Schrock Catalyst Research Articles

Room temperature ionic liquids: new solvents for Schrock's catalyst and removal using polydimethylsiloxane membranes

A room temperature ionic liquid was used as the solvent for metathesis reactions with the Schrock catalyst and a new method to facilitate separation between small molecules and ionic liquids using polydimethylsiloxane thimbles is reported.

Polymer-Supported Non-Chiral and Homochiral Schrock Catalysts

Immobilization of both non-chiral and homochiral Schrock catalysts via the aryl­imido ligand was accomplished for the first time. Thus, a non-chiral Schrock catalyst 1, and homochiral catalysts 2 and 3 were prepared from 4-bromo-2,6-dimethylaniline (for 1 and 2) or 4-bromo-2,6-diisopropylaniline (for 3) with linear polystyrene (Mn = 10,000; PDI = 1.5) in eight steps. The ring-closing metathesis (RCM) and ring-opening cross-metathesis were performed with 1 (0.1-1 mol% Mo) in CH2Cl2 or CDCl3 to give the corresponding products in 64-100% yield (4 examples). The catalyst 2 or 3 (5 mol% Mo) was subjected to the enantioselective RCM to afford the corresponding products in 43-99% conversion with 6-95% ee (7 examples).

Polymer‐Supported Chiral Schrock Catalysts Immobilized via the Arylimido Ligand.

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Synthesis, X-ray structure and reactivity of μ-(CF 3COO) 2-[Mo(N-2,6- i-Pr 2-C 6H 3)(CHCMe 2Ph)(OOCCF 3)(Et 2O)] 2, the first Bis(trifluoroacetate) derivative of a Schrock catalyst

Reaction of Mo(N-2,6- i-Pr 2-C 6H 3)(CHCMe 2C 6H 5)(OSO 2CF 3)(DME) (DME = 1,2-dimethoxyethane) with 2 equiv. of CF 3COOK yields μ-(CF 3COO) 2-[Mo(N-2,6- i-Pr 2-C 6H 3)(CHCMe 2Ph)(OOCCF 3)(Et 2O)] 2 ( 1). Compound 1 crystallizes in the orthorhombic space group Pna2 1 with a = 17.2485(3), b = 17.0336(3), c = 25.4031(5) Å, α = β = γ = 90°, V = 7463.5(2) Å 3, Z = 4. In contrast to alkoxide based Schrock type initiators, 1 is virtually inactive in numerous metathesis reactions including ring-closing metathesis (RCM) and homo metathesis reactions, the cyclopolymerization of 1,6-heptadiynes, and even ring-opening metathesis polymerization (ROMP) of norborn-2-ene. However, addition of quinuclidine results in the in situ formation of 1a (Mo(N-2,6- i-Pr 2-C 6H 3)(CHCMe 2C 6H 5)(OOCCF 3) 2(quinuclidine) which displays moderate activity in ROMP, cyclopolymerization of 1,6-heptadiynes and RCM. Theoretical investigations carried out on the B3LYP/LACVP ∗ level provide substantial explanation for these findings.

Polymer‐Supported Chiral Schrock Catalysts Immobilized via the Arylimido Ligand

AbstractThe immobilization of both chiral and non‐chiral versions of the Schrock catalyst via the arylimido ligand was accomplished for the first time. For this purpose, four different 4‐(6‐halogenohexyl)‐2,6‐R2‐anilines, i.e., {4‐[6‐X‐(CH2)6]‐2,6‐R2‐C6H2‐NH2} (7: R=Me, X=Br; 8: R=Me, X=Cl, 9: R=2‐Pr, X=Cl, 10: R=2‐Pr, X=Br) were prepared and used for the synthesis of the corresponding Mo‐bis(imido)dichloro complexes Mo{N‐2,6‐R2‐4‐[6‐X‐(CH2)6]‐C6H2}2Cl2⋅DME (11: R=Me, X=Br; 12: R=Me, X=Cl; 13: R=2‐Pr, X=Cl; 14: R=2‐Pr, X=Br). Compounds 11–14 were transformed into the corresponding Mo‐bis(imido)dialkyl complexes Mo{N‐2,6‐R2‐4‐[6‐X‐(CH2)6]‐C6H2}2(CH2CMe2Ph)2 (15: R=Me, X=Br; 16: R=Me, X=Cl, 17: R=2‐Pr, X=Cl; 18: R=2‐Pr, X=Br). These were used for the synthesis of the Mo‐imidoalkylidene triflates Mo{N‐2,6‐R2‐4‐[6‐X‐(CH2)6]‐C6H2}(CHCMe2Ph)(OTf)2⋅DME (19: R=Me, X=Br; 20: R=2‐Pr, X=Br, 21: R=2‐Pr, X=Cl). Compounds 19‐‐21 were used in the synthesis of the Schrock type catalysts Mo{N‐2,6‐Me2‐4‐[6‐Br‐(CH2)6]‐C6H2}(CHCMe2Ph)[OC(CH3)(CF3)2]2 (22) and Mo{N‐2,6‐(2‐Pr)2‐4‐[6‐Cl‐(CH2)6]‐C6H2}(CHCMe2Ph)[OC(CH3)(CF3)2] (23), Mo{N‐2,6‐Me2‐4‐[6‐Br‐(CH2)6]‐C6H2}(CHCMe2Ph)[(R)‐BIPHEN] (24) and Mo{N‐2,6‐(2‐Pr)2‐4‐[6‐Br‐(CH2)6]‐C6H2}(CHCMe2Ph)[(R)‐BIPHEN] (25). Compounds 22, 24 and 25 were immobilized on Ag‐perfluoroalkylsulfonate‐modified PS‐ and PS‐DVB materials to yield the corresponding immobilized catalysts Mo{N‐2,6‐Me2‐4‐[PS‐CH2‐O‐CF2‐CF(CF3)‐SO3‐(CH2)6]‐C6H2}(CHCMe2Ph)[OCMe(CF3)2]2 (26), Mo{N‐2,6‐Me2‐4‐[PS‐DVB‐CH2‐O‐CF2‐CF(CF3)‐SO3‐(CH2)6]‐C6H2}(CHCMe2Ph)[(R)‐BIPHEN] (27) Mo{N‐2,6‐(2‐Pr)2‐4‐[PS‐DVB‐CH2‐O‐CF2‐CF(CF3)‐SO3‐(CH2)6]‐C6H2}(CHCMe2Ph)[(R)‐BIPHEN] (28). These were used in a series of ring‐closing metathesis (RCM), ring‐opening cross metathesis, asymmetric RCM and desymmetrization reactions. The use of 27 and 28 resulted in values for enantiomeric excess (ee) virtually identical to those obtained with the corresponding homogeneous chiral catalysts.

