Rates reductive dimerization

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

Signal-time behavior of the ESR response following a current pulse has been calculated by Goldberg and Bard [367] for a number of mechanisms, including first-order decomposition, radical ion dimerization, and radical ion-substrate coupling, and working curves from which rate constants can be calculated were presented. Application of this approach, which is very similar to that taken, for example, in transmission spectroelec-trochemistry, was demonstrated for the reductive dimerization of a series of activated olefins (Fig. 58), a reaction that has been studied by a number of different electrochemical... 

Pyryliiim [181-183] and isobenzopyrylium [184] salts have been shown to be polaro-graphically reducible in a one-electron reduction. In cyclic voltammetry in aprotic solvents, 2,4,6-trisubstituted pyrylium salts show two peaks the shape of the peak depends on the rate of dimerization. This process occurs more rapidly at C-4 than at C-2 (C-6) [182, 183], and the dimerization takes place spontaneously for 4-unsubstituted pyrylium salts. The equilibrium between radicals and dimer is displaced in favor of the radicals on introduction of electron-withdrawing substituents such groups enhance the aromatic character of the radical [185]. If the reduction of pyrylium salts is made in the presence of an alkyl iodide a fair yield of the 4-alkylated 4i/-pyrane is isolated with the dimer [182]. 

Under identical experimental conditions (DMF, 0.28 M H2O, 0.1 M Et4NBr), a thorough study of a large series of cinnamic acid esters led to the conclusion that the EHD reaction followed the RR mechanism with rate-determining dimerization [16]. The rate constants for dimerization were shown to correlate closely with the eP values for the reduction process the more easily the substrate is reduced, the faster is the dimerization (Table 3) [16]. The phenomenon can for this closely related series of compounds be rationalized in terms of a systemic change of the unpaired electron density at the position of dimerization [76]. 

In dry MeCN, n = 0.6 was found for 51, but the value increased to 0.94 on addition of water up to 10% [13]. For 52a in very dry MeCN, the mechanism of the reductive dimerization was examined and the experimental results were interpreted as an RS mechanism under these conditions [125]. However, addition of water increased the rate of reaction considerably, and in the presence of water the kinetic measurements were in accord with the RR mechanism [126]. In DMF, addition of water also accelerates the dimerization process for 52 [7,10], similar to what is observed for many monoactivated alkenes (Sec. II.A.7). The accelerating effect of water in DMF on the dimerization of 52a has been studied in greater detail [15]. On the basis of a reaction order in water close to 1, it was suggested that the dimerization reaction takes place between a free radical anion and a hydrogen-bonded radical anion [15]. The involvement of hydrogen bonding between radical anions and water may also account for the low activation energies found for the reductive dimerization of 52a in MeCN [126] and in DMF [10] (see Table 11). 

The derivatives of Meldrum s acid, 59a-b, undergo reductive dimerization in MeCN, and the mechanism has been studied [137,143]. An RS mechanism with rate-determining electron transfer has been invoked either as the only mechanism (for 59b) or as one of two parallel reaction pathways (for 59a) in order to explain the apparent reaction orders found by DCV or LSV in the presence of AcOH [137]. However, AcOH (pA (AcOH) = 12.3 in DMSO [144]) is not likely to be strong enough to protonate the dimer dianion, since the basicity of the dimer dianion is expected to be close to that of the conjugate base of Meldrum s acid (pAT(Meldrum s acid) = 7.3 in DMSO [145]). Consequently, the dimerization may be reversible, and this, in turn, may lead to anomalous apparent reaction orders, although the coupling is of the RR type [76]. 

Reductive dimerization of the NAD" " analogues 104a-c, in MeCN, and the reoxidation of the dimers have been studied in detail by CV and LSV [300]. This allowed the estimation of the values and the dimerization rate constants (Table 22). Also in aqueous medium, 104a undergoes fast reductive dimerization, and the 4,4 -tetrahydrodimer was isolated as a mixture of two diastereomers [301]. 

