Oxidative homo-couplings

Oxidative homo-coupling of terminal alkynes using copper catalyst in the presence of oxygen. 

Oxidative homo-coupling of alkyl magnesium reagents possessing /3-hydrogens is achieved in the presence of silver tosylate (AgOTs, 1 mol%) as a catalyst and 1,2-dibromoethane as a reoxidant " . 

Oxidative homo-coupling of enolates from acyl oxazolidinones to give the corresponding dimers can be achieved in the presence of oxidants. Titanium and ytterbium enolates of 252 were coupled in the presence of a chiral diol or chiral bisoxazoline in the presence of ferrocenium cation 254 (Scheme 63) [166]. The amount of the meso dimer varied with the chiral ligand with a maximum of 5 1. TADDOL 172 performed best providing a 76% ee for the meso product. Ytterbium enolate gave a low ee of 34% with the same ligand. 

Vinyl stannanes also undergo oxidative homo-coupling under transition metal catalysis to result in dienes (equation An intramolecular version of this method... 

Nagano T, Hayashi T (2005) Iron-catalyzed oxidative homo-coupling of aryl grignard reagents. Oig Lett 7 491—493... 

A few examples of the oxidative homo-coupling of two alkenyltin moieties were reported recently. It is essential to use Cul (Scheme 54). As other oxidizing agents, oxygen or ethyl 2,3-dibrcMno-3-phenylpropanoate,f and ( )-l,2-diiodoethene have been used. 

The first method involves oxidative homo-coupling of bis (terminal alkynyl) complexes in the presence of a catalytic amount of a copper(I) halide and O2 as the oxidizing agent (Scheme 5.1, Eq. 5.1) [10]. The use of this catalyst system in organic synthesis is extensive and is better known as Hay s coupling reaction [11]. Extension of this methodology to organometallic synthesis was demonstrated by the conversion of trans-bis(acetylide) monomers into polymeric complexes. It is... 

As early as in 1993, Ohkubo and co-workers reported a photocatalytic system consisting of a chiral Ru complex A-[Ru(menbpy)3] [menbpy = 4,4 -di-(lR,2S,5R)-(-)-memthoxycarbonyl-2,2 -bipyridine] possessing a C3 molecular helicity as the photosensitizer and Co(acac)3 (acac = acetylacetonato) as the oxidant which could catalyze the oxidative homo-coupling of 2-naphthols (Scheme 3.13). Under the radiation of visible light, A-[Ru(menbpy)3] " can... 

Oxidative homo-coupling of terminal alkynes mediated by stoichiometric (or often excess) Cu(OAc)2. A variant of the Glaser coupling leactioa... 

Scheme 1.11 Intermolecular oxidative homo-coupling (van Helden, Verberg, 1965).

Scheme 9.37 Ruthenium-catalyzed oxidative homo-coupling of oxazoline 66.

More recentiy, a rhodium-catalyzed carbonylation of arylzinc compounds was developed by Takagi [117]. In the presence of an Rh-dppf catalyst under 1 bar of CO using 1,2-dibromoethane as the oxidant, carbonylative homo-coupling of arylzinc compounds was achieved, affording symmetrical diaryl ketones in good yields (Scheme 8.33). Under similar conditions, Pd or Ni catalysts induced oxidative homo-coupling of zinc reagents to yield diaryls instead. 

In contrast to the efficient and widely apphed palladium-catalyzed C(sp)-C(sp ) cross-coupling, palladium-catalyzed C(sp)-C(sp) and C(sp)-C(sp ) cross-couplings remain largely underdeveloped. Furthermore, they are hampered by side reactions. In the first case, competitive cross-homo scrambling is problematic. In the second case, as oxidative addition is slower in the case of C(sp )-electrophiles, competitive oxidative homo-coupling of the alkynylzinc partner becomes important It is thus limited to activated electrophiles. 

Monoreaction of H-T2-H 2 with tropylium tetrafluoroborate gave in 40% yield the monosubstituted bithiophene 401 which was isomerized to the corresponding 4H-derivative 402. Oxidative homo-coupling of lithiated bithiophene 402 with CuCl2 gave the disubstituted quaterthiophene 403 in a moderate yield (27%). Following hydride... 

Haley and coworkers have prepared the analogous expanded dehydrobenzan-nulenes as outlined in Scheme 6.1 [10]. Triynes 5 and 6 were oxidatively homo-coupled using a unique, one-pot desilylation/dimerization procedure to provide hexaynes 7 and 8, respectively. Dehydrobenzannulene 9 and the solubilized derivative 10 were achieved in very good yields via the Cu-mediated intermolecular oxidative homocoupling of the appropriate hexayne 7 or 8, subsequent to desilylation using TBAF. The authors noted the remarkable stability of these [32]annulene derivatives. 

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