Molybdenum Alkylidene Complexes Research Articles

Acyclic Diene Metathesis (ADMET) Polymerization for Precise Synthesis of Defect-Free Conjugated Polymers with Well-Defined Chain Ends

This accounts introduces unique characteristics by adopting the acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers, poly(arylene vinylene)s, known as promising molecular electronics. The method is more suitable than the other methods in terms of atom efficiency affording defect-free, stereo-regular (exclusive trans) polymers with well-defined chain ends; the resultant polymers possess better property than those prepared by the conventional methods. The chain ends (vinyl group) in the resultant polymer prepared by ruthenium-carbene catalyst(s) can be modified by treating with molybdenum-alkylidene complex (olefin metathesis) followed by addition of various aldehyde (Wittig type cleavage), affording the end-functionalized polymers exclusively. An introduction of initiating fragment, the other conjugated segment, and one-pot synthesis of end-functionalized block copolymers, star shape polymers can be achieved by adopting this methodology.

N-heterocyclic carbene, high oxidation state molybdenum alkylidene complexes: functional-group-tolerant cationic metathesis catalysts.

We synthesized the first N-heterocyclic carbene (NHC) complexes of Schrock's molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16-electron complexes represent metathesis active, functional-group-tolerant catalysts. Single-crystal X-ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring-closing metathesis (RCM), ring-opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω-diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock's catalyst, e.g., sec-amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers.

DFT study on the reaction mechanism of the ring closing enyne metathesis (RCEYM) catalyzed by molybdenum alkylidene complexes

The ring closing enyne metathesis reaction (RCEYM) catalyzed by molybdenum based monoalkoxy pyrrolyl Schrock type catalysts has been studied by means of DFT (B3LYP-D) calculations. The two potential active alkylidene species as well as the three proposed reaction mechanisms (ene-then-yne, endo-yne-then-ene and exo-yne-then-ene) have been taken into account. Moreover, the influence on the exo- and endo- selectivity of the reactant substituents has also been explored. Results show that, in contrast to what is found for RCEYM processes catalyzed by Ru-based catalysts, the metallacyclobutene is a very short-living reaction intermediate that can be present in two isomeric forms (trigonal bipyramid (TBP) coordination around the metal center and square based pyramid (SPY) coordination). These two isomers are directly involved in the reaction mechanism, and the ring opening takes place from the SPY species. Moreover and regardless of the nature of the reacting metal-alkylidene, the yne-then-ene pathways (endo- or exo-) are computed to present significantly lower energy barriers than the ene-then-yne pathway and thus the latter is computed not to take place. Finally, the exo-/endo- selectivity is predicted to highly depend on the sterics of the two carbon ends of the alkyne fragment. In this way, the carbon bearing the largest group prefers to interact with the carbon end of the metal-alkylidene. This places the bulkiest groups as far away as possible from the metal fragment and overall leads to a generally lower energy barrier for the metallacyclobutene formation, the key step in defining the exo-/endo- selectivity.

New Boron-Containing Molybdenum Imido Alkylidene Complexes for Linear Olefin Homometathesis

The new molybdenum imido alkylidene complex Mo(N(2,6-iPr2C6H3))(CHCMe2Ph)(NC4H2Me2)(OB(Mes)2) (1; Mes = 2,4,6-MePh) containing both boroxide and pyrrolide ligands is reported. Its formation results from the reaction between bis(mesityl)borinic acid ((Mes)2BOH) and the bis-pyrrolide Schrock-type precursor Mo(N-2,6-iPr2C6H3)(CHCMe2Ph)(NC4H2Me2)2. The complex was fully characterized by 1H, 13C, 11B, and 95Mo NMR spectroscopy, X-ray diffraction, and elemental analysis. Complex 1 proved to be active for homometathesis reactions of 1- and 2-octene at 0.1 mol % loading. The synthesis of mixed pyrrolide boroxide imido molybdenum alkylidene complexes was extended to other borinic acids. The catalytic activity of these new complexes was evaluated in the homometathesis of linear olefins.

