Side‐Chain Modification and “Grafting Onto” via Olefin Cross‐Metathesis

Olefin cross‐metathesis is introduced as a versatile polymer side‐chain modification technique. The reaction of a poly(2‐oxazoline) featuring terminal double bonds in the side chains with a variety of functional acrylates has been successfully performed in the presence of Hoveyda–Grubbs second‐generation catalyst. Self‐metathesis, which would lead to polymer–polymer coupling, can be avoided by using an excess of the cross‐metathesis partner and a catalyst loading of 5 mol%. The results suggest that bulky acrylates reduce chain–chain coupling due to self‐metathesis. Moreover, different functional groups such as alkyl chains, hydroxyl, and allyl acetate groups, as well as an oligomeric poly(ethylene glycol) and a perfluorinated alkyl chain have been grafted with quantitative conversions.     

Poly(p‐phenylene iminoborane): A Boron–Nitrogen Analogue of Poly(p‐phenylene vinylene)

Substitution of selected CC units in π‐conjugated organic frameworks by their isoelectronic and isosteric BN units (BN/CC isosterism) has proven to be a successful concept for the development of BN‐doped polycyclic aromatic hydrocarbons (PAHs) with intriguing properties and functions. The first examples have just demonstrated the applicability of this approach to polymer chemistry. Herein, we present the synthesis and comprehensive characterization of the first poly(p‐phenylene iminoborane). This novel inorganic–organic hybrid polymer can be regarded as a BN analogue of the well‐known poly(p‐phenylene vinylene) (PPV). Photophysical investigations on the polymer and a series of model oligomers provide clear evidence of some π‐conjugation across the B=N bonds and extension of the conjugation path with increasing chain length. TD‐DFT calculations provide deeper insight into the electronic structure of the new materials.     

AuCl3-Catalyzed Ring-Closing Carbonyl–Olefin Metathesis

Compared with the ripeness of olefin metathesis, exploration of the construction of carbon–carbon double bonds through the catalytic carbonyl–olefin metathesis reaction remains stagnant and has received scant attention. Herein, a highly efficient AuCl₃-catalyzed intramolecular ring-closing carbonyl–olefin metathesis reaction is described. This method features easily accessible starting materials, simple operation, good functional-group tolerance and short reaction times, and provides the target cyclopentenes, polycycles, benzocarbocycles, and N-heterocycle derivatives in good to excellent yields.     

Crystalline Divinyldiarsenes and Cleavage of the As=As Bond

The first divinyldiarsenes [{(NHC)C(Ph)}As]₂ (NHC=IPr 3 a , SIPr 3 b ; IPr=C{(NAr)CH}₂; SIPr=C{(NAr)CH₂}₂; Ar=2,6-iPr₂C₆H₃) are reported. Compounds 3 a and 3 b were prepared by the reduction of corresponding chlorides {(NHC)C(Ph)}AsCl₂ (NHC=IPr 2 a , SIPr 2 b ) with Mg. Calculations revealed a small HOMO–LUMO energy gap of 3.86 ( 3 a ) and 4.24 eV ( 3 b ). Treatment of 3 a with (Me₂S)AuCl led to the cleavage of the As=As bond to restore 2 a , which is expected to proceed via the diarsane [{(IPr)C(Ph)}AsCl]₂ ( 4 ). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C₂Cl₆. Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh)₂ (E=Se 5 a ; Te 5 b ), revealing the cleavage of As=As and E−E bonds and the formation of As−E bonds. Such highly selective stepwise oxidation ( 3 a → 4 → 2 a ) and bond metathesis ( 3 a → 5 a , b ) reactions are unprecedented in main-group chemistry.     

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂, Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂IMe, Me₂IⁱPr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n; n=3, 4, 5, 6] and Ph₂C=CPh₂. Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂CPh₂ and cyclo-(ArP)₄CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.     

The Reactivities of Iminoboranes with Carbenes: BN Isosteres of Carbene–Alkyne Adducts

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N‐heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (?45 °C). The adducts show short B?N bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di‐tert‐butyliminoborane was reacted with 1,3‐bis(isopropyl)imidazol‐2‐ylidene (IPr), the initially formed Lewis acid–base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4‐position. Warming the iminoborane–CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2‐azaborilidine compound.     

Synthesis and characterization of polyoctenamer with WCl6?e−?Al?CH2Cl2 catalyst system via ring‐opening metathesis polymerization

A typical low‐strain monomer, cyclooctene, was polymerized via ring‐opening metathesis polymerization with electrochemically produced active species. The structural properties of the polyoctenamer were determined by NMR, gel‐permeation chromatography and differential scanning calorimetry. Analysis of the polyoctenamer microstructure by ¹H and ¹³C NMR spectroscopy indicates that the polymer contains a highly cis stereoconfiguration of the double bonds (σ_c = 0.75). The resulting polymer is of low molecular weight and has a reasonably broad molecular weight distribution (M_w = 18 000, PDI = 1.9). The glass transition temperature and melting point of the polyoctenamer are ?11.3 °C and 36.5 °C respectively. Copyright © 2005 John Wiley & Sons, Ltd.     

