RESUMO
This Account provides an overview of our recent efforts to uncover the fundamental charge transport properties of Si-Si and Ge-Ge single bonds and introduce useful functions into group 14 molecular wires. We utilize the tools of chemical synthesis and a scanning tunneling microscopy-based break-junction technique to study the mechanism of charge transport in these molecular systems. We evaluated the fundamental ability of silicon, germanium, and carbon molecular wires to transport charge by comparing conductances within families of well-defined structures, the members of which differ only in the number of Si (or Ge or C) atoms in the wire. For each family, this procedure yielded a length-dependent conductance decay parameter, ß. Comparison of the different ß values demonstrates that Si-Si and Ge-Ge σ bonds are more conductive than the analogous C-C σ bonds. These molecular trends mirror what is seen in the bulk. The conductance decay of Si and Ge-based wires is similar in magnitude to those from π-based molecular wires such as paraphenylenes However, the chemistry of the linkers that attach the molecular wires to the electrodes has a large influence on the resulting ß value. For example, Si- and Ge-based wires of many different lengths connected with a methyl-thiomethyl linker give ß values of 0.36-0.39 Å-1, whereas Si- and Ge-based wires connected with aryl-thiomethyl groups give drastically different ß values for short and long wires. This observation inspired us to study molecular wires that are composed of both π- and σ-orbitals. The sequence and composition of group 14 atoms in the σ chain modulates the electronic coupling between the π end-groups and dictates the molecular conductance. The conductance behavior originates from the coupling between the subunits, which can be understood by considering periodic trends such as bond length, polarizability, and bond polarity. We found that the same periodic trends determine the electric field-induced breakdown properties of individual Si-Si, Ge-Ge, Si-O, Si-C, and C-C bonds. Building from these studies, we have prepared a system that has two different, alternative conductance pathways. In this wire, we can intentionally break a labile, strained silicon-silicon bond and thereby shunt the current through the secondary conduction pathway. This type of in situ bond-rupture provides a new tool to study single molecule reactions that are induced by electric fields. Moreover, these studies provide guidance for designing dielectric materials as well as molecular devices that require stability under high voltage bias. The fundamental studies on the structure/function relationships of the molecular wires have guided the design of new functional systems based on the Si- and Ge-based wires. For example, we exploited the principle of strain-induced Lewis acidity from reaction chemistry to design a single molecule switch that can be controllably switched between two conductive states by varying the distance between the tip and substrate electrodes. We found that the strain intrinsic to the disilaacenaphthene scaffold also creates two state conductance switching. Finally, we demonstrate the first example of a stereoelectronic conductance switch, and we demonstrate that the switching relies crucially on the electronic delocalization in Si-Si and Ge-Ge wire backbones. These studies illustrate the untapped potential in using Si- and Ge-based wires to design and control charge transport at the nanoscale and to allow quantum mechanics to be used as a tool to design ultraminiaturized switches.
RESUMO
The enantioselective allylation of ketones is a problem of fundamental importance in asymmetric reaction design, especially given that only a very small number of methods can generate tertiary carbinols. Despite the vast amount of attention that synthetic chemists have given to this problem, success has generally been limited to just a few simple ketone types. A method for the selective allylation of functionally complex ketones would greatly increase the utility of ketone allylation methods in the chemical synthesis of important targets. Here we describe the operationally simple, direct, regioselective and enantioselective allylation of ß-diketones. The strong tendency of ß-diketones to act as nucleophilic species was overcome by using their enol form to provide the necessary Brønsted-acid activation. This reaction significantly expands the pool of enantiomerically enriched and functionally complex tertiary carbinols that may be easily accessed. It also overturns more than a century of received wisdom regarding the reactivity of ß-diketones.
Assuntos
Técnicas de Química Sintética/métodos , Cetonas/química , Ácido Clorídrico/química , Macrolídeos/química , Metanol/química , Pentanonas/química , Silanos/química , EstereoisomerismoRESUMO
Because of both their synthetically challenging and stereochemically complex structures and their wide range of often clinically relevant biological activities, nonaromatic polyketide natural products have for decades attracted an enormous amount of attention from synthetic chemists and played an important role in the development of modern asymmetric synthesis. Often, such compounds are not available in quantity from natural sources, rendering analogue synthesis and drug development efforts extremely resource-intensive and time-consuming. In this arena, the quest for ever more step-economical and efficient methods and strategies, useful and important goals in their own right, takes on added importance, and the most useful syntheses will combine high levels of step-economy with efficiency and scalability. The nonaromatic polyketide natural product zincophorin methyl ester has attracted significant attention from synthetic chemists due primarily to the historically synthetically challenging C(8)-C(12) all-anti stereopentad. While great progress has been made in the development of new methodologies to more directly address this problem and as a result in the development of more highly step-economical syntheses, a synthesis that combines high levels of step economy with high levels of efficiency and scalability has remained elusive. To address this problem, we have devised a new synthesis of zincophorin methyl ester that proceeds in just nine steps in the longest linear sequence and proceeds in 10% overall yield. Additionally, the scalability and practicability of the route have been demonstrated by performing all of the steps on a meaningful scale. This synthesis thus represents by a significant margin the most step-economical, efficient, and practicable synthesis of this stereochemically complex natural product reported to date, and is well suited to facilitate the synthesis of analogues and medicinal chemistry development efforts in a time- and resource-efficient manner.
