-based centers. An enantioselective variant of this has recently come to light utilizing aluminum as a Lewis acid complexed to chiral BINOL ligand 26 in the allylation of a-iodolactones 24 (Eq. 9, Table 1) [13]. 

The iV-hydroxypyridine-2(///)thione derivatives (PTOC carbamates and PTOC imidates) permit facile generation of neutral, protonated, Lewis acid-complexed aminyl radicals and amidyl radicals. For cyclization reactions, the PTOC protocol was comparable or superior in yield to those involving Af-chloro or Al-thioaryl compounds. The thioxothiazolyloxycarbonyl (TTOC) carbamates containing a primary amine group would appear to be the most useful precursors now available for generating monoalkylaminium cation radicals [55]. Some representative examples are collected in Table 6. 

The reactivity of Ceo is that of a fairly localized electron deficient polyolefin. The main type of chemical transformations are therefore additions to the [6-6] double bonds, especially, nucleophilic-, radical-and cycloadditions and the formation of tj -transition metal complexes but also, for example, hydroborations and hydrome-talations, hydro-genations, halogenations and Lewis acid complex formations are possible. 

Guindon Y, Guerin B, Chabot C, Mackintosh N, Ogilvie WW. Stereoselective radical aUylations of a-iodo-3-alkoxy esters - reversal of facial selectivity by Lewis-acid complexation. Synlett. 1995 449—451. 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

Lewis acids 436 metal complex-mediated radical polymerization 484-6 molecular weight distributions 251,453-4, 458-60,490-1.499-501 molecular weight conversion dependence 452-3,455... 

Radical cyclization to triple bonds is used as the key step for the synthesis of oxygen heterocycles. This methodology can benefit from a Lewis acid, such as aluminum fns(2,6-diphenyl phenoxide) (ATPH), which forms a complex with the... 

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