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

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Palladium-Catalyzed Cross-CouplingAngewandteChemiechemists who search for more effective, practical catalysts tolower metal loading, increase selectivity and thereby minimizewaste. These are important factors from reagenteconomy and sustainability point of views. A significantpoint to mention is that although academic discoveries on therole of bulky electron-rich trialkylphosphines as highlyeffective ligands for coupling reactions, many of these ligandsare highly pyrophoric liquids or solids. This was a considerabledrawback for their use in scale-up of reactions in the chemicalindustry. For example, Fu demonstrated the superior propertiesof the sterically demanding trialkylphosphine, PtBu 3ligand for a wide range of coupling reactions. However, thepyrophoric property of this ligand detracted from its practicaluse in many, inadequately equipped, industries. In addition,typically these ligands are used in excess. A solution to itspyrophoric nature proved to be the use of preformedpalladium complexes incorporating the PtBu 3 ligand. Twofront-line solid crystalline precatalysts, [{Pd(m-Br)P(tBu 3 )} 2 ](1) and [Pd(PtBu 3 ) 2 ](2; Scheme 26) evolved, both of whichwere shown to be only slightly air-sensitive (comparable to[Pd(PPh 3 ) 4 ]), and thus attractive for use on large scalesynthesis. Hartwig, among others, demonstrated the efficacyof the Pd I dimer 1 in Suzuki–Miyaura couplings andBuchwald–Hartwig aminations. [147] Furthermore, the Pd 0catalyst 2 has been used, for example, in many reactionsincluding Heck, Suzuki, and Negishi reactions, displayingexcellent reactivity even at low palladium loadings. [148] Theseobservations triggered the development and commercialavailability of a large range of preformed [L 2 Pd 0 ] catalysts,bearing various tertiary phosphine ligands (2–8) for use insmall and large-scale synthesis. [149] All of these new Pd 0catalysts turned out to be unique for certain chemistries, anexample being a new C C bond forming carbohalogenationreaction using [Pd(Q-Phos) 2 ]. [150]The use of preformed palladium complexes as catalystsleads, unavoidably, to a second point for consideration: theoften observed, more efficient reaction using a preformedcomplex as opposed to one formed in situ. Prashad and coworkersshowed that the use of precatalyst 1 results in higheryields of isolated tertiary amine in Buchwald–Hartwigaminations than those obtained using a Pd(OAc) 2 /PtBu 3catalyst system (Scheme 27). [151] As further recent examples,catalyst 2 was found to give 98% conversion into productScheme 27. Preformed catalysts in Prashad’s Buchwald–Hartwig aminations.[151]compared to 69% yield obtained by an in situ [Pd(dba) 2 ]/2PtBu 3 system for a one-pot conversion of isoindolines to 1-arylisoindoles, [152] and Shaughnessy and Colacot reportedthat, in amination reactions, the preformed p-allyl catalyst 9 isnot only air-stable but shows a performance superior to thatof the in situ [Pd 2 (dba) 3 ]/Dt-BNpP catalyst system whenexcess ligand is used. [153] A number of highly active preformedPd p-allyl and crotyl catalysts containing dialkylaryl phosphineligands, have recently been reported by Colacot and coworkers.[154]Similarly, the preformed air-stable palladium(II) catalystbearing the 1,1’-bis(di-tert-butylphosphino)ferrocene(DtBPF) 10 shows definite superior activity in both Suzuki–Miyaura and a-arylation reactions, compared to the catalystgenerated in situ from [Pd 2 (dba) 3 ] and DtBPF(Scheme 28). [155]Scheme 28. Advantage of preformed catalysts. [155] Ar = p-tolyl.Scheme 26. Selected preformed palladium catalysts for various crosscouplingreactions. [147–150, 153–159] Amphos = PtBu 2 C 6 H 4 NMe 2 ;Dt-BNpP = di(tert-butyl)neopentylphosphine.In recent years, a number of research groups haveintroduced other preformed Pd complexes as highly effectivecatalysts for cross-coupling reactions. Among these, Buchwaldspalladacycles 11, [156] the PEPPSI catalyst 12 [157] andNolans [137] carbene-containing complexes 13 [158] and 14, [159]are noteworthy.In addition to the practicality and efficiency aspects, thethird point that is of higher significance in industry comparedAngew. Chem. Int. Ed. 2012, 51, 5062 – 5085 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org5077