Precise Synthesis of End-Functionalized Oligo(2,5-dialkoxy-1,4-phenylene vinylene)s with Controlled Repeat Units via Combined Olefin Metathesis and Wittig-Type Coupling

The precise synthesis of chemically, analytically pure oligo(2,5-dialkoxy-1,4-phenylene vinylene)s [OPVs, alkoxy = O(CH2)2OSi(i)Pr3] with strictly controlled repeat units (up to 15 repeat units) and well-defined end groups has been achieved by a combined olefin metathesis reaction of 2,5-dialkoxy-1,4-divinylbenzene or their derivatives with a molybdenum-alkylidene complex and the subsequent Wittig-type cleavage with their dicarboxaldehyde analogues. The effects of the repeat units and the end functional groups toward their UV-vis and the fluorescence spectra have been clearly demonstrated.

ChemInform Abstract: Rendering Schrock‐Type Molybdenum Alkylidene Complexes Air Stable: User‐Friendly Precatalysts for Alkene Metathesis.

AbstractThe decomplexation of phenanthroline or bipyridine ligands from several molybdenum alkylidene complexes using ZnCl2 as activation agent releases tetracoordinate molybdenum complexes, which catalyze high‐yielding metathesis reactions under mild conditions (the activation is carried out at temperatures of 60 to 100°C, the metathesis reaction in the most cases at 25°C).

ChemInform Abstract: Asymmetric Ring-Closing Metathesis Catalyzed by Chiral Molybdenum Alkylidene Complexes.

Kinetic resolution was observed in ring-closing metathesis of racemic dienes catalyzed by the newly developed chiral molybdenum alkylidene complexes (R,R)-Mo(CHCMe2Ph)(NAr)(TBEC) 1 (Ar = 2,6-i-Pr2C6H3, TBEC = 2‘,2‘,2‘‘,2‘‘-tetrakis(trifluoromethyl)-1,2-bis(2‘-hydroxyethyl)cyclopentane) and (R,R)-Mo(CHCMe2Ph)(NAr)(TBEH) 2 (Ar = 2,6-i-Pr2C6H3, TBEH = 2‘,2‘,2‘‘,2‘‘-tetrakis(trifluoromethyl)-1,2-bis(2‘-hydroxyethyl)cyclohexane). In the case of a prochiral symmetric triene substrate, optically active cyclized product was formed by catalytic ring-closing metathesis with 1, which opens the possibility of a new version of two-directional synthesis. Although the observed enantiomeric excesses were modest to low, this data demonstrates the feasibility of asymmetric induction by chiral alkylidene catalysts in ring-closing metathesis.

Ring‐Closing Metathesis of Heteroatom‐Substituted Dienes.

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Ring-Closing Metathesis of Heteroatom-Substituted Dienes

The ring-closing metathesis (RCM) of dienes that contain at least one participating heteroatom-substituted olefin is reviewed (olefins substituted with O, N, S, P, Si, B, and halogen atoms will be considered). Efforts have been made to include as many relevant examples of RCM of heteroatom-substituted dienes as possible, but due to the increasing volume of research in this area, an exhaustive coverage is not intended. Attention has focussed on diene RCM reactions catalysed by ruthenium and molybdenum alkylidene complexes, although a brief discussion of some ring-closing reactions of titanium alkylidene species is provided. Other metathesis reactions such as enyne RCM, cross-metathesis and ROMP are not included.

Molybdenum‐Catalyzed Ring‐Closing Metathesis of Allenynes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Molybdenum-Catalyzed Ring-Closing Metathesis of Allenynes

A ring-closing metathesis reaction of allenynes occurred at room temperature in the presence of a molybdenum alkylidene complex to give ring-closed vinylallenes. The vinylallene skeletons were constructed by a metathesis-type reaction between the alkyne moiety and the proximal carbon-carbon double bond of the allene moiety. [reaction: see text]

Recent developments in design and synthesis of well-defined ruthenium metathesis catalysts – a highly successful opening for intricate organic synthesis

This paper surveys the latest and fast growing developments in the design and synthesis of homogeneous and immobilized ruthenium metathesis catalysts. These novel ruthenium carbene complexes, devised as convenient counterparts of the well-defined tungsten and molybdenum alkylidene complexes, display comparable activity and selectivity in metathesis reactions combined with good tolerance towards organic functionalities, air and moisture. Due to their valuable attributes, they have been applied successfully in a multitude of complex organic and polymer syntheses involving ring-opening and ring-closing metathesis reactions, cross- metathesis, enyne metathesis and ring-opening metathesis polymerization.

