Palladium mono-N-protected amino acid complexes: experimental validation of the ligand cooperation model in C-H activation.

Mechanistic proposals for the C-H activation reaction enabled by mono-N-protected amino acid ligands (MPAAs) have been supported by DFT calculations. The direct experimental observation of the ligand-assisted C-H activation has not yet been reported due to the lack of well-defined isolated palladium complexes with MPAAs that can serve as models. In this work, palladium complexes bearing chelating MPAAs (NBu4)[Pd(κ2-N,O-AcN-CHR-COO)(C6F5)py] (Ac = MeC(O); R = H, Me) and [Pd(κ2-N,O-MeNH-CH2-COO)(C6F5)py] have been isolated and characterized. Their evolution in a solution containing toluene leads to the C-H activation of the arene and the formation of the C6F5-C6H4Me coupling products. This process takes place only for the ligands with an acyl protecting group, showing the cooperating role of this group in a complex with a chelating MPAA, therefore experimentally validating this working model. The carboxylate group is inefficient in this C-H activation.

Multifaceted role of silver salts as ligand scavengers and different behavior of nickel and palladium complexes: beyond halide abstraction.

The reaction of [NiArBr(PPh3)2] with AgBF4 brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH2Cl2/toluene a mixture of the cationic aquo derivatives [NiAr(H2O)(PPh3)2]BF4 (2) and [NiAr(H2O)2(PPh3)]BF4 (3) is formed, along with AgBr and [Ag(PPh3)n]BF4. When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ2-O, O-MeC(O)CH2C(OH)Me2)(PPh3)] (5), bearing a chelating ligand formed by the aldol self-condensation of acetone. Phosphine abstraction by silver is less favorable for the analogous palladium(II) complexes and only occurs if a large excess of AgBF4 is used. Thus, silver salts can be safely used as halide scavengers for palladium derivatives. However, the generation of cationic Ni complexes from neutral precursors by halide extraction with a silver salt may produce naked species, different than those expected, and highly reactive in certain media.

Palladium-Catalyzed Ortho C-H Arylation of Unprotected Anilines: Chemo- and Regioselectivity Enabled by the Cooperating Ligand [2,2'-Bipyridin]-6(1H)-one.

Metal-catalyzed C-H functionalizations on the aryl ring of anilines usually need cumbersome N-protection-deprotection strategies to ensure chemoselectivity. We describe here the Pd-catalyzed direct C-H arylation of unprotected anilines with no competition of the N-arylation product. The ligand [2,2'-bipyridin]-6(1H)-one drives the chemoselectivity by kinetic differentiation in the product-forming step, while playing a cooperating role in the C-H cleavage step. The latter is favored in an anionic intermediate where the NH moiety is deprotonated, driving the regioselectivity of the reaction toward ortho substitution.

Palladium hydrazonato complexes and their role in the Pd-catalyzed cross-coupling reactions of hydrazones as carbene precursors.

Hydrazones are common carbene precursors in many palladium-catalyzed cross coupling reactions of carbenes as a coupling partner, but their interaction with palladium has been overlooked. We have found that hydrazonato ligands readily coordinate to Pd aryl complexes leading to [PdAr(L-L){(TolSO2)N-NCHR}] (Ar = Ph, C6F5; L-L = dppe, dppf; R = CHCHPh, Ph). Ligand substitution reactions on [PdAr(dppe)X] (X = Br, TolSO2) show that the hydrazonato ligand coordinates preferentially so the hydrazonato complexes are likely resting states in catalytic carbene coupling reactions using hydrazones as reactants. The decomposition of the hydrazonato moiety to a diazoalkane is needed during the catalysis and the analysis of the evolution of the hydrazonato complexes shows that it is not promoted by coordination to the metal and it does not occur in the coordination sphere of palladium. The substitution of diazoalkane for the metal-bound hydrazonato is possible and the steps that follow to form a new C-C bond, including the carbene migratory insertion, are fast.

Diazo compounds and palladium-aryl complexes: trapping the elusive carbene migratory insertion organometallic products.

The reactions of Pd-aryl complexes with diazo compounds N2CH-CHCHPh and N2CHPh allowed us to isolate the organometallic products formed right after the migratory insertion of a non-stabilized CHR carbene into the Pd-aryl bond. η3-Allylic and η3-benzylic palladium complexes were formed respectively. This is compelling experimental evidence for the key step in the palladium-catalyzed cascade transformations of diazo derivatives leading to multiple C-C or C-X bond formation.

