Radicals homocoupling

The addition of substituted allylic zinc reagents to aldehydes is usually unselective" . Furthermore, the direct zinc insertion to substituted allylic halides is complicated by radical homocoupling reactions. Both of these problems are solved by the fragmentation of homoallylic alcohols. Thus, the ketone 166 reacts with BuLi providing a lithium alcoholate which, after the addition of ZnCl2 and an aldehyde, provides the expected addition product... 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Scheme 5.28 Reduction of N-acyliminium ion pool. Radical homocoupling...

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

The anion of methyl phenylacetate, formed by an electrogenerated base, was homocoupled with iodine or anodically mediated by iodide to afford dimethyl 2,3-diphenylsuccinate in high yield and high d, /-selectivity. This reaction probably does not involve free radicals but an iodination-nucleophilic substitution sequence [194,195]. 

Oxidation of arylolefins, enolethers, or dienes yields intermolecular homocoupling products in moderate to good yield (see Sect. 13.2.1.4) however, no pronounced diastereoselectivity was observed. This is also due to the fact that the coupling sites do not tolerate substituents that would make up a prostereogenic center. Furthermore, the fairly stable cations of the dimerized radical cation solvolyze stereounselectively. The same holds for the intermolecular coupling of aromatic compounds, in... 

Although difluoroenoxysilanes are typical nucleophiles, they can also react with other nucleophiles via their oxidation into radical cations. The oxidative homocoupling and the cross-coupling with heteroaromatics and alcohols proceeds very well in the presence of Cu triflate as the oxidizing reagent (Figure 2.24). " ... 

Radical arylations of phenols differ in some respects from those of phenolates (Scheme 37). First, the decreased nucleophilicity of the phenol, such as 100, allows the use of unmasked aryl diazonium chlorides 101 as radical sources. Given that an efficient reductant is present in the reaction mixture and that the diazonium salt is added slowly, biphenyl alcohols 102 can be prepared in moderate to good yields [153,154]. In this way, the concentration of the salt 101 is kept low at any time and homocoupling reactions (addition of the aryl radical to diazonium ions) as well as azo coupling to the phenol 100 can be successfully overcome. 

Radical intermediates and transition metal-catalyzed reactions are in principle ideally suited to be linked together. A prerequisite to perform successful radical reactions is that the concentration of radicals has to be kept low to promote the desired reaction and to avoid competing homocoupling and disproportionation, which occur often diffusion-controlled. Including radical intermediates in the regime of a transition metal catalyzed process is thus ideal to keep their concentrations low, since their maximum concentration cannot exceed that of the metal catalyst. On the other hand, radicals are much more reactive than closed-shell organotransition metal intermediates. Thus, the involvement of radicals in transition metal catalysis often leads to a strong acceleration of the reactions compared to a process where only closed-shell intermediates are involved [101]. 

Homocoupling of terminal alkynes in the presence of cuprous chloride and O2 involves radical coupling, as shown in Scheme 2.50. 

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

The MHCs are normally additional to the two LHDs formed by homocoupling. Mechanistic studies on cross-couplings are scant and inconclusive. The two main mechanistic proposals are (1) Michael addition of the radical anion (or dianion) of the more easily... 

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