Homo-coupling product

Total basicity is measured by standard acid—base titration techniques. The activity divided by the total basicity should be greater than 90%. If it is not, then the Grignard reagent should be checked for unreacted alkyl or aryl haUde, homo-coupled product, hydrolysis products, and oxidation products. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

The electrochemical reduction of W-acyliminium ion pool 2 gave rise to the formation of the corresponding homo-coupling product 13 (Scheme 8).23 Presumably, a radical intermediate 14 was generated by one electron reduction of 2 and homo-coupling of the radical led to the formation of the dimer 13. However, a mechanism involving two-electron reduction to give anion 15 followed by the reaction with cation 2 cannot be ruled out. 

The procedure described here incorporates a number of modifications to the Suzuki coupling that result in a sound, efficient and scaleable means of synthesizing biaryls. First, the catalytic use of palladium acetate and triphenylphosphine to generate palladium(O) eliminates the need for the expensive air and light sensitive tetrakis(triphenylphosphine)palladium(0). No purification of reagents is necessary, no special apparatus is required, and rigorous exclusion of air from the reaction mixture is not necessary. Furthermore, homo-coupled products are not present in significant levels (as determined by 500 MHz 1H NMR). 

The Grignard is made from 4-bromoanisole, Mg (3 % excess), I2 catalyst, in tetrahydrofuran (3 M), at 70 °C. The major problem is the formation of the homo coupled product during the Grignard synthesis. Up to 6% of 4,4 -dimethoxybiphenyl may be formed. The amount depends on the type of... 

Similarly, electron-rich benzylic bromides such as 25a often lead to homo-coupling products. The use of the corresponding phosphate 25b and catalytic amounts of Lil in dimethyltetrahydropyrimidinone (DMPU) provides the corresponding zinc reagent in quantitative yield (Scheme 10) . The presence of Lil generates small concentrations of the benzylic iodide, which is converted to the zinc reagent. Little homo-coupling is observed under these conditions. 

Since silyl enol ethers have a silyl group ji to the jr-system, anodic oxidation of silyl enol ethers takes place easily. In fact, anodic oxidation of silyl enol ethers proceeds smoothly to provide the homo-coupling products, 1,4-diketones (equations 37 and 38)42. This dimerization of the initially generated cation radical intermediate is more likely than the reaction of acyl cations formed by two electron oxidation of unreacted silyl enol ethers in these anodic reactions. 

Organoaurates react with organic halides to form isolable trialkylgold derivatives which on thermolysis form cross- and/or homo-coupled products ... 

The reaction of organolithiums or organomagnesium halide reagents with silver halides produces organosilvers, but these are less stable than those containing Cu and Au and readily decompose to form homo-coupled products via alkyl radicals ... 

This reaction is known as the Ullmann coupling. It is believed to involve the intermediacy of aryl copper complexes rather than radical species. The reaction is best suited to the preparation of symmetrical biaryls ( homo-coupled products). Attempts to couple two different halides (Ar X and Ar X) in this way can lead to mixtures of the desired cross-coupled product (Ar -Ar ) and the two homo-coupled species (Ar -Ar and Ar -Ar ). 

The corresponding silyl enol ethers are likewise readily available carbonyl umpolmg substrates which can be oxidized by a variety of chemical oxidants and also by cathodic oxidation. If not trapped by nucleophiles, the radical cations can dimerize and subsequently hydrolyze to give 1,4-dicarbonyl (homo)coupling products [195]. 

The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. Transient metalated aryl complexes can be formed that react with another aromatic compound. Aryl iodides reacted with benzene to form a biaryl in the presence of an iridium catalyst. Aniline derivatives reacted with TiCLj to give the para-homo coupling product (RaN-Ar-Ar-NRj). ... 

Organotin compounds may be synthesized by the cathodic reduction of organic compounds in the presence of tin halides. For example, the reduction of allylic halides in the presence of chlorostannanes gives the corresponding allylstannanes in good yields [69]. Combination of this reaction with in situ palladium-catalyzed reaction with allylic halides leads to effective formation of the head-to-tail homo coupling products as shown in Eq. (17). 

Benzylic and allylic halides dimerize (homo-couple) readily in the presence of a number of metal species. Synthetically useful yields of homo-coupled products have been obtained using chlorotris(tri-phenylphosphine)cobalt(I), nickel(O) complexes generated in situ, ° Te " species, VCb/LAH, CrCh/LAH and TiCb or TiCl4/LAH. Representative examples are given in equations (37) and (38). Treatment of 8-bromocrotonates with zinc(O) in DMSO leads to dimerization. ... 

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