Tandem Ring‐Opening–Ring‐Closing Metathesis for Functional Metathesis Catalysts

Use of a tandem ring‐opening–ring‐closing metathesis (RORCM) strategy for the synthesis of functional metathesis catalysts is reported. Ring opening of 7‐substituted norbornenes and subsequent ring‐closing metathesis forming a thermodynamically stable 6‐membered ring lead to a very efficient synthesis of new catalysts from commercially available Grubbs’ catalysts. Hydroxy functionalized Grubbs’ first‐ as well as third‐generation catalysts have been synthesized. Mechanistic studies have been performed to elucidate the order of attack of the olefinic bonds. This strategy was also used to synthesize the ruthenium methylidene complex.     

Poly(iminoborane)s: An Elusive Class of Main‐Group Polymers?

The significance of inorganic main‐group polymers is demonstrated most clearly by the commercial relevance of polysiloxanes (silicones). Organoboron‐based materials such as π‐conjugated organoborane polymers and BN‐doped polycyclic aromatic hydrocarbons are currently attracting considerable attention. Surprisingly, poly(iminoborane)s (PIBs; [BRNR′]_n), that is, the parent unsaturated BN polymers, which are formally isoelectronic to polyacetylene, have not been convincingly characterized thus far. Herein, we present the synthesis and comprehensive characterization of a linear oligo(iminoborane), which comprises a chain of 12–14 BN units on average. With our synthetic approach, unwanted side reactions that result in borazine formation are effectively suppressed. Supporting DFT and TD‐DFT calculations provide deeper insight into the microstructure and the electronic structure of the oligomer.     

Ruthenium carbene initiated ring-open metathesis polymerization of endo-bicyclo[3.2.0]hept-6-en-3-yl benzoates with tacticity studies

<正>After tentative ring-opening cross metathesis(ROCM) of endo-bicyclo[3.2.0]hept-6-en-3-yl 4-bromobenzoate with PhCH=CH_2 initiated by(Cy_3P)_2Cl_2Ru=CHPh in CH_2Cl_2,ring-opening metathesis polymerization(ROMP) of a series of endo-bicyclo[3.2.0]hept-6-en-3-yl benzoates was achieved under the same conditions,furnishing a variety of the corresponding polymers.Then some of them were selected to hydrogenate the C=C double bonds in their backbone with p-tosylhydrazide,affording saturated and thus more flexible polymers.All of these polymers were well characterized by spectroscopic means including GPC,UV-Vis,NMR and IR,based on which the tacticity of these polymers was investigated together with nonlinear optical(electric-field-induced second-harmonic generation,EFISH)) analysis.     

Olefin Metathesis in Organic Chemistry

Transition metal catalyzed C? C bond formations belong to the most important reactions in organic synthesis. One particularly interesting reaction is olefin metathesis, a metal-catalyzed exchange of alkylidene moieties between alkenes. Olefin metathesis can induce both cleavage and formation of C? C double bonds. Special functional groups are not necessary. Although this reaction—which can be catalyzed by numerous transition metals—is used in industry, its potential in organic synthesis was not recognized for many years. The recent abrupt end to this Sleeping-Beauty slumber has several reasons. Novel catalysts can effect the conversion of highly fictionalized and sterically demanding olefins under mild reaction conditions and in high yields. Improved understanding of substrate–catalyst interaction has greatly contributed to the recent establishment of olefin metathesis as a synthetic method. In addition to the preparation of polymers with fine-tuned characteristics, the metathesis today also provides new routes to compounds of low molecular weight. The highly developed ring-closing metathesis has been proven to be key step in the synthesis of a growing number of natural products. At the same time interesting applications can be envisioned for newly developed variants of bimolecular metathesis. Improvements in the selective cross-metathesis of acyclic olefins as well as promising attempts to include alkynes as viable substrates provide for a vivid development of the metathesis chemistry.     

Ethenolysis: A Green Catalytic Tool to Cleave Carbon–Carbon Double Bonds

Remarkable innovations have been made in the field of olefin metathesis due to the design and preparation of new catalysts. Ethenolysis, which is cross‐metathesis with ethylene, represents one catalytic transformation that has been used with the purpose of cleaving internal carbon–carbon double bonds. The objectives were either the ring opening of cyclic olefins to produce dienes or the shortening of unsaturated hydrocarbon chains to degrade polymers or generate valuable shorter terminal olefins in a controlled manner. This Review summarizes several aspects of this reaction: the catalysts, their degradation in the presence of ethylene, some parameters driving their productivity, the side reactions, and the applications of ethenolysis in organic synthesis and in potential industrial applications.     

The sigma-CAM Mechanism: sigma complexes as the basis of sigma-bond metathesis at late-transition-metal centers

Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.     