Assuntos
Ácidos Carboxílicos/síntese química , Ácidos Carboxílicos/química , Conformação MolecularRESUMO
A highly conducting electronic contact between a strained disilane and Au is demonstrated through scanning tunneling microscope-based single-molecule measurements. Conformationally locked cis diastereomers of bis(sulfide)-anchor-equipped 1,2-disilaacenaphthenes readily form high-conducting junctions in which the two sulfide anchors bind in a bipodal fashion to one gold electrode, providing enough stability for a stable electrical contact between the Si-Si σ bond and the other electrode.
RESUMO
The stability of chemical bonds can be studied experimentally by rupturing single molecule junctions under applied voltage. Here, we compare voltage-induced bond rupture in two Si-Si backbones: one has no alternate conductive pathway whereas the other contains an additional naphthyl pathway in parallel to the Si-Si bond. We show that in contrast to the first system, the second can conduct through the naphthyl group when the Si-Si bond is ruptured using an applied voltage. We investigate this voltage induced Si-Si bond rupture by ab initio density functional theory calculations and molecular dynamics simulations that ultimately demonstrate that the excitation of molecular vibrational modes by tunneling electrons leads to homolytic Si-Si bond rupture.
RESUMO
An approach to the validation of linker strategies for polyketide natural products with few or no obvious handles for linker attachment, and its application to dictyostatin, are described. Analogues in which the C(6)- and C(12)-methyl groups were replaced by 4-azidobutyl groups were prepared and shown to retain the low nanomolar potency of dictyostatin. Further, conjugation of the C(6) analogue with a cyclooctyne resulted in only minor attenuations in potency. Together, these results shed light on the binding of dictyostatin to ß-tubulin, establish a validated linker strategy for dictyostatin, and set the stage for the synthesis and study of dictyostatin conjugates.
Assuntos
Produtos Biológicos/química , Macrolídeos/química , Policetídeos/química , Sítios de Ligação , Modelos Moleculares , Conformação Molecular , Reprodutibilidade dos Testes , Estereoisomerismo , Tubulina (Proteína)/químicaRESUMO
An approach to the synthesis of the (iso)cyclocitrinol core structure is described. The key step is a tandem Ireland Claisen/Cope rearrangement sequence, wherein the Ireland Claisen rearrangement effects ring contraction to a strained 10-membered ring, and that strain in turn drives the Cope rearrangement under unusually mild thermal conditions. A major side product was identified as resulting from an unexpected and remarkably facile [1,3]-sigmatropic rearrangement, and a tactic to disfavor the [1,3] pathway and increase the efficiency of the tandem reaction was rationally devised.
Assuntos
Terpenos/síntese química , Lactonas/síntese química , Lactonas/química , Modelos Moleculares , Conformação Molecular , EstereoisomerismoRESUMO
Here we demonstrate for the first time that strained silanes couple directly to gold electrodes in break-junction conductance measurements. We find that strained silicon molecular wires terminated by alkyl sulfide aurophiles behave effectively as single-molecule parallel circuits with competing sulfur-to-sulfur (low G) and sulfur-to-silacycle (high G) pathways. We can switch off the high conducting sulfur-to-silacycle pathway by altering the environment of the electrode surface to disable the Au-silacycle coupling. Additionally, we can switch between conductive pathways in a single molecular junction by modulating the tip-substrate electrode distance. This study provides a new molecular design to control electronics in silicon-based single molecule wires.
RESUMO
The enantioselective crotylation of aldehydes with 1,2-diaminochlorocrotylsilane reagents is effectively catalyzed by Sc(OTf)(3). The one significant limitation on the utility of these reagents--substrate scope--has thus been addressed. The net result is the most comprehensive and highly practical method for enantioselective aldehyde crotylation yet advanced.
Assuntos
Aldeídos/química , Silanos/química , Catálise , EstereoisomerismoRESUMO
A highly efficient and step-economical synthesis of zincophorin methyl ester has been achieved. The unprecedented step economy of this zincophorin synthesis is principally due to an application of the tandem silylformylation-crotylsilylation/Tamao oxidation-diastereoselective tautomerization reaction, which achieves in a single step what would typically require a significant multistep sequence.
Assuntos
Produtos Biológicos/economia , Ácidos Carboxílicos/síntese química , Macrolídeos/síntese química , Produtos Biológicos/síntese química , Ácidos Carboxílicos/química , Macrolídeos/química , Estrutura Molecular , EstereoisomerismoRESUMO
Enantioselective (formal) aza-Diels-Alder reactions between acylhydrazones and non-Danishefsky-type dienes have been developed. The reactions are promoted by a simple and economical chiral silicon Lewis acid and are typically conducted at ambient temperature. Both glyoxylate- and aliphatic aldehyde-derived hydrazones may be employed, as may variously substituted dienes, leading to the synthesis of a diverse array of tetrahydropyridines with good to excellent levels of enantioselectivity.