Synthesis of higher, trans configured oligomers of diisoalkyloxysubstituted divinylbenzenes (PV-oligomers) via metathesis telomerization of the corresponding lower oligomers

Abstract In this contribution the preparation of higher, all trans configured oligomers of diisoalkyloxysubstituted divinylbenzenes (PV-oligomers) via metathesis polycondensation of the corresponding low oligomers (telomerization) is described. The main concern was with the selectivity of the telomerization process. In this context two highly active metathesis catalysts were investigated. Two 2,5-disubstituted divinylbenzene trimers (with isopentyloxy resp. isooctyloxy substituents) were used as feed component. The time dependent product distribution was determined by means of MALDI TOF mass spectrometry. Results reveal that the molybdenum complex Mo(NPh Me 2 )(neoPh)[OCMe(CF 3 ) 2 ] 2 is much better suitable than the ruthenium based catalyst Ru(=CHPh)(PCy 3 )[1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]Cl 2 . With the molybdenum alkylidene complex higher conversions and above all considerably higher average degrees of polymerization were obtained before “side reactions” (splitting of the internal double bonds) occur.

Molybdenum alkylidene complexes with linked cycloheptatrienyl–phosphane ligands for potential use in olefin metathesis

The synthesis of cycloheptatrienyl molybdenum complexes incorporating the linked cycloheptatrienyl–phosphane ligand [2-(diisopropylphosphanyl)phenyl]cycloheptatrienyl, o- iPr 2P-C 6H 4-C 7H 6, is described. Reaction of the 17-electron dibromide [( o- iPr 2PC 6H 4-η 7-C 7H 6)MoBr 2( P-Mo)] ( 1) with NaBH 4 furnishes a tetrahydroborate complex, which can be directly converted into the acetate complex [( o- iPr 2PC 6H 4-η 7-C 7H 6)Mo(η 2-O 2CCH 3)( P-Mo)] ( 2). The acetate ligand can be cleaved upon treatment with diazoalkanes, N 2CHR, in the presence of Me 3SiCl affording the molybdenum alkylidenes [( o- iPr 2PC 6H 4-η 7-C 7H 6)MoCl(CHR)( P-Mo)] ( 4a, R=Ph; 4b, R=SiMe 3). Complexes 4 might serve as a source for the preparation of potential olefin metathesis catalysts of the type [( o- iPr 2PC 6H 4-η 7-C 7H 6)Mo(CHR)( P-Mo)] + by halide abstraction. CH 2 insertion into the metal–carbon bond of the diphenylacetylene complex 5 resulted in the formation of the cationic η 3-vinylcarbene complex 6, which can be regarded as an alkene adduct of a 16-electron complex of just that kind. In addition, the X-ray crystal structures of 2 and 6 are reported.

An Enantiomerically Pure Adamantylimido Molybdenum Alkylidene Complex. An Effective New Catalyst for Enantioselective Olefin Metathesis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

An enantiomerically pure adamantylimido molybdenum alkylidene complex. An effective new catalyst for enantioselective olefin metathesis.

An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral alkylimido catalyst; all previous chiral complexes are arylimido systems. These studies show that the chiral Mo catalyst exists exclusively as the syn isomer and that it offers unique reactivity and selectivity profiles in asymmetric olefin metathesis.