Electrochemically generated tungsten‐based active species as catalysts for metathesis‐related reactions: 1. Acyclic diene metathesis polymerization of 1,9‐decadiene

The application of the WCl₆–e^?–Al–CH₂Cl₂ system to acyclic diene metathesis polymerization of 1,9‐decadiene is reported. The polyoctenamer formed is of a weight‐average molecular weight of 9000 with a polydispersity of 1.92. IR and NMR spectral analyses indicate the retention of the double bonds in the polymer structure with high trans content as expected from a step condensation reaction. This relatively stable catalytic system, however, also activates the competing vinyl addition reactions while being inactive in ring‐closure metathesis reactions. Copyright © 2002 John Wiley & Sons, Ltd.     

Dynamic Diselenide Bonds: Exchange Reaction Induced by Visible Light without Catalysis

Dynamic covalent bonds are extensively employed in dynamic combinatorial chemistry. The metathesis reaction of disulfide bonds is widely used, but requires catalysis or irradiation with ultraviolet (UV) light. It was found that diselenide bonds are dynamic covalent bonds and undergo dynamic exchange reactions under mild conditions for diselenide metathesis. This reaction is induced by irradiation with visible light and stops in the dark. The exchange is assumed to proceed through a radical mechanism, and experiments with 2,2,6,6‐tetramethylpiperidin‐1‐yloxyl (TEMPO) support this assumption. Furthermore, the reaction can be conducted in different solvents, including protic solvents. Diselenide metathesis can also be used to synthesize diselenide‐containing asymmetric block copolymers. This work thus entails the use of diselenide bonds as dynamic covalent bonds, the development of a dynamic exchange reaction under mild conditions, and an extension of selenium‐related dynamic chemistry.     

Bis(Cyclic Alkyl Amino Carbene) Ruthenium Complexes: A Versatile,Highly Efficient Tool for Olefin Metathesis

The state‐of‐the‐art in olefin metathesis is application of N‐heterocyclic carbene (NHC)‐containing ruthenium alkylidenes for the formation of internal C=C bonds and of cyclic alkyl amino carbene (CAAC)‐containing ruthenium benzylidenes in the production of terminal olefins. A straightforward synthesis of bis(CAAC)Ru indenylidene complexes, which are highly effective in the formation of both terminal and internal C=C bonds at loadings as low as 1 ppm, is now reported.     

Bis(Cyclic Alkyl Amino Carbene) Ruthenium Complexes: A Versatile,Highly Efficient Tool for Olefin Metathesis

The state‐of‐the‐art in olefin metathesis is application of N‐heterocyclic carbene (NHC)‐containing ruthenium alkylidenes for the formation of internal C=C bonds and of cyclic alkyl amino carbene (CAAC)‐containing ruthenium benzylidenes in the production of terminal olefins. A straightforward synthesis of bis(CAAC)Ru indenylidene complexes, which are highly effective in the formation of both terminal and internal C=C bonds at loadings as low as 1 ppm, is now reported.     

Short eight-steps total synthesis of racemic asteriscunolide C

A concise total synthesis of racemic asteriscunolide C in eight steps has been described starting from neopentane diol involving an efficient Yamaguchi esterification using an aldehyde-acid, intramolecular Horner–Wittig–Emmons olefination, and a late stage ring-closing metathesis to construct the strained 11-membered ring with one Z- and two E-double bonds.     

Hydroboration of the Amino(imino)borane [Me3C(Me3Si)N]B[N(CMe3)]

The formation of four products of the type Me₃C(Me₃Si)N=BH–N(CMe₃)=BR'₂ [BR'₂ = B(CHMeiPr)₂ ( 1 ), B(c‐C₆H₁₁)₂ ( 2 ), B(C₈H₁₄) ( 3 ), B(O₂C₆H₄) ( 4 )] from the iminoborane Me₃C(Me₃Si)N–···B=···N(CMe₃) and the hydroboranes (R'₂BH)₂ is described. Crystal structure analysis reveals the molecule 1 to have an N=B–N=B backbone with two orthogonal N=B bond planes and, hence, no conjugation between the two B–N double bonds.     

Bis-Phosphaketenes LM(PCO)2 (M=Ga,In): A New Class of Reactive Group 13 Metal-Phosphorus Compounds

Phosphaketenes are versatile reagents in organophosphorus chemistry. We herein report on the synthesis of novel bis-phosphaketenes, LM(PCO)₂ (M=Ga 2 a , In 2 b ; L=HC[C(Me)N(Ar)]₂; Ar=2,6-i-Pr₂C₆H₃) by salt metathesis reactions and their reactions with LGa to metallaphosphenes LGa(OCP)PML (M=Ga 3 a , In 3 b ). 3 b represents the first compound with significant In−P π-bonding contribution as was confirmed by DFT calculations. Compounds 3 a and 3 b selectively activate the N−H and O−H bonds of aniline and phenol at the Ga−P bond and both reactions proceed with a rearrangement of the phosphaethynolate group from Ga−OCP to M−PCO bonding. Compounds 2–5 are fully characterized by heteronuclear (¹H, ¹³C{¹H}, ³¹P{¹H}) NMR and IR spectroscopy, elemental analysis, and single crystal X-ray diffraction (sc-XRD).