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1.
J Am Chem Soc ; 2020 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-32905701

RESUMO

Unidirectional molecular rotation based on alternating photochemical and thermal isomerizations of overcrowded alkenes is well established, but rotary cycles based purely on photochemical isomerizations are rare. Herein we report three new second-generation molecular motors featuring a phosphorus center in the lower half, which engenders a unique element of axial chirality. These motors exhibit unusual behavior, in that all four diastereomeric states can interconvert solely photochemically. Kinetic analysis and modeling reveal that the behavior of the new motors is consistent with all-photochemical unidirectional rotation. Furthermore, X-ray crystal structures of all four diastereomeric states of two of these new motors were obtained, which constitute the first achievements of crystallographic characterization of the full 360-degree rotational cycle of overcrowded-alkene based molecular motors. Finally, the axial phosphorus stereoelement in the phosphine motor can be thermally inverted, and this epimerization enables a "short-cut" of the traditional rotation-al cycle of these compounds.

2.
Angew Chem Int Ed Engl ; 59(2): 785-789, 2020 01 07.
Artigo em Inglês | MEDLINE | ID: mdl-31736200

RESUMO

A photoresponsive chiral catalyst based on an oligotriazole-functionalized unidirectional molecular motor has been developed for stereodivergent anion binding catalysis. The motor function controls the helical chirality of supramolecular assemblies with chloride anions, which by means of chirality transfer enables the enantioselective addition of a silyl ketene acetal nucleophile to oxocarbenium cations. Reversal of stereoselectivity (up to 142 % Δee) was achieved through rotation of the motor core induced by photochemical and thermal isomerization steps.

3.
Chemistry ; 26(8): 1759-1762, 2020 Feb 06.
Artigo em Inglês | MEDLINE | ID: mdl-31872488

RESUMO

A method for the nucleophilic fluorination of heptamethyl aryl trisiloxanes to form fluoroarenes is reported. The reaction proceeds in the presence of Cu(OTf)2 and KHF2 as the fluoride source under mild conditions for a broad range of heptamethyltrisiloxyarenes with high functional group tolerance. The combination of this method with the silylation of aryl C-H bonds enables the regioselective fluorination of non-activated arenes controlled by steric effects following a two-step protocol.

4.
Acc Chem Res ; 52(7): 1812-1823, 2019 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-31264834

RESUMO

The quest for organic materials with improved optoelectronic properties has stimulated the development of new strategies for the preparation of polycyclic aromatic hydrocarbons. Within this context, transition metal catalysis offers unparalleled opportunities for the assembly of complex molecular architectures. The palladium-catalyzed direct C-H arylation provides straight access to biaryls without the need of prefunctionalization at the nucleophilic site, which is attractive from the perspective of the synthesis of polyarenes. Mechanistically, this reaction was found to be different from an electrophilic aromatic substitution, involving the abstraction of a proton by an external base in the key metalation step. Using readily available C27 truxene as the starting material, a concise synthetic route consisting of a threefold benzylation and subsequent palladium-catalyzed arylation led to C60 polyarenes, also referred to as "crushed fullerenes", which could be converted into C60 fullerene by laser-induced cyclodehydrogenation in the gas phase or by thermal cyclodehydrogenation on a platinum surface. A conceptually related strategy based on the use of a highly electrophilic gold(I) complex as the catalyst for the threefold intramolecular hydroarylation of truxene derivatives was applied for the synthesis of decacyclenes. Using gold(I) catalysis, we have also developed a variety of synthetically useful protocols for the cycloisomerization of readily available 1,n-enynes as well as for the addition for nucleophiles to these unsaturated substrates. In one of these transformations, 1,7-enynes bearing aryl-substituted alkynes undergo formal [4 + 2] cycloaddition reactions via gold(I)-catalyzed 6-exo-dig cyclization and intramolecular Friedel-Crafts-type reaction to form tricyclic compounds bearing a dihydronaphthalene core. A related transformation led to a general synthesis of hydroacenes, which are known to be stabilized precursors of the corresponding conjugated acenes with enhanced solubility. A wide variety of dihydrotetracenes featuring electron-donating and electron-withdrawing groups, as well as dihydropentacene and dihydrohexacene could be easily obtained. A simple variation of our synthetic route led to tetrahydro-derivatives of higher acenes with up to 11 linearly fused six-membered rings. The dehydrogenation of tetrahydroacenes on a metallic substrate using the tip of a scanning tunneling microscopy instrument or by thermal annealing enabled the preparation of the whole series of higher acenes from heptacene up to previously unknown undecacene, whose structure was confirmed by noncontact atomic force microscopy. This work provided a unique opportunity for the analysis of the evolution of the transport gap in the acene series on Au(111). Furthermore, heptacene was also generated by on-surface dehydrogenation on Ag(001) from tetrahydroheptacene and a dibrominated derivative.