Theoretical studies on the ring-opening metathesis reaction of norbornadiene with molybdenum alkylidenes

The mechanism of ring-opening metathesis reaction of molybdenum alkylidene complexes with norbornadiene has been studied by quantum mechanics method. Both cycloaddition and ring-opening reactions have a twisted trigonal bipyramidal transition structure. The cycloaddition is the rate-determining step while the ring-opening step has very low activation energy. When the alkoxyl ligands are small (OCH 3, OCF 3), there is only a small preference for norbornadiene attacking syn to the imido group. When the size of the alkoxyl ligands increases, the syn preference for norbornadiene addition becomes more favorable. Our calculations strongly support Schrock's proposal for the stereoselectivity of ring-opening metathesis polymerization. That is, the stereochemistry is determined by the relative rates of alkylidene syn/ anti interconversion and polymerization reaction.

Fine-Tuning of Molybdenum Imido Alkylidene Complexes for the Cyclopolymerization of 1,6-Heptadiynes To Give Polyenes Containing Exclusively Five-Membered Rings

Diethyl dipropargylmalonate (1) was cyclopolymerized by various well-defined molybdenum-based Schrock initiators to produce polymers exclusively based on 1,2-cyclopent-1-enylenvinylene units. For this purpose, variations in both the imido and alkoxy ligand in molybdenum alkylidene complexes of type Mo(NAr‘)(CHCMe2Ph)(OR)2 (Ar‘ = 2,6-i-Pr2C6H3; 2,6-Me2C6H3; OR = OCH(CF3)2, OCH(CH3)2, OC(CH3)3) were carried out. Polymers containing >95% 1,2-cyclopent-1-enylenvinylene units were obtained by low-temperature-initiated cyclopolymerization of 1 by Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCH(CH3)2)2 (2). In the presence of quinuclidine, >95% five-membered ring structures were realized at room temperature using Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OC(CH3)3)2 (3) and Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OC(CH3)3)2 (4). In contrast, molybdenum initiators containing electron-withdrawing alkoxides such as Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CF3)2)2(quinuclidine) (5) or the nonfluorinated analogue Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CH3)2)2(quinuclidin...

Olefin metathesis in supercritical carbon dioxide.

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.

Organomolybdenum(VI) and Lithium Organomolybdate(VI) and -(V) Complexes with C,N-Chelating Aminoaryl Ligands

The synthesis and characterization of new, five-coordinate molybdenum bis(imidoaryl) complexes [Mo(NAr)2(C-N)X] (Ar = C6H3i-Pr2-2,6; C-N = [C6H4(CH2NMe2)-2]-; X = Cl (1), Me (2), Et (3), Bu (4), CH2SiMe3 (5), (p-tolyl) (6), (C-N) (7)) is reported. The solid-state structure of 2 has been elucidated by single-crystal X-ray analysis. Compounds 2, 3, 4, 5, and 6 react with alkyl- or aryllithium compounds to form lithium molybdate(VI) derivatives, of which [Li(DME)Mo(NAr)2(C-N)(Me)(p-tolyl)] (10), formed by the reaction of 2 with [Li(p-tolyl)], has been structurally (X-ray) characterized. Thermal activation of these lithium molybdates leads to the formation of paramagnetic lithium molybdate(V) compounds instead of the anticipated molybdenum(VI) alkylidenes. The actual temperature (between -10 and 80 C) at which paramagnetic Mo(V) radical anions are formed is dependent on both the type of alkyl or aryl substituent (introduced via LiR') and the solvent. The synthesis of [LiMo(NAr)2(C-N)2] (11) by reaction of 7 with n-BuLi is described. The initially formed lithium molybdate(VI) compound [Li(DME)nMo(NAr)2(C-N)2(n-Bu)] is not stable at room temperature and converts directly to the lithium molybdate(V) derivative 11. The solid-state structure of 11 has been elucidated by single-crystal X-ray analysis. None of the lithium molybdate(VI) nor -(V) derivatives described herein are active catalysts for ROMP, as thermal activation does not lead to the formation of a molybdenum alkylidene complex but to electron transfer and formation of a lithium molybdate(V) instead. However, upon treatment of a solution of any of the molybdate(V) derivatives with dry air, catalytic ROMP is observed.