A different polynorbornene backbone by combination of two polymer growth pathways: vinylic addition and ring opening via ß-C elimination.

A new polynorbornene skeleton has been found that contains bicyclic norbornane units and cyclohexenyl methyl linkages. The polymers have been synthesized using a nickel catalyst in the presence of a controlled amount of ligands with low or moderate coordination ability. The backbone structure is the result of a vinylic addition polymerization, via sequential insertions of norbornene into a Ni-C bond (bicyclic units) combined with an unusual ring opening of the norbornene structure by a ß-C elimination (cyclohexenyl methyl units) to give a new Ni-C(alkyl) bond that continues the polymerization. The ring opening events are favored when the rate of propagation of the vinylic addition polymerization decreases, and this can be modulated by making the coordination of norbornene to the metal center less favorable using additional ligands.

Trispyrazolylborate Ligands Supported on Vinyl Addition Polynorbornenes and Their Copper Derivatives as Recyclable Catalysts.

Polynorbornenes prepared by vinyl addition polymerization and bearing pendant alkenyl groups serve as skeletons to support trispyrazolylborate ligands (Tpx ) built at those alkenyl sites. Reaction with CuI in acetonitrile led to VA-PNB-Tpx Cu(NCMe) (VA-PBN=vinyl addition polynorbornene) with a 0.8-1.4 mmol incorporation of Cu per gram of polymer. The presence of tetracoordinated copper(I) ions was been assessed by FTIR studies on the corresponding VA-PNB-Tpx Cu(CO) adducts, in agreement with those on discrete Tpx Cu(CO). The new materials were employed as heterogeneous catalysts in several carbene- and nitrene-transfer reactions, showing a behavior similar to that of the homogeneous counterparts but also being recycled several times maintaining a high degree of activity and selectivity. This is the first example of supported Tpx ligands onto polymeric supports with catalytic applications.

[2,2'-Bipyridin]-6(1 H)-one, a Truly Cooperating Ligand in the Palladium-Mediated C-H Activation Step: Experimental Evidence in the Direct C-3 Arylation of Pyridine.

The ligand [2,2'-bipyridin]-6(1 H)-one (bipy-6-OH) has a strong accelerating effect on the Pd-catalyzed direct arylation of pyridine or arenes. The isolation of relevant intermediates and the study of their decomposition unequivocally show that the deprotonated coordinated ligand acts as a base and assists the cleavage of the C-H bond. Mechanistic work indicates that the direct arylation of pyridine with this ligand occurs through a Pd(0)/Pd(II) cycle. Because of this dual ligand-intramolecular base role, there is no need for an available coordination site on the metal for an external base, a difficulty encountered when chelating ligands are used in coupling reactions that involve a C-H cleavage step.

Polyphosphazenes for the Stille reaction: a new type of recyclable stannyl reagent.

A random phosphazene copolymer {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}n (2) and a block copolyphosphazene {[N = P((CH2)7-Br)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}n (3) and {[N = P((CH2)7-SnBu2An)Ph]0.5[N = PMePh]0.5}45-b-[N = P(O2C12H8)]55 (6, An = p-MeOC6H4). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with ArI, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without a significant release of tin, but its efficiency decreased after three consecutive cycles. This effect was explained by studying the self-assembly of the polymer under the same conditions used for the catalytic experiments, which evidenced the progressive coalescence of the polymeric vesicles (polymersomes) leading to stable and bigger core-shell aggregates by the attraction of the [N = P(O2C12H8)] rich membranes, thus decreasing the accessibility of the tin active centers.

Formal Gold-to-Gold Transmetalation of an Alkynyl Group Mediated by Palladium: A Bisalkynyl Gold Complex as a Ligand to Palladium.

The reaction of [Au(C≡C-n-Bu)]n with [Pd(η(3) -allyl)Cl(PPh3 )] results in a ligand and alkynyl rearrangement, and leads to the heterometallic complex [Pd(η(3) -allyl){Au(C≡C-n-Bu)2 }]2 (3) with an unprecedented bridging bisalkynyl-gold ligand coordinated to palladium. This is a formal gold-to-gold transmetalation that occurs through reversible alkynyl transmetalations between gold and palladium.

Detection and reactivity of a palladium alkoxycarbene.

A characterful carbene: The high electrophilicity of a genuine palladium alkoxycarbene, obtained by transmetalation, is evident in its reactivity. Nucleophilic attack on two electrophilic centers (red) is observed. Alkyl abstraction and addition to the carbene carbon by different nucleophiles occur. This palladium(II) alkoxycarbene also undergoes comproportionation with palladium(0) to give an unprecedented palladium(I) dimeric carbene (see scheme).

