Palladium-Catalyzed Cross-Coupling - A Historical Contextual Perspective to the 2010 Nobel Prize

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Palladium-Catalyzed Cross-CouplingAngewandteChemiewhich, today, is a highly promising C C bond formingreaction with application in industry. [166]4.3. Decarboxylative Coupling ReactionsThe original decarboxylative cross-coupling reaction wasreported by Nilsson in 1966 employing nearly stoichiometricquantities of copper under harsh conditions (Scheme 31). [171]Almost 40 years later, Myers disclosed a catalytic processemploying palladium(II) salts, [172] and, in further variants,other researchers showed that the organometallic componentmay be generated in situ from a carboxylic acid by, forexample, the action of an additional metal (Scheme 32) [173] oran additive, such as nBu 4 NCl. [174]reagents, the only by-product is H 2 , and the term high atomeconomymay be used without embarrassment. This dream israpidly and undeniably becoming reality, as numerousresearch groups are recognizing the opportunities to developnew synthetic methods by processes given the umbrella titleof C H activation. The first hint in the literature concerningsuccessful C H activation reactions predate the palladiummediatedcross-couplings. Thus, in 1963 Kleiman and Dubeckreported the isolation of a nickel Cp complex (Cp = C 5 H 5 )resulting from ortho C H activation of azobenzene formed bythe action of stoichiometric quantities of nickel(Scheme 33). [175] Shortly after, Chatt and Davidson notedScheme 31. The first examples of the decarboxylative cross-coupling[171, 172]reaction.Compared to the complementary C H activation methodologydiscussed below, the decarboxylative coupling doesnot suffer from regioselectivity issues since the site forcoupling is predefined. The elevated temperatures for thereaction, availability of benzoic acid derivatives, and functional-grouptolerance need to be addressed for furtherevolution of the decarboxylative coupling as a competingcoupling methodology. The interested reader is referred tothe Review by Goossen [173b] for a detailed account of thistopic.4.4. C H Activation Reactionsthe equilibrium of a ruthenium naphthalene species betweenthe p-complex and the C H insertion complex. [176] In fact, CH activation had successfully moved into the catalytic realmeven before the discovery of catalytic palladium-mediatedcross-coupling. This is evidenced by the work of Fujiwarawho, in 1969, reported on the oxidative Heck reaction [177] twoyears before the initial disclosures of Heck and Mizoroki.Following these early results, C H activation has witnesseda surge of activity in the last 20 years, pioneered by thecontributions of Murai and his school, using rutheniumcatalysis. [178] Bergman and Graham initiated the quest forthe “holy grail” of cross-coupling, investigating alternativemetals for these transformations. [179] Today, the direct C Hactivation wave employing palladium catalysis stimulated byresults from the laboratories of Du Bois, Fagnou, [180] Gaunt,Hartwig, Miura, Sanford, Yu, among others, are pushing theboundaries of the probable and possible. [181] We will leave thistopic here at the origins and developments, and refer thereader to Reviews in the rapidly evolving C H activation[182, 183]area.Scheme 33. Early C H activation processes. [175–177]Scheme 32. The Pd–Cu catalyzed decarboxylative cross-coupling reaction.[173]A dream of organic chemists has been the discovery ofcoupling reactions with no prefunctionalization of the couplingpartners. In other words, imagine a route which avoidsthe use of aryl halides or pseudohalides and organometallic5. Industrial ApplicationsIn organic synthesis methodology, the ultimate testimonyof value is utility. The Nobel Prize winning Heck, Suzuki, andAngew. Chem. Int. Ed. 2012, 51, 5062 – 5085 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org5079
