Homo-couplings

It has been shown that even the labile monosubstituted butadiynyl derivatives ean be used as the starting terminal alkynes. Thus, the oxidative eoupling of 3-butadiynyl-l-methyl-, 5-butadiynyl-l-methyl-, and 4-butadiynyl-l,3,5-dimeth-ylpyrazole in pyridine by oxygen in the presenee of CuCl at room temperature leads to eonjugated oetatetraynes in 75%, 83%, and 84.0% yields (69IZV2546 69KGS1055) (Seheme 65, Table XVIII). 

Moreover, it has been established that dehydrocondensation can also be applied to 3,5-diethynyl-l-methylpyrazole, which makes it possible to produce polymer (88%) with an extended system of conjugate bonds possessing semiconductor properties (2001UP2). 

The same type of reactions includes the Chodkiewicz-Cadiot reaction, i.e., a coupling of terminal acetylenes with bromoacetylenes, which is performed in 

Little is known concerning the mechanism of this reaction, but most probably the intermediates are acetylides. 

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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. 

However, attempts to couple (A-acetyl)-4-iodopyrazole 36 under the same conditions withphenylacetylene,p-methoxyphenylacetylene, andoct-l-yne, once again, were unsuccessful, instead, reductive deiodination to give 5-(iV-acetylamino)-3-methyl-l-ethylpyrazole and homo-coupling of alk-l-yne occurred (Scheme 49). The isomeric 3-(A-acetylamino)pyrazole 37 was somewhat less inclined to deiodination. 

Early development of the homo-coupling reactions of aryl halides involves the use of stoichiometric amounts of air-sensitive Ni(0) complexes.54 The reaction could also be realized with a catalytic amount of Ni(0) complexes formed in situ when a stoichiometric amount of Zn was present. Besides aryl iodides, tosylates,... 

The rate-determining step in the homo-coupling reaction of aryl halides could be the oxidative step or the reduction of Ni(II) to Ni(I) step. 

NMR spectroscopy, 93. See also Proton NMR integrations Hoechst continuous process, 548 Homo-coupling reactions, aryl halide, 486-487 Homopolymers, 7 Hot-cast prepolymer method, 211 Hot phosgenation, 222 Houvink-Sakurada equation, 286 HTMAB. See Hexadecyltrimethylammonium bromide (HTMAB)... 

The ketone 15 was eventually prepared by Grignard addition to Weinreb amide 21, as shown in Scheme 5.5. The Weinreb amide 21 was prepared from p-iodobenzoic acid (20). The phenol of readily available 3-hydroxybenzaldehyde (22) was first protected with a benzyl group, then the aldehyde was converted to chloride 24 via alcohol 23 under standard conditions. Preparation of the Grignard reagent 25 from chloride 24 was initially problematic. A large proportion of the homo-coupling side product 26 was observed in THF. The use of a 3 1 mixture of toluene THF as the reaction solvent suppressed this side reaction [7]. The iodoketone 15 was isolated as a crystalline solid and this sequence was scaled up to pilot plant scale to make around 50 kg of 15. 

The ternary system consisting of a metallic catalyst, a chlorosilane, and a stoichiometric co-reductant has been reported by us for the first time to achieve the catalytic pinacol coupling. The homo coupling of aliphatic aldehydes is catalyzed by CpV(CO)4, Cp2VCl2, or Cp2V in the presence of a chlorosilane and Zn in DME to give the 1,3-dioxolanes 1 via the coupling and acetalization (Scheme 3) [18,19]. 

The homo-coupling reaction of benzylic halides by metallic nickel proceeded at room temperature. 1,2-Diarylethanes having a variety of functional groups could be easily prepared by this method in good to high yields. 

The homo-coupling of benzylic halides appears to proceed via benzylnickel intermediates similar to that of aryl halides. The intermediate, benzylnickel halide, was successfully trapped with electron deficient olefins. 

It has been shown recently that the selective reductive homo-coupling polymerization of aromatic diisocyanates via one electron transfer promoted by samarium iodide in the presence of hexamethylphosphoramide [PO(NMe2)3] (HMPA) can produce poly(oxamide)s in nearly quantitative yield (Scheme 9). 

Homo-coupling of vinylic mercurials occurs readily under palladium195 or rhodium196 catalysis, but with the stoichoimetric amount of a reagent (equation 111)195. Divinylpal-ladium intermediates may be involved in this reaction. This reaction is also of limited synthetic scope since organomercurials are usually prepared via vinylboranes, which... 

Direct homo-coupling of vinyl halides is a simple way of generating 1,3-dienes. This transformation can be achieved employing various transition metal catalysts such as nickel(O) reagent in the presence of phosphine ligand202 or a nickel(O) reagent in the presence of potassium iodide and thiourea (equation 116)203. 

Besides the above electrophiles, the acetylene—titanium complexes react regioselectively with other acetylenes providing the corresponding titanacyclopentadienes. An example of a homo-coupling reaction is shown in Eq. 9.11 [30], which also displays some synthetic applications [30,31]. Especially noteworthy is the highly regioselective cross-coupling reaction of unsymmetrical internal and terminal acetylenes, which is illustrated in Eq. 9.12... 

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. 

Costanzo et al. <2002MI87> published the synthesis of the furyl-substituted pyrazolo[3,2-f][l,2,4]triazine derivative 146 by Suzuki coupling of the iodo compound 145 with 2-furylboronic acid. The yield was found to be moderate (38%). It may be important to mention that these authors also tried to transform 145 to a heteroaromatic boronic acid and to carry out cross-coupling of this compound with 3-bromofuran. Unfortunately, however, this approach failed and only homo-coupling occurred. 

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 homo-coupling reaction of 1,1-dimethylallene proceeds at -50 °C to give a symmetrically substituted diene in good yield (Scheme 16.99) [105], In sharp contrast, the reaction of a monosubstituted allene proceeds via a titanabicycle to give an unsymmetrical diene. 

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). 

Side reactions are exchange of organic groups, followed by homo coupling, P-hydrogen elimination with alkene formation, isomerisation followed by coupling, or benzene formation and alkene liberation. Examples are shown in Figure 13.22. 

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... 

Figure 13.24. Slow addition of Grignard suppresses homo-coupling...

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