Palladacycle-phosphine complexes

One of the design ideas was to develop a good source of monophosphine palladium complexes formed from some convenient precursor. As soon as palladacycles were shown to release palladium, it seemed promising to design 1 1 palladacycle-phosphine hybrid complexes that were supposed to serve as a source of monophosphine species upon cleavage. 

This design idea, however, has not led to clear improvement yet. Indeed, if this principle were operative, then a hybrid complex would perform superior to the respective homoleptic phosphine complex or a mixture of a simple palladium source and a phosphine. In fact, this design idea implies that during the preactivation stage the hybrid complex 199 loses palladacychc ligand while keeping phosphine to furnish monophosphine palladium species 

Indolese and coworkers [132, 133] have shown that simple CN-palladacycles 203 or 204 can be used in equimolar mixtures with bulky secondary or tertiary phosphines at 0.5 mol% loading (Na2C03, DMA, 140 °C) in order to process deactivated aryl chloride 

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Zim and Buchwald synthesized the palladacyclic phosphine complex 44 based on their ligand P(Bu-f)2(o-biphenyl) 13 (equation 46)157. This air- and moisture-stable palladium complex is a convenient one-component precatalyst for animation of aryl chlorides when combined with sodium tert-butoxide or sodium methoxide. For coupling of anilines, the addition of NEt3, which is possibly acting as the reducing agent to produce Pd(0), is necessary. 

A novel racemic palladacyclic dimeric complex 136 was resolved via recrystallization of its diastereomeric (3 )-prolinate derivatives 137 and 138 (Scheme 20). The chemistry and resolution of chiral PMeBuTh was studied using 139 and 140. Complex 139 was also used in the resolution of stilbenediamine. The P-chiral phosphine PBuTh(4-BrC6H4) was resolved using 141. ... 

It appears that a modified mechanism operates when tr .s-(o-tolyl)phosphine is used as the ligand,133 and this phosphine has been found to form a palladacycle. Much more stable than noncyclic Pd(0) complexes, this compound is also more reactive toward oxidative addition. As with the other mechanisms, various halide adducts or halide-bridged compounds may enter into the overall mechanism. 

Milstein et al. found that Pd complexes with chelating alkylphosphines such as bis(diisopropylphosphino)butane (dippb) efficiently catalyze the olefmation of aryl chlorides with styrenes in the presence of elemental zinc [29]. Unfortunately, these electron-rich phosphines are apparently incompatible with electron-poor olefins such as acrylic acid derivatives. The latter were successfully coupled with activated chloroarenes by Herrmann et al., who used palladacycles or Pd-catalysts with heterocyclic carbenes [30]. 

Here again, the reaction involved an intramolecular displacement of the iodide by an ester enolate (Scheme 10). Preparation of stable azapalladacycle ( )-93 commenced with treatment of sulfonamide 90, accessible via A -alkylation of A -trifluoromethanesulfonyl-2-iodoaniline with palladium(O) (Pd2(DBA)3 DBA = dibenzylideneacetone) and tetramethylethylenediamine (TMEDA) to afford palladium(ll) complex 91. An easy ring closure of complex 91 provided palladacycle ( )-92 in 92% yield via addition of /-BuOK (IM in solution in THE, 1.2equiv) at room temperature. Displacement of tetramethylethylenediamine with triphenyl-phosphine delivered palladacycle ( )-93 in quantitative yield. 

Many types of palladacyclic complexes have been used as precursors to catalysts for a variety of coupling processes146,147. Mixtures of dimeric palladacycles containing bridging halide, acetate or trifluoroacetate ions and a phosphine or carbene ligand have been studied as catalysts for the animation of aryl halides. The isolated phosphine adducts can also be applied in catalysis. 

These catalysts are more active in some cases than complexes generated from the same ligands and either palladium salts or dba adducts. In general, palladacycles have a higher thermal stability compared to the Pd/phosphine system and precipitation of palladium black is reduced. 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers. They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles and tuned catalysts (TunaCat). A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity. ... 

