Coupling, 2-Alkyl

TrialkyIboranes (p. 9), which can be synthesized from olefins and diborane, undergo alkyl coupling on oxidation with alkaline silver nitrate via short-lived silver organyls. Two out of three alkyl substituents are coupled in this reaction. Terminal olefins may be coupled by this reaction sequence in 40 - 80% yield. With non-terminal olefins yields drop to 30 - 50% (H.C. Brown, 1972C, 1975). 

The most interesting and difficult cross-coupling is alkyl-alkyl coupling, because oxidative addition of alkyl halides having /i-hydrogen is slow. In addition, easy elimination of /d-hydrogen is expected after the oxidative addition. 

SULFIDE CONTRACTION via ALKYLATIVE COUPLING 3-METHYL-2.4-HEPTANEDIONE... 

ALKYLATIVE COUPLING, 55, 127 2-ALKYNOlC ACIDS, methyl esters, 55,... 

Somewhat similar observations have been made in the reaction of alkyl halides with sodium mirrors (the Wurtz reaction) in which alkyl coupling occurs. Thus, ethane formed on treatment of methyl iodide with sodium in a field of 20 G shows n.m.r. emission (Garst and Cox, 1970). The phase is consistent with polarization via T j-S mixing,... 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

R = H (reductive coupling), R = alkyl or aryl (alkylative coupling)... 

Scheme 24 Other classes of nickel-catalyzed reductive and alkylative couplings...

The opposite regioselectivity is observed in intermolecular alkylative coupling of terminal alkynes and aldehydes with diethylzinc and with alkenylzirconium/ZnCl2 (Scheme 88). 

Furstner and Leitner " have recently described the synthesis of (f )-(-t-)-muscopyridine. This elegant strategy is based on two regioselective iron-catalyzed heteroaryl-alkyl coupling reactions (Scheme 65). 

In summary, the Pd-catalyzed alkynylation with alkynylzincs has finally become generally applicable with the exception of the alkynyl-alkyl coupling and often capricious... 

SCHEME 64. Pd- or Ni-catalyzed alkyl-alkenyl and alkenyl-alkyl coupling reactions... 

SCHEME 75. Alkyl-alkyl coupling catalyzed with Ni, Pd or Cu complexes with butadiene dimer... 

Alkylation, enolates, 56, 52 ALKYLATIVE COUPLING, 55, 127 2-Alkyl-l,3-cyclopentanediones, 58, 82 Alkyl 2,2-dimethylpropanethioate, 58, 146 Alkyl disulfides, 58, 142 Alkyl fluoride, 58,77 Alkyl halide, 58, 145... 

Patel and Jamison [40] reported a catalytic three-component coupling of alkynes, imines, and triethylborane using a Ni complex and (S)-(+)-(neomenthyl)diphenyl-phosphane [(S)-NMDPP] 159 forming the alkylative coupling product (Scheme 9.28). No yields were reported. 

In this last section we wish to illustrate the use of TAMREAC with three examples. The first, ethylene dimerization, is of importance since it is involved in the oligomerization and polymerization reactions of olefins. The second example concerns the hydroformylation of ethylene, and the last example is the allyl—alkyl coupling reaction induced by a Ni(II) complex. 

In a related reaction, Caddick et al. expanded this Suzuki-type coupling to other alkyl-alkyl couplings.21 Although the yields were still modest, the reaction seemed compatible with various functional groups and required only mild conditions (Table 10.1). 

The Caddickprotocol setup for alkyl-alkyl coupling (see Section 10.2) could also be used for the coupling of a vinylborane with an alkyl bromide (Scheme 10.7). The yield was, however, modest.17... 

Figure 8.4 Schematic presentation of the mechanism for the aryl-alkyl coupling reaction...

Electrophilic haloarene function and nucleophilic areneboronic acid function undergo carbon-carbon coupling only in the presence of Pd(0) catalytic species. The general catalytic cycle for this aryl-aryl coupling, involving an oxidative addition-transmetallation-reductive elimination sequence, follows the mechanistic scheme shown in Figure 8.4 for aryl-alkyl coupling. 

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