Stannylated polynorbornenes as new reagents for a clean Stille reaction.

New functionalized polynorbornenes have been obtained in good yields by vinylic copolymerization of norbornene with a (norbornenyl)SnBu(2)Cl monomer, catalyzed by [Ni(C(6)F(5))(2)(SbPh(3))(2)]. Subsequent functionalization produces a wide variety of polymers with different --SnBu(2)R groups (R=aryl, vinyl, alkynyl). The polymers can be used as R-transfer reagents in Stille couplings, thereby providing easy workup and separation of the polymeric tin byproducts from the coupling products. Tin contents of around 0.05 wt % are found in the Stille products. The stannylated polymers can be recycled and reused with good efficiency.

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines.

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.

Pd-H elimination reactions in palladium(II) allylic complexes.

The ease of H elimination from the 4- (beta-) position in a series of allylic complexes [Pd(5-C(6)F(5)-1-3-eta(3)-cyclohexenyl)XL](n+) (X, L = Br, N-, P-donor, C(6)Cl(2)F(3); n = -1, 0, +1) was compared by analyzing their decomposition products at 50 degrees C. Pd-H elimination does not occur for cationic complexes, whereas it is the main decomposition pathway for neutral and anionic complexes. In addition to the charge of the complex, the ease of this Pd-H elimination is determined by the trans influence of the ligands (aryl > PMe(3) > Br, N-donor).

Aryl palladium carbene complexes and carbene-aryl coupling reactions.

Transmetalation of an aminocarbene moiety from [W(CO)5{C(NEt2)R}] to palladium leads to isolable monoaminocarbene palladium aryl complexes [{Pd(mu-Br)Pf[C(NEt2)R]}2] (R = Me, Ph; Pf = C6F5). When [W(CO)5{C(OMe)R}] is used, the corresponding palladium carbenes cannot be isolated since these putative, more electrophilic carbenes undergo a fast migratory insertion process to give alkyl palladium complexes. These complexes could be stabilized in the eta3-allylic form for R = 2-phenylethenyl or in the less stable eta3-benzylic fashion for R = Ph. Hydrolysis products and a pentafluorophenylvinylic methyl ether (when R = Me) were also observed. The monoaminocarbenes slowly decompose through carbene-aryl coupling to produce the corresponding iminium salts and, depending on the reaction conditions, the corresponding hydrolysis products. The electrophilicity of the carbene carbon, which is mainly determined by the nature of the heteroatom group, controls the ease of evolution by carbene-aryl coupling. Accordingly, no carbene-aryl coupling was observed for a diaminocarbene palladium aryl complex.

Dimeric palladium complexes with bridging aryl groups: when are they stable?

Stable dimeric palladium(II) complexes of general formula [Pd(2)(mu-R)(2)(eta(3)-allyl)(2)] (R=haloaryl, mesityl) have been prepared. Their X-ray crystal structures, determined for some of the complexes, show that the two coordination square planes are usually coplanar. The haloaryl complexes are fluxional in solution, showing exchange between cis and trans isomers (relative to the orientation of the two allyl groups in the dimer) through solvent-assisted associative bridge splitting. A number of other ancillary ligands (O,O, S,S, or C,N donors) failed to stabilize the bridging situation. Also, bridging phenyls were unstable. The reasons for this behavior and the formation of alternative compounds in attempts at synthesizing them are fully analyzed and explained. Stable aryl bridges seem to be favored by a combination of factors: the use of ancillary ligands of small size and lacking electron lone pairs, and the use of aryl ligands reluctant to homo and hetero C--C coupling. These seem to be more important factors in the stabilization of bridging aryl complexes than the strength of the bridges themselves.

A warning on the use of radical traps as a test for radical mechanisms: they react with palladium hydrido complexes.

Typical radical traps (galvinoxyl, TEMPO, DPPH) react with palladium hydrides, sometimes at rates competitive with those of palladium hydride catalyzed reactions that follow an insertion mechanism (for example, alkene isomerization). Thus, positive results for radical reaction tests can be misleading. The complexes with more polarizable (neutral complexes rather than cationic) and more accessible hydrides, and the less sterically protected radical traps, react faster.

Oxidative coupling of platinum arylamides: temperature dependent C-H or C-F cleavage.

The oxidation of an arylamido platinum complex leads to C-C coupling with selective C-H versus C-F bond cleavage depending on the reaction temperature.