A step forward in the design of catalysts enabling the N-arylation of amines is to eliminate the activation step (reduction of palladium(II) to palladium(O) prior to oxidative addition). One approach is to use well-defined palladium(O) complexes of (NHC)2Pd or mixed phosphine/NHC,(R3P)Pd(NHC) type [80]. These complexes are efficient catalysts for this transformation at mild temperatures. A second approach is to sidestep two required activation stages in the catalytic cycle and eliminate the need for the preactivation and the oxidative addition processes by using well-defined catalysts that are oxidative addition adducts such as NHC-stabilized palladacycles (Scheme 25) [81]. 

In conclusion, for C-C bond forming reactions on aryl bromides and iodides there is no need for the use of complexes, palladacycles or pincers as simple ligand-free Pd(OAc)2 will perform as well. For C—C bond forming reactions on aryl chlorides some palladacycles may be the catalyst of choice. For problematic cases, the palladium complexes based on bulky electron-rich phosphines or catalysts based on carbene Hgands are likely to give the best results. 

Bulky tri(o-tolyl)phosphine was used first by Heck [11]. A palladacycle obtained from it is known as the Herrmann complex (XVIII-1) and is used extensively in HR [12]. Also, palladacycles XVIII-7 [13] and XVIII-2 [14] are high performance catalysts. Turnover numbers as high as 630-8900 were achieved by tetraphosphine Tedicyp (X-1) [15]. Recently, the remarkable effect of electron-rich and bulky phosphines, typically P(t-Bu)3 and other phosphines shown in Tables 1.4, 1.5 and 1.6, have been vmveiled. Smooth reactions of aryl chlorides using these ligands are treated later. Electron-rich ligands accelerate oxidative addition of aryl chlorides, and reductive elimination is accelerated by bulky ligands. HR can be carried out in an aqueous solution by use of a water-soluble sulfonated phosphine (TPPMS, II-2) [16]. 

Apart from palladacycles, a number of catalyst systems are currently known that show productivities up to 100,000 for Heck and Suzuki reactions of all kinds of aryl bromides. It is important to note that coupling reactions of electron-deficient aryl bromides (e.g., 4-bromoacetophenone), which are often used in academic laboratories, are not suitable as test reactions to judge the productivity of a new catalyst, because simple palladium salts without any Ugand give turnover numbers up to 100,000 with these substrates. Recently, palladium complexes in combination with sterically congested basic phosphines (e.g., tri-tcrt-butylphosphine), carbenes, and also phosphites led to productive palladium catalysts for the activation of various aryl chlorides. 

It emerges that the main steps of the former textbook mechanism proposed by Heck have been confirmed (Scheme 1.8). However, the catalytic cycle may involve intermediate palladium complexes whose structures differ from those originally proposed, depending on the experimental conditions. One must also take into account the fact that new reagents (aryl triflates), new ligands (bidentate ligands, carbenes, bulky phosphines, etc.) and new precursors (palladacycles) have been introduced a long time after Heck s proposal. 

The in situ formation of Pd(0) complexes takes place when Pd(OAc)2 is associated with various phosphines (i) aromatic phosphines (p-Z—C6H4)3P (Z = EDG or EWG). The formation of the Pd(0) complex follows a Hammett correlation with a positive slope [20]. The more electron-deficient the phosphine, the faster the reduction process this is in agreement with the intramolecular nucleophilic attack of the acetate onto the ligated phosphine as proposed in Scheme 1.13 (ii) aliphatic phosphines [20] (iii) water-soluble phosphines, triphenylphosphine trisulfonate (trisodium salt) [25] and triphenylphosphinetricarboxylate (trilithium salt) [26]. One major exception is the tri-o-tolylphosphine P(o-Tol)3, which cannot reduce Pd(OAc)2 to a Pd(0) complex in DMF or THE. Instead, an activation of one C-H bond of the tolyl moieties by Pd(OAc)2 takes place, leading to a dimeric P,C-palladacycle (see Section 1.5), as reported by Herrmann et al. in 1995 [27]. Such a Pd(ll) P,C-palladacycle catalyses Mizoroki-Heck reactions [27]. It is, however, a reservoir of a Pd(0) complex, as recently established by d Orly6 and Jutand [28] in 2005 (see Section 1.5). 

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