Reduction of acridizinium ion (106) and substituted acridizinium ions in MeCN or DMF gives a dimer ( 80%) [306]. Although not confirmed experimentally, the most likely positions for coupling are indicated. The results of LSV measurements were in agreement with rate-determining dimerization of neutral radicals, and for 106 a lower limit for the rate constant of 10 M s was obtained by CV measurements [306]. The dimer could be quantitatively reoxidized to the substrate cations either electrochemically (at a potential 0.5 V anodic relative to the initial reduction peak) or by action of oxygen [306]. 

In 1999, Cozzi and Umani-Ronchi described a diastereoselective intermolecular pinacol coupling of aromatic and aliphatic aldehydes in the presence of a catalytic quantity of TiCl4(THF)2/Schiff base (Eq. 3.38) [60]. Manganese is employed as the stoichiometric reductant with the Cozzi/Umani-Ronchi system, zinc generally affords a lower yield of the diol. The reaction is believed to proceed via a pathway analogous to that illustrated in Fig. 3-5. The observations of Cozzi and Umani-Ronchi that the Schiff base affects reaction diastereoselectivity and increases the reaction rate bode well for studies of asymmetric variants. In an initial investigation, these workers obtained 10% ee in a reductive dimerization of benzaldehyde (Eq. 3.39). 

A.G.J. Buma, A.H. Engelen, W.W.C. Gieskes (1997). Wavelength dependent induction of thymine dimers and growth rate reduction in the marine diatom Cyclotella sp. exposed to ultraviolet radiation. Mar. Ecol. Progr. Ser., 153,91-97. 

Very active and selective catalysts for oligomerization of dienes are formed during electrochemical reduction of complexes. The rate of dimerization of 1,3-butadiene to 4-vinylcyclohexene in the presence of the catalyst which is obtained by electrochemical reduction of [ FeCl(NO)2 2] is 20,000 moles of BD per mole of Fe per 1 h at 353... 

The 2,6-DHMP condensation produced only one dimer and a significant amount of trimer as depicted in Scheme 8. The structure of the trimer was not reported. The reaction path is analogous to that of 2-HMP, but occurred at a faster rate. 2,6-DHMP was the only derivative to form a significant amount of trimer under the reaction conditions studied. This supports the idea that ortho-linked PF polymers should have a faster cure than others. It also points out the futility of attempting to manufacture an ortho-Ymkcd polymer under alkaline conditions. Extension of the polymerization process as depicted in Scheme 8 leads to a continual reduction in the amount of para functionality available for condensation as shown in Table 7. 

Evans found that molecular hydrogen was efficiently generated by the reaction of a simple diiron complex [CpFe(CO)2]2 (Fp2) with acetic acid (pA a = 22.3) in acetonitrile [202]. Electrochemical simulations revealed that Ep2, [CpEe(CO)2] (Fp ), and [CpFe(CO)2H] (FpH) were key intermediates in this catalytic mechanism (Scheme 61). Reduction of Fp2 produces both an Fp anion and an Fp radical, which is further reduced to give an Fp anion. The oxidation of the Fp anion by proton affords FpH. This protonation was found to be the rate-limiting step. The dimerization of the FpH generates Fp2 and H2. Alternatively, the FpH is reduced to afford the FpH anion, which is subsequently protonated... 

These present an interesting dichotomy in their reductions by tm(l,10-phen-anthroline)iron(ri) (ferroin) °. That of CIO2 to CIOJ is rapid, is first-order in each component ki = 1.86 0.13 l.mole sec at 35 °C) and is independent of acidity. Ferriin is the immediate product and an outer sphere electron-transfer is proposed. The reduction of CIO2 is much slower, proceeding at the same rate as dissociation of ferroin at high chlorite concentrations and a major product is feriin dimer, possibly [(phen)2Fe-0-Fe(phen)2] . Clearly the reaction depends on ligand-displacement followed by an inner-sphere electron transfer. 

This concerted reduction by two ferrous species eliminates H02- (or O2 ) as an intermediate and explains the weak catalysis by Cu(II) (which is strong for V([II) and V(IV) autoxidations). Weiss has suggested that the species Fe. 02.Fe may be a stable intermediate, but Wells explains the presence of two Fe(Il) species in the rate law in terms of a pre-existing dimeric form of Fe(lf) containing an H2O bridge, for which there is evidence . The reduction is completed via the Fenton reaction vide infra). The hydrogen peroxide dianion is probably never free but is protonated whilst complexed to Fe(III). 

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

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