Angewandte .ReviewsNegishi coupling chemistries, alongside the other discoveriesand technologies outlined above, amply substantiate thismaxim. Thus, syntheses of natural products and drug moleculesabound where the key step(s) consists of metalcatalyzedcross-coupling reactions as selectively exemplifiedby the well-established BMS (originally Dupont–Merck)synthesis of Losartan, [184] the Merck Singulair process (aHeck coupling involving isomerization of an allylic alcohol),[185] and the total synthesis of discodermolide by Smith(Negeshi coupling) and later Novartis (Suzuki coupling;Scheme 34). [186] Examples are continuously appearing, such asthe Novartis route to Gleevec (Imatinib) which employsa Buchwald–Hartwig amination as the key step. [187] Manleyand co-workers have demonstrated large-scale Corriu–Kumada and Negishi couplings in the synthesis of PDE472,a potential drug for the treatment of asthma. [189] Pfizersprocess for the production of a hepatitis C polymeraseinhibitor incorporates a Heck reaction carried out on 40 kgscale. [188] In a very recent report, Koning and co-workersprovide an example of a Suzuki–Miyaura coupling on a 50 kgscale in the synthesis of Crizotinib, a potent anti-canceragent. [190] A large number of Reviews published on theScheme 34. Cross-coupling reactions in total synthesis. [184–190]T. J. Colacot, V. Snieckus et al.application of cross-coupling methodologies in total synthesis[191] reflect the fertile evolution of the subject and theobvious revolution that these methodologies have broughtabout for natural-product and drug-molecule synthesis forwhich the 2010 Nobel Prize has been fittingly awarded.6. Coupling Reactions: Unsolved Problems,Challenges, and OutlookDespite the many significant discoveries and developmentssince the first reports of transition-metal-catalyzedorganic transformations, there remain, as expected in theincremental waves of scientific progress, numerous unsolvedproblems. Thus, only partial success in C(sp 3 ) C(sp 3 ) andC(sp 2 ) C(sp 3 ) cross-coupling reactions involving alkyl halideshas been achieved. Further, achievement of selective mono a-arylation requires effort in cases where the product of thereaction can partake in secondary coupling (e.g., a-arylationof CH 3 COR and arylation of primary amines). In the area ofasymmetric catalysis, although simple methods using chiralligands to form enantioenriched products have been developed,fundamental studies are essential to achieve practicaland hence widely applied methodology. Finally, determinationof the mechanism is essential for successful use insynthesis. Thus, although the steric and electronic effects ofthe ligands involved in the various steps of the catalytic cycleare now reasonably well rationalized, there remains, in manycases, a lack of understanding of the mechanism of formationof the active Pd 0 catalytic species from preformed Pd IIcomplexes. The newest wave of C H activation reactionswill increasingly find advantages by the use of alternativemetals, such as ruthenium, iridium, and—the oldest catalyticmetal—copper. To predict whether or not palladium willalways be the metal of choice is, as always in scienceprognosis, a treacherous undertaking. For now, the future ofpalladium is bright since, in a vast array of conditions fora multitude of disconnections, the unique qualities ofpalladium are demonstrable, serving only to reinforce itspower especially in C C bond construction.To conclude, gigantic progress has been achieved in thelast 40 years within the palladium-catalyzed cross-couplingfield using functionalized leaving group and metal couplingpartners. Assessed by recent developments, the increasingfocus on the development of direct C H activation processeswill become the next prevailing wave of reactions forsynthesis. This reaction also awaits the development ofmore practical and economical conditions and proceduresfor application in large-scale syntheses whilst finding robustsolutions to the selectivity issues encountered for lessstraightforwardcoupling partners. The sometime expressednotion that the market is becoming saturated with catalystsand methods for coupling reactions can easily be dismissed byanother observation by Woodward who, in 1969, noted thatcomments such as “… well, now you have shown thatanything can be synthesized, so the field has had it…” werecommon place. [146] Three Nobel Prizes in Chemistry forhomogeneous catalysis during the last decade, [192] culminatingwith the 2010 Nobel prize for palladium-catalyzed cross-5080 www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 5062 – 5085
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Palladium-Catalyzed Cross-Coupling - A Historical Contextual Perspective to the 2010 Nobel Prize

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