Radical reductively initiated

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

Tin-free radical reduction by an organophosphite [17] and phosphinic acid can also be initiated by Et3B/02. Radical cyclizations using phosphinic acid neutralized with sodium carbonate and Et3B/02 as a radical initiator... 

Electron transfer to the protein metal center is monitored spectroscopically. In the case of a heme (FeP), a fast increase in absorbance due to direct reduction of Fe(III)P by Ru(bpy)f is followed by a slower increase in absorbance due to reduction of Fe(III)P by the Ru(II) on the protein surface. Control flash experiments with unmodified proteins show only the fast initial increase in absorbance due to Fe(III)P reduction by Ru(bpy)3. Such control experiments demonstrate for horse heart cytochrome c [21], azurin [28], and sperm whale myoglobin [14] that slow reduction of the heme by the EDTA radical produced in the scavenging step does not occur in competition with intramolecular ET. However, for Candida krusei cytochrome c, the control experiment shows evidence for slow EDTA radical reduction of the heme after initial fast reduetion by Ru(bpy)i+ [19]. 

Oxygen has two possible interactions during the polymerization process [94], and these reactions are illustrated in Fig. 2. The first of these is a quenching of the excited triplet state of the initiator. When this quenching occurs the initiator will absorb the light and move to its excited state, but it will not form the radical or radicals that initiate the polymerization. A reduction in the quantum yield of the photoinitiator will be observed. The second interaction is the reaction with carbon based polymerizing radicals to form less reactive peroxy radicals. The rate constant for the formation of peroxy radicals has been found to be of the order of 109 1/mol-s [94], Peroxy radicals are known to have rate constants for reaction with methyl methacrylate of 0.241/mol-s [100], while polymer radicals react with monomeric methyl methacrylate with a rate constant of 5151/mol-s [100], This difference implies that peroxy radicals are nearly 2000 time less reactive. Obviously, this indicates that even a small concentration of oxygen in the system can severely reduce the polymerization rate. 

In redox methods, radicals are generated and removed either by chemical or electrochemical oxidation or reduction. Initial and final radicals are often differentiated by their ability to be oxidized or reduced, as determined by substituents. In oxidative methods, radicals are removed by conversion to cations. Such oxidations are naturally suited for the additions of electrophilic radicals to alkenes (to give adduct radicals that are more susceptible to oxidation than initial radicals). Reductive methods are suited for the reverse addition of alkyl radicals to electron poor alkenes to give adducts that are more easily reduced to anions (or organometallics). 

The radical anion initially formed on reduction of 65 could be trapped with tert-butyl chloride in DMF solvent.101 The yields of alkylated amide were low, and the problem was traced to the rate of competitive dimerization. 

In comparison, the stereoselectivity of the radical reduction slightly decreases if the anomeric center is substituted by an electron withdrawing substituent (CN) using AIBN as radical initiator [17 b]. The corresponding radical is supposed to be more planar than pyramidal to explain the lower degree of stereoselectivity for the reduction process of 8 b into 9. 

If the mechanism in Scheme 10.24 is correct and cyclohexadienyl radicals do indeed react with the initiator, their presence should retard other radical processes. To test this, the reduction of 1-bromo-octane was investigated in the presence of 24 (Fig. 10.7), which readily undergoes cyclisation so should form intermediate cyclohexadienyl radicals. The initial reaction rates were measured between 2 and 12 minutes. During this period, less than 10% of the BusSnH in the reaction was consumed and so, to a fair approximation, the... 

Allylation of these P-alkoxy-a-halo esters with allyltributyltin initiated with AIBN at 60° also shows good to impressive stereoselectivity, which can be improved by use of triethylborane as initiator at —78°. However, these reactions are generally slower than radical reductions. 

Desulfonylation of fi-keto sulfones.13 Radical reduction of these sulfones with Bu3SnH and AIBN can be more efficient than the conventional method with Al/Hg. An excess of initiator is necessary for ready and complete reduction, but yields of 80-95% can be obtained by use of 4 equiv. of the stannane and 2 equiv. of AIBN in refluxing toluene. 

Radical reductions with [(CH ) Si]3SiH.1 In combination with an initiator, this silane (I) reduces not only alkyl halides, but also thionoesters (used for deoxygenation of secondary alcohols) and selenides. However, cleavage of C-S bonds is inefli-... 

Hydrostannylation This reaction with a tin hydride normally requires an initiator, usually AIBN but also B(C2H5)3 (14, 314). It can also be initiated with high intensity ultrasound, and such reactions show large rate acceleration (100-600 times) and take place even at temperatures of -50°. Sonication is also effective for radical reductions and cyclizations (last example). 

The use of the pseudohalogen nitryl iodide, prepared in situ from iodine and silver nitrite, has been found to add to an alkene in what is strictly an anti-Markownikov fashion. The explanation for this lies in that nitryl iodide adds in a radical manner, initially forming the more stable secondary radical after addition of NO2.115 Treatment of 3-0-acetyl-5,6-dideoxy-1,2-0-isopropylidene-a-D-xy/o-hex-5-enofuranose with nitryl iodide was found to afford an unstable adduct, with the nitro group appended to C-6, and iodine attached to the more substituted C-5.116-118 Similarly, treatment of benzyl 2-0-benzyl-3,4-dideoxy-a-D-g/ycero-pent-3-enopyranoside (70, Scheme 19) with nitryl iodide afforded the unstable adduct 71, which, upon exposure to mild base (NaHC03), afforded the eliminated product, namely benzyl 2-0-benzyl-3,4-dideoxy-4-nitro-a-D-g(ycew-pent-3-enopyranoside (72). The eliminated product was then readily converted into benzyl 2-0-benzyl-3,4-dideoxy-(3-L-r/ireo-pentopyranoside (73) by reduction with sodium borohydride. Addition of deuteride using NaBD4 led to axial deuteration atC-3. 

Any reduction initiated by electron transfer might be completed by either H - or H+/e-transfer. In one case at least, it has been shown that the H+/e sequence would be followed. Photochemical reaction of fluorenone with NMAH (Peters et al., 1982) involves initial electron transfer from NMAH to singlet fluorenone yielding a contact radical ion pair, presumably identical to that expected from a hypothetical ground state reaction with initial electron transfer. Fast time-resolved spectroscopy shows that this decays to the ketyl-NMA radical pair by proton transfer. 

Radical reduction is followed by a rapid reaction of the 2-hydroxyphenoxyl radical with the boronate 46. In this manner, chain propagation is ensured by the regeneration of the initial alkyl radical and the formation of Meulenhoff s free acid 47 (Scheme 40). 

Malacria s group recently developed a new method for the synthesis of allenes based on the radical reduction of bromides 7181. Equation 32 shows the general reaction for the terminally substituted allenes. Eight examples were reported with yields in the range of 30-80%. As pointed out by the authors, the use of large excesses of initiator suggests that these transformations do not involve straightforward chain reactions. 

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