5.
Angew Chem Int Ed Engl ; 58(48): 17118-17129, 2019 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-31166642

RESUMO

The Pd-catalyzed coupling of aryl (pseudo)halides and amines is one of the most powerful approaches for the formation of C(sp2 )-N bonds. The pioneering reports from Migita and subsequently Buchwald and Hartwig on the coupling of aminostannanes and aryl bromides rapidly evolved into general and practical tin-free protocols with broad substrate scope, which led to the establishment of what is now known as the Buchwald-Hartwig amination. This Minireview summarizes the evolution of this cross-coupling reaction over the course of the past 25 years and illustrates some of the most recent applications of this well-established methodology.

6.
Chem Commun (Camb) ; 55(46): 6477-6486, 2019 Jun 04.
Artigo em Inglês | MEDLINE | ID: mdl-31099809

RESUMO

Photoswitchable catalysis is a young but rapidly evolving field that offers great potential for non-invasive dynamic control of both activity and selectivity in catalysis. Within this context, the E/Z photoisomerisation of double bonds in molecular switches and motors is one of the most popular tools to control the catalytic activity essentially due to its reversible nature, the large concomitant geometrical changes, and the high tunability of such photochromic entities. This Feature Article summarises the key developments accomplished over the past years through the incorporation of photoswitchable double bonds into the structure of catalytically competent molecules and shows some perspectives on the remaining challenges and possibilities arising from this, yet still somehow immature, exciting area of research.

7.
Chem Commun (Camb) ; 54(73): 10260-10263, 2018 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-30152499

RESUMO

Achieving the Ag(001)-supported synthesis of heptacene from two related reactants reveals the effect of the presence of Br atoms on the reaction process. The properties of reactants, intermediates and end-products are further characterized by scanning tunneling microscopy and spectroscopy.

8.
Org Lett ; 20(13): 3715-3718, 2018 07 06.
Artigo em Inglês | MEDLINE | ID: mdl-29878791

RESUMO

The design and synthesis of an overcrowded-alkene based molecular motor featuring a crown ether integrated in its stator structure has been accomplished. The photostationary state ratios and rotational speed of this motor can be modulated by cation coordination to the crown ether moiety, which can be reversed upon the addition of a competing chelating agent, thus achieving a dynamic control over the rotational behavior of the motor.

9.
Angew Chem Int Ed Engl ; 57(33): 10500-10505, 2018 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-29791082

RESUMO

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on-surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on-surface annealing. The structure of the generated acenes has been visualized by high-resolution non-contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

10.
Org Lett ; 19(20): 5561-5564, 2017 10 20.
Artigo em Inglês | MEDLINE | ID: mdl-28976200

RESUMO

The alkynylation of naphthols takes place with total regiocontrol at the peri position of the hydroxyl group in the presence of [RuCl2(p-cymene)]2 as the catalyst. This reaction features high functional group tolerance. The related ortho-alkynylation of benzoic acids proceeds under similar conditions and also shows wide functional group tolerance. Both reactions proceed through metalation, insertion of the alkyne, and bromide elimination.

11.
ACS Nano ; 11(9): 9321-9329, 2017 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-28817255

RESUMO

The on-surface synthesis of nonacene has been accomplished by dehydrogenation of an air-stable partially saturated precursor, which could be aromatized by using a combined scanning tunneling and atomic force microscope as well as by on-surface annealing. This transformation allowed the in-detail analysis of the electronic properties of nonacene molecules physisorbed on Au(111) by scanning tunneling spectroscopy measurements. The spatial mapping of molecular orbitals was corroborated by density functional theory calculations. Furthermore, the thermally induced dehydrogenation uncovered the isomerization of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.

12.
European J Org Chem ; 2017(1): 14-24, 2017 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-28747846

RESUMO

The outstanding performance of pentacene-based molecules in molecular electronics, as well as the predicted enhanced semiconducting properties of extended acenes, have stimulated the development of new synthetic methods and functionalization strategies for the preparation of stable and soluble acenes larger than tetracene with the aim of obtaining improved functional materials.

