Single-molecule conductance in a unique cross-conjugated tetra(aminoaryl)ethene.

A 1,1,2,2-tetrakis(4-aminophenyl)ethene with three paths of π-conjugation, linear-cis, linear-trans and a cross-conjugation, has been prepared. The molecule is able to bind to gold electrodes forming molecular junctions for single-molecule conductance measurements. Only two regimes of conduction are found experimentally. The modelling of the conductance allows to assign them to through-bond transmission in the linear case, while the cross-conjugated channel is further assisted by through-space transmission, partially alleviating the destructive quantum interference.

The sequential building of chiral macrocyclic bis-beta-lactams by double staudinger-Cu-catalyzed azide-alkyne cycloadditions.

A novel approach for the synthesis of macrocyclic bis-beta-lactams based on the Cu-catalyzed alkyne-azide cycloaddition (CuAAC) is reported. The procedure is general and allows access to a full range of diastereomerically or enantiomerically pure macrocyclic cavities in good yields. The incorporation of chiral oxazolidinone fragments at C3 in the beta-lactam rings allows the total enantiocontrol of the process.

Synthesis of ferrocene tethered open and macrocyclic bis-beta-lactams and bis-beta-amino acid derivatives.

New bioorganometallic ferrocene derivatives are synthesized through a Diversity Oriented Synthesis strategy. Easily available ferrocene bisimines have been transformed into open ferrocenyl bis-beta-lactams. These compounds have demonstrated to be versatile synthons used in further transformations into new ferrocene bis-beta-amino acids. Carefully selected substituents submitted to ring closing metathesis (RCM) and Cu-catalyzed oxidative alkyne coupling conditions have also allowed the conversion of open substrates into ferrocenic macrocyclic bis-beta-lactams.

The assembly of macrocyclic bis- and tetra-beta-lactams with embedded platinum or palladium square-planar centers.

The synthesis, isolation, and full characterization of different types of stable, metal-assembled macrocyclic beta-lactams are reported. By using adequately functionalized bis-beta-lactams with defined stereochemistry as building blocks, a series of mono- and bimetallic Pd and Pt macrocycles has been prepared in good to quantitative yields. These novel structures combine the beta-lactam moiety with transition-metal fragments with cis-square-planar geometry and constitute a new class of metal-assembled cavities involving molecules with biological relevance as building blocks. By combining the adequate ligands, metallic fragments, and tuning the reaction conditions, different mono- and bimetallic macrocyclic beta-lactam cavities can be selectively obtained. Macrocycles with Pt-ethynyl groups are suitable to form host-silver triflate guest complexes in a tweezer fashion.

Synthesis of highly functionalized macrocycles by the peripheral functionalization of macrocyclic diimines.

The easily available macrocyclic diimines 4-7 can be stereoselectively transformed to macrocyclic bis-beta-amino acids 13-17, macrocyclic bisazetidines 18-20, and macrocyclic bisamides 21 and 22 by means of the corresponding bis-beta-lactam scaffolds 8-12. These key intermediates are available through standard Staudinger reaction and obtained as the cis-cis diastereomers, exclusively. An interesting relation between the proximity of the reactive C=N bonds and the selectivity in the formation of the bis-beta-lactams 8-12 is observed. Thus, diimine 4 leads to low selectivities, producing a 1:1 mixture of cis-syn-cis and cis-anti-cis diastereomers, while diimines 5-7 having the diimine sites more separated lead almost exclusively to the cis-anti-cis diastereomers. The stereochemistry of all the products was unambiguously assigned by X-ray diffraction analysis of compounds cis-syn-cis 8 and cis-anti-cis 12-Co2CO6 complex.

Catalytic transmetalation from group 6 Fischer carbene complexes: an emerging powerful tool in organic synthesis.

The chemistry of metal carbene complexes has experienced an enormous development in the past decades. Despite this fact, the use of transition metals as catalysts in reactions involving group 6 Fischer carbene complexes was virtually neglected. Here, we describe how the reactivity of these compounds can be enhanced or modified in the presence of catalytic amounts of a transition metal, leading to new forms of reactivity and others offering clear advantages in terms of efficiency over the uncatalyzed reactions. The key step for these reactions is the transmetalation from the stoichiometric metal carbene reagent to the catalyst. This process generates a new metal-carbene complex that leads to enhanced reactivity of new reaction modes. Two examples of Fischer carbene complexes obtained by transmetalation to Pd and Cu have been isolated during the last 2 years, showing the flexibility if the mechanistic hypothesis for these reactions. The work presented in this Account shows how an area, which was practically unexplored 5 years ago, has emerged as a new and powerful field of research.

Synthesis and electrochemical properties of novel tetrametallic macrocyclic Fischer carbene complexes.

[reaction: see text] Metallomacrocyclic compounds can be easily prepared by 1,4-addition of diamines to alpha,beta-unsaturated Fischer bis-carbene templates. This method allows the preparation of a new family of homo- and heterotetrametallic compounds having macrocyclic cyclophanic structures.

Reduction of cyclic group 6 Fischer carbene complexes: a case of exquisite regioselectivity.

The reduction of alpha,beta-unsaturated cyclic group 6 metal-carbene complexes strongly depends on the electronic profile of the groups attached both to the carbene and the beta-carbon and occurs with exquisite regioselectivity. Thus, for complexes 8 the reduction does not take place at the carbene carbon but exclusively at the gamma-carbonyl group. The resulting alkoxide 20 evolves to a tricyclic epoxide structure 21, which precludes additional hydride transfers. Complex 9 experiences the exclusive 1,4-reduction because of the imino character of the beta-carbon (due to the participation of the aromatic resonance form 22). In contrast, monocyclic carbene complexes 10 behave as their acyclic congeners and experience 1,2-hydride addition followed by the 1,3-migration of the metal center. In this case, the participation of eta(3)-Cr(CO)(5) species 31 allows us to understand the labeling pattern found in the final products.

Synthesis of o,p-EDDHA and its detection as the main impurity in o,o-EDDHA commercial iron chelates.

Ethylenediamine-N,N'bis(o-hydroxyphenyl)acetic acid (o,o-EDDHA) is one of the most efficient iron chelates employed to relieve iron chlorosis in plants. However, the presence of positional isomers of EDDHA in commercial iron chelates has been recently demonstrated, and among them, it has been claimed that ethylenediamine-N(o-hydroxyphenylacetic)-N'(p-hydroxyphenylacetic) acid (o,p-EDDHA) is the main impurity present in EDDHA fertilizers. Here we report the preparation of o,p-EDDHA, a compound whose synthesis had not been previously reported. The synthetic o,p-EDDHA is able to form ferric complexes, and it has been used as a standard in the analysis of the impurities of commercial iron fertilizers. The presence of o,p-EDDHA/Fe(3+) in commercial samples has been unambiguously demonstrated by HPLC.