Angewandte .ReviewsT. J. Colacot, V. Snieckus et al.to academic research is the amount of remaining palladiumcontamination in the isolated organic product. [160] Typically,approximately 5 ppm is the allowable limit. This can criticallyinfluence the route selection by which a pharmaceuticalmaterial is synthesized. Apart from the evident solution ofchoosing a highly active catalytic system which can beoperated at a lower loading, this issue has been addressedby the development of several solid-supported preformedpalladium complexes which have been launched onto thecatalyst market. [161] Often, as noted previously, the additionalbeneficial effects, such as air-stability (for pyrophoric phosphineligands) and higher activity, are observed using thesepolymer supported catalysts. An example of this technology isthe development of air-stable tunable Pd-FibreCat series,which contains air-sensitive or pyrophoric mono- and bidentateligands, such as tBu 3 P and 1,1’-bis(diisopropylphosphino)ferrocene(dippf). [162]Scheme 29. The Tsuji–Trost allylation reaction. [163] Optical yield is theratio of optial purity of product to that of the catalyst.1977 which marked his defining contributions to the field. bis(di-o-tolylphosphino)ferrocene.Before this seminal work, Pd was not known to participate inchiral induction reactions because of the b-hydride eliminationcomplications.4. Cross-Coupling Reactions Involving AlternativeCoupling Partners: History Repeats ItselfAs is evident from the above discussion, a common trendis apparent in the historical development of cross-couplingprocesses involving organohalides and organometallic components:a coupling reaction is studied using stoichiometricamount of transition metal, usually copper or nickel andoptimization of the procedure leads to conditions of catalyticloadings. Often these observations prompt further researchinto these processes, ultimately yielding catalytic palladiummediatedvariants of these transformations. As this storyunfolded in the 20th century, punctuated by the discovery of4.2. a-Arylation of Carbonyl Compounds [166]In the a-arylation process, an enolate coupling partner,generated in situ from a carbonyl compound and a base, istreated with an aryl halide or pseudohalide in the presence ofa nickel(0) or palladium(0) catalyst to achieve a formalC(sp 3 ) H–C(sp 2 ) coupling result. This reaction also followsthe historical sequence noted above, namely the movementfrom stoichiometric to catalytic conditions. Discovered bySemmelhack in 1973 with a stoichiometric organonickelspecies (Scheme 30), the a-arylation process was appliedalso in stoichiometric fashion in the total synthesis ofthe named reaction processes, other types of new coupling Cephalotaxus alkaloids. [167] Almost 20 years later, simultaneousprocedures emerged with this recurring theme and with rarelya divergence from this chronology.reports from Hartwig, [168] Buchwald, [169] and Miura [170]set the stage for the development of the Pd-based a-arylationThe cross-coupling reactions discussed so far are mechanisticallyrationalized to follow the general catalytic cycledepicted in Scheme 1, with the common features of anorganohalide (or pseudohalide) and an organometallicreagent (or a nucleophilic heteroatom) as coupling partners.A number of other cross-coupling reactions in which one (orboth) of the coupling partners is replaced by a reagent notexhibiting these general features have been developed.Although not the focus of this Review, these methods arehighlighted briefly below.4.1. Allylic Alkylation[163, 164]The Tsuji–Trost allylation, is conceptually alsoa palladium(0)-mediated coupling reaction but is mechanisticallydifferent from the conventional cross-coupling processesand achieves an allylic substitution via an intermediatep-allylpalladium complex. This type of reactivity was firstdescribed in 1965 by Tsuji, who observed the reaction ofdiethyl malonate with the stoichiometric preformed palladiumallyl complex (Scheme 29). [50] Following initial results in1973, [164] Trost reported the first asymmetric transformation inScheme 30. History of the a-arylation reaction. [167–170] DTPF = 1,1’-5078 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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