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The addition of excess H2CC(H)ER to (PCy3)2Cl2RuC(H)R (1a,b) afforded a series of well-defined ruthenium carbene complexes, (PCy3)2Cl2RuC(H)ER (ER = OEt (5), SEt (6), SPh (7), N(carbazole) (8), N(pyrrolidinone) (9)) in yields ranging from 66 to 90%. Such complexes containing an electron-donating group on the carbene carbon are often referred to as Fischer-type carbenes. Replacement of one phosphine ligand with 1,3-dimesitylimidazolylidene (IMes) afforded the respective mixed-ligand complexes (IMes)(PCy3)Cl2RuC(H)ER (11−14) in 48−89% yield. Alternatively, addition of H2CC(H)OEt to (H2IMes)(PCy3)Cl2RuC(H)Ph (3a; H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H2IMes)(PCy3)Cl2RuC(H)OEt (15) in 93% yield. The crystal structures of complexes 5, 7−9, and 11 were determined and found to be structurally similar to the parent ruthenium alkylidene ([Ru]C(H)R) complexes. In solution, the chemical shift of the [Ru]C(H)ER resonance in 1H NMR spectra was found to be inversely related to the electronegativity of the α-heteratom; however, no trends were evident in the 31P or 13C NMR spectra. Intramolecular coordination of the pendant amide carbonyl group to the Ru center established a temperature-dependent equilibrium between complexes 9 and 14 and their cyclometalated forms. All Ru electron-rich complexes initiated the ring-opening metathesis polymerization (ROMP) of strained cyclic olefins and the ring-closing metathesis (RCM) of diethyl diallylmalonate. A general trend in the relative reactivities and thermal stabilities of the (PCy3)2Cl2RuC(H)ER complexes followed the order C > N > S > O. In addition, complexes coordinated with an N-heterocyclic carbene ligand (e.g., (IMes)(PCy3)Cl2RuC(H)ER) displayed enhanced activities in olefin metathesis and were thermally more stable than their bis(phosphine) analogues. Finally, the thermal decomposition product of (PCy3)2Cl2RuC(H)OEt was isolated and determined by X-ray analysis to be (PCy3)2ClRu(H)CO (10).

General information. All manipulations were performed in a N 2 filled drybox or using standard Schlenk techniques. Allyl benzene, methyl vinyl ketone, p-chlorostyrene, methyl methacrylate, diethyl diallylmalonate (3), and 1,6-heptadien-4-ol (6) were purchased from Aldrich and used without further purification. cis-4-Cyclopentene-1,3-diol and (+)-citronellal were purchased from Fluka and used without further purification. cis-2-Butene-1,4-diol diacetate and 5-acetoxy-1-hexene were purchased from TCI America and used without further purification. Complexes (PCy 3 ) 2 Cl 2 Ru=CHPh (1) 1 and (H 2 IMes)(PCy 3 )Cl 2 Ru=CHPh (2) 2 and substrates 1,6-heptadien-4-one (6), 3 1,8-nonadien-5-one (12), 4 2-allyl-2-(2-methyl-allyl)-malonic acid diethyl ester (15), 5 1,4-bis-allyloxy-but-2-yne (21), 4 2-methyl-acrylic acid but-3-enyl ester (24), 6 N-allyl-N-isopropyl-acrylamide (27), 7 and undec-10-enoic acid but-3-enyl ester (30) 8 were prepared as previously reported.Substrates 4,4-dicarbethoxycyclopentene (4), 2 cyclopent-3-enone (7), cyclopent-3-enol (10), 2 cyclohept-4-enone (13), 4,4-dicarbethoxy-1-methylcyclopentene (16), 5 2,5,2 ',5'-tetrahydro-[3,3']bifuranyl (22), 9 3-methyl-5,6-dihydro-pyran-2-one (25), 6 1-isopropyl-1,5-dihydro-pyrrol-2-one (28), 10 oxacyclotetradec-11-en-2-one (31), 8 acetic acid 4-phenyl-but-2-enyl ester (33), 11 acetic acid 7-oxo-oct-5-enyl ester (35), 6 4-phenyl-but-3-en-2-one (37), 12 and 3-(4-chloro-phenyl)-acrylic acid methyl ester

A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO(2) and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO(2)'s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO(2) to decarboxylate increases. The comparison of NHC-CO(2)'s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO(2) thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C-CO(2) bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of I(t)BuPrCO(2) (2f) indicate that the C-CO(2) bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of the C-CO(2) bond may be involved in the bond breaking event to release CO(2).