Assuntos
Hidrazonas/química , Polienos/química , Hidrazonas/síntese química , Polienos/síntese química , Piridinas/síntese química , Piridinas/química , EstereoisomerismoRESUMO
The addition of a stabilized sulfur ylide (generated by the rhodium-catalyzed reaction of Ph(2)S with ethyl diazoacetate) to N-acylhydrazones promoted by a chiral silane Lewis acid leads to the highly diastereo- and enantioselective synthesis of beta-chloro-alpha-hydrazido esters. The addition of electron-rich arenes and ZnCl(2) to the reaction mixture leads to the highly diastereo- and enantioselective one-pot synthesis of diarylalanine derivatives. In both cases, the silane Lewis acid responsible for the first reaction performs the second function of activating the aziridine intermediate toward nucleophilic attack.
RESUMO
"Quat's" the story? N-Aryl amides are effective directing/activating groups for chlorosilane Lewis acids. This aspect has been exploited for the development of the first simple and general method for the highly enantioselective Pictet-Spengler reaction of ketimines derived from alpha-ketoamides leading to quaternary alpha-amino acid derivatives (see scheme).
Assuntos
Compostos Heterocíclicos/síntese química , Silanos/química , Amidas/química , Aminoácidos/química , Catálise , Compostos Heterocíclicos/química , EstereoisomerismoRESUMO
The first general method for the highly enantioselective Mannich reaction of aliphatic ketimines is reported. A new, second generation chiral silane Lewis acid has been developed that promotes the reaction between ketone-derived hydrazones and silyl ketene acetals, providing the beta,beta-disubstituted beta-amino esters with good enantioselectivity even for the hydrazone derived from 2-butanone (methyl vs ethyl, 91% ee). Several examples are provided, including a reaction with a substituted (propanoate-derived) silyl ketene acetal.
Assuntos
Aminoácidos/síntese química , Hidrazonas/química , Cetonas/química , Silanos/química , Acetais/química , Ésteres/síntese química , Etilenos/química , Hidrazonas/síntese química , Iminas/química , Silanos/síntese química , EstereoisomerismoRESUMO
A brief synthesis of manzacidin C based on a chiral silane-promoted diastereo- and enatioselective acylhydrazone-alkene [3 + 2] cycloaddition reaction has been achieved. This synthesis is the first synthesis of any of the manzacidins wherein the C(4) and C(6) stereocenters are established in a single highly stereoselective step.
Assuntos
Pirimidinas/síntese química , Pirróis/síntese química , Ciclização , Estrutura Molecular , Pirimidinas/química , Pirróis/química , EstereoisomerismoRESUMO
Spongistatin 1 is among the most potent anti-proliferative agents ever discovered rendering it an attractive candidate for development as a payload for antibody-drug conjugates and other targeted delivery approaches. Unfortunately, it is unavailable from natural sources and its size and complex stereostructure render chemical synthesis highly time- and resource-intensive. As a result, the design and synthesis of more acid-stable and linker functional group-equipped analogs that retain the low picomolar potency of the parent natural product requires more efficient and step-economical synthetic access. Using uniquely enabling direct complex fragment coupling crotyl- and alkallylsilylation reactions, we report a 22-step synthesis of a rationally designed D-ring modified analog of spongistatin 1 that is characterized by GI50 values in the low picomolar range, and a proof-of-concept result that the C(15) acetate may be replaced with linker functional group-bearing esters with only minimal reductions in potency.
Assuntos
Desenho de Fármacos , Macrolídeos/química , Macrolídeos/síntese química , Antimitóticos/síntese química , Antimitóticos/química , Antimitóticos/farmacologia , Linhagem Celular Tumoral , Proliferação de Células/efeitos dos fármacos , HumanosRESUMO
The first general method for the enantioselective cinnamylation of aldimines is reported. The method utilizes a simple chiral cinnamylsilane reagent and is characterized by experimental simplicity and extraordinarily high levels of enantioselectivity. Further, a remarkable and unprecedented diastereochemical reversal has been realized whereby either diastereomer may be accessed from the same trans-cinnamylsilane based upon a subtle change to the imine, thus obviating the usual requirement for both the trans- and cis-cinnamylsilanes.
Assuntos
Cinamatos/química , Iminas/química , Benzeno/química , Estrutura Molecular , EstereoisomerismoRESUMO
A new chiral allylchlorosilane has been developed that allows the highly enantioselective allylation and crotylation of a range of 2-imidazolylaldimines and ketimines. The method may be exploited for the protecting group-free synthesis of a diverse array of imidazole-bearing chiral carbinamines and, when coupled with ring-closing metathesis reactions, allows the one-pot synthesis of unusual heterocyclic motifs with potential relevance in medicinal chemistry.