Ana C. Albéniz

The Pd-Catalyzed Coupling of Allyl Halides and Tin Aryls: Why the Catalytic Reaction Works and the Stoichiometric Reaction Does Not

Arylallylpalladium complexes [Pd(5-C6F5-eta3-cyclohexenyl)(C6Cl2F3)(NCMe)] (10) and [Pd2(mu-C6Cl2F3)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (13) have been synthesized. Complex 13 is an example of a rare class of metal complexes with aryl bridges and its X-ray crystal diffraction structure has been determined. These arylallylpalladium complexes are involved in the coupling of Bu3SnRf (1, Rf = dichlorotrifluorophenyl) and [Pd2(mu-Br)2(5-C6F5-1,3-eta3-cyclohexenyl)2] (2); complex 10 has been detected in the course of the stoichiometric coupling reaction in acetonitrile. Decomposition experiments of 10 and 13 in different conditions, and comparison with the reactions of 1 and 2, allow us to determine that reductive elimination does not occur in the absence of additives. p-Benzoquinone coordinates to Pd to give complex 15 and promotes reductive elimination to give the coupling products selectively. The outcome of the coupling reaction is controlled by the reductive elimination step, but the overall rate is controlled by the faster preequilibrium, which determines the concentration of 10 or 13. Palladium-catalyzed coupling of allyl halides and tin aryls works better than the stoichiometric allyl-aryl reductive coupling on isolated allylarylpalladium complexes, because they benefit from the presence in the solution of substrate allylic halides acting as electron-withdrawing olefins and promoting reductive elimination. More efficient allyl-aryl couplings, whether stoichiometric or catalytic, can be achieved upon addition of p-benzoquinone to the reaction mixture in a noncoordinating solvent.

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.

Ionic Silver Amino Complexes Displaying Liquid Crystalline Behavior Close to Room Temperature

The liquid crystalline properties of a series of ionic silver complexes [Ag(NH2n-CnH2n+1)2]X (X = NO3, n = 6, 8, 10, 12, 14; X = BF4, n = 8, 10, 12, 14), derived from silver nitrate or tetrafluoroborate and aliphatic amines, have been investigated by optical microscopy, DSC, and X-ray diffraction. The materials exhibit a birefringent fluid phase identified as a smectic A mesophase by optical microscopy and X-ray diffraction. Some of the complexes are liquid crystalline at temperatures close to room temperature. The X-ray results are consistent with the existence in the mesophase of a bilayer organization. The silver cations adopt a U-shape, which allows for the adoption of this bilayer arrangement, with alternating ionic and apolar regions and a short-range square-planar array inside the cationic sublayers. The thermodynamic parameters suggest that the melting transition clearly involves only disorder (melting) of the chains, whereas the clearing transition implies breaking the anion-cation arrangement.

Synthesis and phase behaviour of mesomorphic transition-metal complexes of alkoxydithiobenzoates

Reaction of 4-alkoxydithiobenzoates (n-odtb, where n refers to the number of carbon atoms in the alkoxy chain) with divalent salts of nickel, palladium and zinc leads to complexes of the formula [M(n-odtb)2] where M = Ni, Pd and [Zn2(n-odtb)4] which are mesomorphic. The shortest-chain homologues of the Ni and Pd complexes show a nematic phase while at longer chain lengths, an SC phase is observed. Most homologues of the zinc complexes show a nematic phase with an SC phase appearing only for the longest homologues. The gold(III) complexes [AuX2(n-odtb)] are also mesomorphic and show the SA phase. Single-crystal X-ray structure determinations have been carried out for [Pd(8-odtb)2], [Zn2(4-odtb)4] and [Zn2(8-odtb)4], showing the first to be planar and the last two dinuclear with pseudo five-co-ordinate zinc atoms.

Palladium(II) complexes of 3,3′-, 5,5′-, and 7,7′-dimethyl-2,2′-biindazole

Palladium-chelate complexes of the type [PdX2(L-L)] [X = Cl; L-L = 3,3′- (1), 5,5′- (2), or 7,7′-dimethyl-2,2′-biindazole (3) have been prepared, but are too insoluble for structural studies. When X = C6F5 the position of the methyl substituents is critical: thus, 3 is unable to give the corresponding complex due to severe methyl-C6F5 interactions, 2 gives a planar complex, and 1 gives a non-planar (likely helicene-type) complex due to methyl-methyl interaction, as shown by 19F NMR spectroscopy. [Pd(C6F5)X(L-L)][X = Br; L-L = 1] is also non-planar.

Asymmetric organocatalysts supported on vinyl addition polynorbornenes for work in aqueous media

In an effort to identify novel polymer architectures suitable for a covalent support for catalysts, L-proline derivatives have been immobilized onto rationally designed vinyl addition polynorbornene (VA-PNB) resins through copper-catalyzed azide–alkyne cycloaddition (CuAAC) reactions. These fully saturated resins have been found to be optimal catalyst supports, the resulting proline-functionalized resins behave as very active, easily recoverable and highly reusable organocatalysts for the asymmetric direct aldol reaction of benzaldehydes with ketones in aqueous media. The results show that, the combination of modular VA-PNB resins with proline derivatives and triazole linkers represents a promising strategy for the immobilization of organocatalytic species.

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.

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).

N‐Heterocyclic Carbenes Supported on Vinylic Addition Polynorbornene: A Recyclable and Recoverable Organocatalyst

A bromoalkyl functionalized vinylic addition polynorbornene has been used as an inert new scaffold to support imidazolium salts by straightforward nucleophilic substitution using different imidazoles ImR (R=Me, mesityl, 2,6‐iPr2C6H3). These polymers, VA‐PNB‐(CH2)4ImRBr, are precursors of N‐heterocyclic carbenes which are useful as heterogeneous organocatalysts. The polymeric carbenes have been tested in two model reactions: The synthesis of saturated esters from α,β‐unsaturated aldehydes and the conjugate umpolung reaction of α,β‐unsaturated aldehydes for the synthesis of γ‐butyrolactones. The supported catalyst (R=mesityl) is fully recyclable and can be recovered as an imidazolium salt and stored for further use.

Palladium-Mediated Organofluorine Chemistry

Abstract The importance of organofluorine compounds in pharmaceuticals and agrochemicals has encouraged the study of their synthesis since the middle of the twentieth century. In recent years, there has been a huge increase in the work devoted to improve the use of transition metals as catalysts for the formation of C F bonds and its activation, and also to the development of fluoroalkylation and fluoroarylation reactions. Palladium complexes, as undisputed protagonists in the catalyzed reactions for the formation of C C or C heteroatom bonds, have been employed for these reactions. However, the particular properties of fluorine and fluorinated hydrocarbons do not allow a straightforward translation of the palladium-catalyzed methodologies. This chapter provides an overview of the progress in palladium-catalyzed synthesis of organofluorinated compounds. It also accounts for those studies on the reactivity of palladium complexes bearing fluoride or fluorinated groups that allow to rationalize why some catalysts are effective and why some others are not. Although progress in the understanding of these chemical systems has been enormous in recent years, many aspects of their reactivity are still poorly understood, so, predictably, the field will remain an active focus of research in coming years.