A family of 2D coordination polymers were successfully synthesized through "bottom-up" techniques using Ni, Cu, Co, and hexaaminobenzene. Liquid-liquid and air-liquid interfacial reactions were used to realize thick (∼1-2 μm) and thin (<10 nm) stacked layers of nanosheet, respectively. Atomic-force microscopy and scanning electron microscopy both revealed the smooth and flat nature of the nanosheets. Selected area diffraction was used to elucidate the hexagonal crystal structure of the framework. Electronic devices were fabricated on thin samples of the Ni analogue and they were found to be mildly conducting and also showed back gate dependent conductance.

Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.

Recent progress in the synthesis of benzene and 1,3-cyclohexadiene derivatives, and heterocyclic compounds such as pyridines, pyridones, pyrans, pyrimidine diones, etc, has been reviewed. The general mechanistic aspects of the [2 + 2 + 2] cycloaddition reaction are discussed. The asymmetric variants of these reactions are also discussed along with the proposed models of asymmetric induction. Keywords: arenes; catalysis; cycloaddition; heterocycles; metallacycles; transition metals IntroductionThe significance and increasing popularity of [2 + 2 + 2] cycloaddition reactions is evident from the number of reviews that have recently appeared in the literature. [1][2][3] The [2 + 2 + 2] cycloaddition reaction is remarkable in terms of its ability to utilize various unsaturated substrates such as alkynes, diynes, alkenes, imines, isocyanates, isothiocyanates, and CO 2 in the synthesis of a broad variety of highly substituted cyclic molecules such as benzenes, pyridines, pyridones, 1,3-cyclohexadienes, pyrones, thiopyridones and cyclohexanes. Multisubstituted benzenes and pyridines have traditionally been synthesized by aromatic electrophilic substitution (AES) reactions and a variety of metal-mediated coupling reactions. Although, these reactions are extremely efficient, they generally involve multistep syntheses. The application of AES reactions in the synthesis of polysubstituted aromatic rings is limited by the effect of the substituent groups and, hence, it may be very difficult or even impossible to add various functionalities at a specific position on the aromatic ring. On the other hand, [2 + 2 + 2] cycloaddition reactions are extremely atom-efficient and involve the formation of several C À C bonds in a single step. Another important feature of the [2 + 2 + 2] cycloaddition reaction is tolerance of a myriad of functional groups such as alcohols, amines, alkenes, ethers, esters, halogens, and nitriles. Moreover, the availability of numerous catalytic systems that have Adv. Synth. Catal. 2006, 348, 2307 -2327 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2307 REVIEWS been efficient in tedious syntheses highlights the applicability of the [2 + 2 + 2] cycloaddition reaction. These are important requirements for the [2 + 2 + 2] cycloaddition reaction to become a universal synthetic tool for the synthesis of benzene, pyridine, and other cyclic derivatives. An important problem with the [2 + 2 + 2] cycloaddition reaction is the lack of chemoand regioselectivity observed in earlier reported reactions. However, significant effort has been focused on attaining a high degree of chemo-, regio-and even enantioselectivity with considerable success as evident from recent reports. Excellent reviews on [2 + 2 + 2] cycloaddition reactions are available. Kotha and co-workers have reviewed the synthesis of benzene derivatives by the cyclotrimerization of three alkyne functionalities utilizing transition metal systems as catalysts.[1] Their review is divided into: (a) intermolecular reactionsin which the three pi (alkyn...

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.