13.
J Am Chem Soc ; 139(6): 2216-2223, 2017 02 15.
Artigo em Inglês | MEDLINE | ID: mdl-28099010

RESUMO

Silver(I) promotes the highly chemoselective N-amidation of tertiary amines under catalytic conditions to form aminimides by nitrene transfer from PhI═NTs. Remarkably, this transformation proceeds in a selective manner in the presence of olefins and other functional groups without formation of the commonly observed aziridines or C-H insertion products. The methodology can be applied not only to rather simple tertiary amines but also to complex natural molecules such as brucine or quinine, where the products derived from N-N bond formation were exclusively formed. Theoretical mechanistic studies have shown that this selective N-amidation reaction proceeds through triplet silver nitrenes.

14.
European J Org Chem ; 2016(19): 3171-3176, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27774038

RESUMO

The synthesis of a new C3v -symmetric crushed fullerene C60H24 (5) has been accomplished in three steps from truxene through sixfold palladium-catalyzed intramolecular arylation of a syn-trialkylated truxene precursor. Laser irradiation of 5 induces cyclodehydrogenation processes that result in the formation of C60, as detected by LDI-MS.

15.
J Org Chem ; 81(18): 8444-54, 2016 09 16.
Artigo em Inglês | MEDLINE | ID: mdl-27529429

RESUMO

The [3,5,5,7] tetracyclic skeleton of echinopines has been stereoselectively accessed through a gold(I)-catalyzed alkoxycyclization of cyclopropyl-tethered 1,6-enynes. The key bicyclo[4.2.1]nonane core of the enyne precursors was readily assembled by means of a Co-catalyzed [6 + 2] cycloaddition. Furthermore, the attempted alkoxycyclization of 1,5-enyne substrates revealed an uncovered cyclopropyl rearrangement that gives rise to [3,6,5,7] tetracyclic structures.

16.
Angew Chem Int Ed Engl ; 55(37): 11120-3, 2016 09 05.
Artigo em Inglês | MEDLINE | ID: mdl-27381976

RESUMO

A novel strategy for the synthesis of partially saturated acene derivatives has been developed based on a Au(I) -catalyzed cyclization of 1,7-enynes. This method provides straightforward access to stable polycyclic products featuring the backbone of the acene series, up to nonacene.

17.
J Am Chem Soc ; 138(11): 3671-4, 2016 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-26963149

RESUMO

The total synthesis of lundurines A-C has been accomplished in racemic and enantiopure forms in 11-13 and 12-14 steps, respectively, without protection/deprotection of functional groups, by a novel tandem double condensation/Claisen rearrangement, a gold(I)-catalyzed alkyne hydroarylation, a cyclopropanation via formal [3 + 2] cycloaddition/nitrogen extrusion, and a remarkable olefin migration through a vinylcyclopropane retro-ene/ene reaction that streamlines the endgame.

18.
J Org Chem ; 80(15): 7321-32, 2015 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-26061916

RESUMO

Cycloisomerizations of 1,n-enynes catalyzed by gold(I) proceed via electrophilic species with a highly distorted cyclopropyl gold(I) carbene-like structure, which can react with different nucleophiles to form a wide variety of products by attack at the cyclopropane or the carbene carbons. Particularly important are reactions in which the gold(I) carbene reacts with alkenes to form cyclopropanes either intra- or intermolecularly. In the absence of nucleophiles, 1,n-enynes lead to a variety of cycloisomerized products including those resulting from skeletal rearrangements. Reactions proceeding through cyclopropyl gold(I) carbene-like intermediates are ideally suited for the bioinspired synthesis of terpenoid natural products by the selective activation of the alkyne in highly functionalized enynes or polyenynes.

19.
Angew Chem Int Ed Engl ; 54(31): 9088-91, 2015 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-26079290

RESUMO

Methods for the highly stereoselective and regiodivergent hydrosilylation of 1,3-disubstituted allenes have been developed. The synthesis of E allylsilanes is accomplished with palladium NHC catalysts, and trisubstituted Z alkenylsilanes are accessed with nickel NHC catalysts. Unsymmetrically substituted allenes are well tolerated with nickel catalysis and afford Z alkenylsilanes. Evidence for a plausible mechanism was obtained through an isotopic double-labeling crossover study.

20.
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