Oxidative coupling, palladium-catalysed

There are also palladium-catalysed procedures for allylation. Ethyl 3-bromo-l-(4-methylphenylsulfonyl)indole-2-carboxylate is allylated at C3 upon reaction with allyl acetate and hexabutylditin[27], Ihe reaction presumably Involves a ir-allyl-Pd intermediate formed from the allyl acetate, oxidative addition, transmetallation and cross coupling. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Palladium-catalysed directed C-H oxidation with (diacetoxy)iodobenzene of a series of meta -substituted aryl pyridine and aryl amide derivatives resulted in the formation of the corresponding acetoxy compounds. The reactions generally proceed with high levels of regioselectivity for functionalization of the less sterically hindered ortho-C-H bond.144 The mechanism shown in Scheme 4 has been proposed for the oxidation of 2,6-dimethylphenol with (diacetoxyiodo)benzene for the formation of 3,5,3, 5 -tetramethyl-biphenyl-4,4 -diol, via C-C coupling.145... 

Mori started with the early introduction of the chiral centre [298] in using (3-oxidation of pentanoic acid A by the yeast, Candida rugosa, IFO 0750 [299]. The obtained (R)-3-hydroxypentanoic acid B was transformed into C in a few conventional steps. The second building block was prepared from methyl 2-pentynoate D conjugate addition of lithium dimethyl cuprate yielded E, which was further converted into the frans-configured vinyl bromide F. Hydro-boration of C yielded G which upon Suzuki s palladium catalysed cross-coupling with F furnished 157 after treatment of the reaction product with hydrochloric acid followed by chromatographic purification. The synthesis of ent-157 used (S)-3-hydroxypentanoic acid. 

The palladium-catalysed cross-coupling of aryl halides or vinyl halides with dialkyl phosphonates (31) to yield dialkyl arylphosphonates and dialkyl vinylphosphonates, respectively, was first reported by Hirao and co-workers 69 the halides used most frequently are bromides and the reaction is stereospecific with haloalkenes. Subsequently, analogous reactions of alkyl alkylphosphinates (32), alkyl arylphosphinates (32), alkyl phosphinates (33), and secondary phosphine oxides (34), replacing [P—H] bonds with [P—C] bonds to yield various phosphinates and tertiary phosphine oxides, have been developed (Figure 7.1). Alkyl phosphinates (33) may be mono- or diarylated as desired by the selection of appropriate conditions. Aiyl and vinyl triflates have also found limited... 

Terminal arylalkynes can be prepared by oxidation-decarbonylation of 3-arylpropargyl alcohols using manganese dioxide in the presence of alkali. The corresponding arylpiopargyl dcohols are available by palladium-catalysed cross-coupling of aryl halides with commercial propargyl alcohol. The yield of the second step can be improved by the addition of a phase-transfer catalyst (18-crown-6 Scheme 18). ... 

Coupling of the building blocks and further transformation to 104-109 are shown in Figure 4.60. Palladium-catalysed Wacker oxidation of G yielded 104, 107 and two other ketones. Final oxidation of 105 and 108 to 106 and 109, respectively, was achieved with Dess-Martin periodinane.105 Thus, all the six components 104—109 of the female-produced contact sex pheromone of the German cockroach were synthesized from the enantiomers of citronellal. 

Both 2- and 3-pyridyl stannanes can be prepared by palladium-catalysed stannylation. The M-oxides and qnatemary salts of pyridyl stannanes can be used in Stille couplings. ... 

Halodiazinones undergo palladium-catalysed couplings with boronic acids and stannanes, but the reactions appear to be less consistent than with other systems. Temporary protection as silyl derivatives," or the use of additives such as silver oxide are beneficial in some cases, but it is often preferable to carry out transformations on alkoxydiazines, followed by hydrolysis. Direct coupling with organocopper reagents has also been described." ... 

Peroxide oxidation of At-phenylsulfonylindole-3-boronic acid gives A-phenylsul-fonylindoxyl, which can be converted into the triflate of the 3-hydroxyindole tautomer,and this in turn utilised in palladium-catalysed cross-coupling processes. The same A-protected indoxyl can be prepared by ring synthesis, as shown below. 

Another route to compounds of the same oxidation level involves palladium-catalysed coupling of enol ethers. ... 

The mechanism of palladium-catalysed cross-coupling starts, as in the Heck reaction, with oxidative addition of the halide or triflate to the initial palladium(O) phosphine complex to form a palladium(ll) species. But the next step is new it is a transmetallation, so-called because the nucleophile (Ri) is transferred from the metal in the organometallic reagent to the palladium and the counterion (X=hallde or triflate) moves in the opposite direction. The new palladium(ll) complex with two organic ligands undergoes reductive elimination to give the coupled product and the palladium(O) catalyst, ready for another cycle. 

Furan-2- and 3-boronic acids have been made by reaction of the lithiated species with tributyl borate " they are very useful for palladium-catalysed coupling reactions. Oxidation of a 2-boronate ester is a means for the synthesis of butenolides (section 15.12.1). ... 

Table 8. Palladium-catalysed homo-coupling reaction of arylboronic acids to symmetrical biaryls in the presence of copper(II) nitrate as an oxidant...

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