SYNTHESIS with palladium complexes

Allyl complexes were amongst the earliest studied nomigid complexes, and there are a number of reviews on their rearrangements. Interest has been particularly high owing to their importance in catalysis and synthesis, with palladium and molybdenum complexes playing leading roles. 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

Allylic substitution was initially conducted with palladium complexes lacking any added dative ligands or with palladium complexes containing classic aromatic phosphine ligands. More recently, catalyst development for this reaction has focused on the design and synthesis of new ligands for enantioselective allylic substitution processes. These ligands were presented as part of Section 18.6 on enantioselective allylic substitution. 

Recently, the synthesis of palladium complexes based on 1,8-naphthridine functionalized NHC motif has been reported [16]. This was the first reported case of the successful preparation of a di-palladium complex with double 1,8-naphthridine-based NHC ligands (12). It was reported to catalyze Suzuki-Miyaura coupling reactions for aryl bromides imder refluxing temperatures but was found to be ineffective for activated or unactivated aryl chlorides. 

Sanchez and coworkers reported the synthesis of palladium complexes bearing two imidazolin-2-ylidene moieties linked by a chiral dioxolane backbone (Figure 4.19). This is the first example on the use of a chiral bis(NHC)-metal catalyst in asymmetric hydrogenation with high enantioselectivity [60]. 

Fluoroalkylated heteroaromatic polymers can be prepared from 2,5-dibromo-3-perfluoropropylthiophene with NiCl2 [567]. The cross coupling reaction of dihaloheteroaromatic compounds and l,2-bis(tributylstannyl)ethylene catalyzed with palladium complexes leads in a single step to poly(thienylene vinylene) [353,568, 569]. The synthesis of structurally homogeneous PATs is described in Sect. 5.1.8. 

In the last few years, several groups have developed enantioselective domino reactions catalysed by combinations of organocatalysts with palladium complexes. As an example, Murkheqee and List have reported a domino synthesis of p-all earbon quaternary amines on the basis of a highly enantioseleetive a-allqrlation of a-branched aldehydes, involving an achiral palladium catalyst and a chiral phosphoric acid. Under the catalysis of phosphoric acid, a secondary allylamine reacted with an a-branehed aldehyde to form an enammonium phosphate salt, which upon reaction with palladium catalyst afforded a cationic 7t-allyl palladium complex (Scheme 7.2). This intermediate resulted in the formation of an a-allylated iminium ion, whieh eould be reduced to the corresponding final chiral amine in high yield and exeellent enantioselectivity of 97% ee. The synthetic utility of this transformation was also demonstrated hy a formal synthesis of (+)-cuparene. 

These studies were then extended to the synthesis of palladium complexes of symmetric azines. In contrast to the above-mentioned compounds the di-jU-acetato complexes, 61, showed SmC, and in two cases, also nematic, mesophases. The jU-acetato ligands constrain the complexes to be nonpla-nar, and a novel type of structure based upon open-book-shaped molecules [143] has been proposed. With R optically active, a mixture of cis- and rrans-isomers was observed. 

J. Tsuji, Organic Synthesis by Means of Transition Metal Complexes, Springer-Verlag, New York (1975). J. Tsuji, Organic Synthesis with Palladium Compounds, Springer-Verlag, New York (1980). 

Seven procedures descnbe preparation of important synthesis intermediates A two-step procedure gives 2-(HYDROXYMETHYL)ALLYLTRIMETH-YLSILANE, a versatile bifunctional reagent As the acetate, it can be converted to a tnmethylenemethane-palladium complex (in situ) which undergoes [3 -(- 2] annulation reactions with electron-deficient alkenes A preparation of halide-free METHYLLITHIUM is included because the presence of lithium halide in the reagent sometimes complicates the analysis and use of methyllithium Commercial samples invariably contain a full molar equivalent of bromide or iodide AZLLENE IS a fundamental compound in organic chemistry, the preparation... 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

The systematic synthesis and spectral examination of a large series of complexes, M(ol)n, ra = 1-3, M = Ni or Pd, has been performed (140 -142), with special reference to the optical spectra of the products, again affirming the usefulness of the technique for observing spectral trends as a function of substituent (see later). A number of interesting points emerged from this study, some of which have already been alluded to. The optical data for the nickel and palladium complexes respectively are reported in Tables XVI and XVII. 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

Reinhoudt and coworkers studied the synthesis of hyper-branched polymers composed of organopalladium complexes with an SCS pincer ligand [11]. Removal of acetonitrile ligands on palladium led to the self-assembly of dinu-clear palladium complex (9) to give hyper-branched polymer (10), which was... 

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