Lewis catalyzed electron transfer

The catalytic role of Mg + in Scheme 12 is ascribed to the 1 1 and 1 2 complex formation of Q and Mg +, which results in an increase in kobs with an increase in [Mg +], exhibiting first- and second-order dependences on [Mg ], respectively (Figure 7). This contrasts with the Lewis acid catalysis in conventional concerted Diels-Alder reactions, in which the catalyst is believed to activate dienophiles (not the radical anions) by coordination to the acidic metal center [208-212]. However, the exact catalytic mechanism of numerous Lewis acid-catalyzed Diels-Alder reactions [208-212] has yet to be clarified, including a possible contribution of Lewis-acid catalyzed electron transfer step. 

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]. 

For the oxygenation of 40 to 41, in the dark and in the presence of catalytic amounts of tris(p-bromophenyl)ammoniumyl tetrafluoroborate, (p-BrC6H4)3N BF4, Tang and co-workers [140] proposed a chin electron-transfer pathway, suggesting at the same time that the same mechanism could operate in the trityl cation and Lewis add-catalyzed oxygenations. 

Metal cation-catalyzed photoadditions of radical species to radical cations of electron rich alkenes have been reported. Lewis found that the radical cation of norbornene generated by photoinduced electron transfer from this alkene to Ag(I) reacts with acetonitrile to produce 2-cyanomethylnorbomane (Scheme 42)... 

For this comparison die [4 + 2] cycloadditions of methyl vinyl ketone to cyclopentadiene and to furan are chosen as reference reactions (Scheme 60, Table 9). Smectile clays, with the interlayer cations exchanged by CH" or Fe" ions, catalyzed the addition of cyclopentadiene in providing the endo product (258 Y = CH2) at a drastically reduced reaction temperature in high yield together with 10% of the exo product (259 Y = CH2) (entries 3,4). These results are comparable with those obtained under aqueous reaction conditions (entry 2), which supports the idea that die presence of water pockets in clays could account for their catalytic activity. The Lewis acidity as well as one-electron-transfer processes, involving the internal Fe " or Ci" cations, have also been invoked as possible explanations. 

Compared with the electron transfer reduction of neutral radicals, the reduction of radical anions may be more readily catalyzed by metal ions which have weak Lewis acidity, since the binding to dianions produced in the electron transfer reduction of radical anions should be much stronger than the binding to monoanions. As such, even Na+ has been reported to accelerate electron transfer from 1-benzyl-1,4-... 

Each electron transfer generates a proton or Lewis acid at the anode and a hydroxyl ion or Lewis base at the cathode. This creates strongly acidic or basic media in the reaction layers adhering to the anode or cathode, whereby acid- [24] or base-catalyzed reactions [25] can be induced (see Chapter 30). 

M. R. Hofemann N. S. Lewis, Fluxmatching condition at Ti02 photoelectrodes Is interfadal electron transfer to O2 rate-limiting in the Ti02-catalyzed degradation of organics J. Phys. Chem. 1994, 98, 13385-13395. 

Scheme 4-2 outlines an activation cycle that is based on the argument that within an aprotic matrix the iron(II) center is a strong Lewis acid that weakens the 0-0 bond of electron transfer from the iron(II).12 The products for the Fe I/HOOH oxidations are consistent with those that result from some peroxidase-catalyzed processes. 

Electron transfer from metal oxide surfaces to CO can be quite facile, occurring at room temperature. This process can be important as an initial CO activation step in metal oxide catalyzed reduction schemes. We have attempted to clarify what types of metal oxides interact (MO CO MO. . . CO -) with CO in this way, and what surface features these active metal oxides possess. Only MgO, CaO, SrO, BaO, and Th02 were electron transfer active. These oxides have in common the possession of both Lewis basic sites and one electron reducing site. It appears that CO is first adsorbed on Lewis base sites followed by slow migration to electron transfer reducing sites. The studies leading to this conclusion are discussed. 

This reaction was first reported by Mukaiyama et al. in 1974. It is a Lewis acid-catalyzed Michael conjugate addition of silyl enol ether to o ,/3-unsaturated compounds. Therefore, it is generally referred to as the Mukaiyama-Michael reaction. Because this reaction is essentially a conjugate addition, it is also known as the Mukaiyama-Michael addition or Mukaiyama-Michael conjugate addition. This reaction is a mechanistic complement for the base-catalyzed Michael addition, j and often occurs at much milder conditions and affords superior regioselectivity. s Besides silyl enol ether, silyl ketene acetals are also suitable nucleophiles in this reaction.For the hindered ketene silyl acetals, the Lewis acid actually mediates the electron transfer from the nucleophiles to o ,/3-unsaturated carbonyl molecules.On the other hand, the Q ,j8-unsaturated compounds, such as 3-crotonoyl-2-oxazolidinone, alkylidene malonates, and a-acyl-a,/3-unsaturated phosphonates are often applied as the Michael acceptors. It has been found that the enantioselectivity is very sensitive to the reactant structures —for example, Q -acyl-Q ,j8-unsaturated phosphonates especially prefers the unique syn- vs anft-diastereoselectivity in this reaction. In addition,... 

It should be noted that addition of Lewis acidic salts, such as MgBt2, is critical in order to achieve an effective catalytic transformation when using arylzinc compounds. This observation indicates that the difficult step of the catalytic cycle is the transmetaUation of the aryl group from the zinc reagent to the catalytically active iron complex [42]. While the involvement of an intermediate radical species or a single electron-transfer process is suspected, mechanistic details of these iron-catalyzed cross-coupling reactions remain unclear. 

The photoaddition of alcohols to epoxides which is catalyzed by Fe203 (Fe3+ and other metal ions) [39], Scheme 14(c) functions similarly. The metal catalysis of this reaction was detected when the solvent and reagent methanol had been distilled from EDTA to remove metal ion traces and the reaction found to proceed orders of magnitude slower in the purified solvent. Photo-electron transfer from the epoxide to Fe(III) as acceptor generates an oxonium cation that is highly susceptible to nucleophilic attack by the alcohol. Since the catalysis by metal ions in this case is quite general, their functioning simply as Lewis acids cannot be excluded. 

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