A reação de metátese de olefinas: reorganização e ciclização de compostos orgânicos

THE OLEFIN METATHESIS REACTION: REORGANIZATION AND CICLIZATION OF ORGANIC COMPOUNDS. The olefin metathesis reaction allows the exchange of complex alkyl units between two olefins, with the formation of a new olefinic link and a sub-product olefin usually ethylene. This reaction has found extensive application in the last ten years with the development of the Grubbs and Schrock catalysts, in total synthesis of complex organic molecules, as opposed to the very important use in the petrochemical industry with relatively simple molecules. This review intends to trace a historical and mechanistic pathway from industry to academy, before illustrating the more recent advances.

The Asymmetric Schrock Olefin Metathesis Catalyst. A Computational Study

The mechanism of the transition metal catalyzed olefin metathesis reaction with the Schrock catalyst is investigated with pure (BP86) and hybrid (B3LYP) density functional theory. On the free-energy surface there is no adduct between ethylene and model catalyst (MeO)2Mo(CH2)NH but instead a single transition on a flat surface giving a metallacyclobutane with a trigonal bipyramidal conformation that has a sizable barrier for conversion to the square pyramidal form. The model was expanded to the active asymmetric Schrock catalyst and investigated with QM/MM, using BP86 for the QM part and AMBER 95 for the MM part, for the experimentally known ring-closing metathesis of a symmetrical triene. The factors effecting chiral induction to the observed product are delineated.

Sequential Ring‐Closing Metathesis/Pd‐Catalyzed, Si‐Assisted Cross‐Coupling Reactions: General Synthesis of Highly Substituted Unsaturated Alcohols and Medium‐Sized Rings Containing a 1,3‐cis—cis Diene Unit.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Sequential ring-closing metathesis/Pd-catalyzed, Si-assisted cross-coupling reactions: general synthesis of highly substituted unsaturated alcohols and medium-sized rings containing a 1,3- cis– cis diene unit

A sequential ring-closing metathesis/silicon-assisted cross-coupling protocol has been developed. Alkenyldimethylsilyl ethers of allylic, homoallylic and bis(homoallylic) alcohols undergo facile ring closure with Schrock's catalyst to afford 5-, 6-, and 7-membered cycloalkenylsiloxanes, respectively, in some cases with substituents on both alkenyl carbons. These siloxanes are highly effective coupling partners that afford styrenes and dienes (with various aryl and alkenyl halides) in high yield and specificity as well as good functional group compatibility. The siloxanes bearing a Z-iodoalkenyl tether undergo an intramolecular coupling process in the presence of [allylPdCl] 2 which constitutes a powerful method for the construction of medium-sized rings with an internal 1,3- cis– cis diene unit. The formation of 9-, 10-, 11-, and 12-membered carbocyclic dienes is achieved in good yield. Extension to the synthesis of 9-membered ring unsaturated ethers has also been accomplished. Noteworthy features of this process include: (1) highly stereospecific intramolecular coupling, (2) flexible positioning of the revealed hydroxy group, and (3) potential extension to other medium-sized carbocycles and heterocycles.

A ROMP‐Derived, Polymer‐Supported Chiral Schrock Catalyst for Enantioselective Ring‐Closing Olefin Metathesis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cross‐Metathesis Reaction of Vinyl Sulfones and Sulfoxides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Carbohydrate‐Based Oxepines: Ring Expanded Glycals for the Synthesis of Septanose Saccharides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cross-metathesis reaction of vinyl sulfones and sulfoxides

Cross-metathesis reactions of α,β-unsaturated sulfones and sulfoxides in the presence of molybdenum and ruthenium pre-catalysts were tested. A selective metahesis reaction was achieved between functionalized terminal olefins and vinyl sulfones by using the ‘second generation’ ruthenium catalysts 1c– h while the highly active Schrock catalyst 1b was found to be functional group incompatible with vinyl sulfones. The cross-metathesis products were isolated in good yields with an excellent ( E)-selectivity. Both the molybdenum and ruthenium-based complexes were, however, incompatible with α,β- and β,γ-unsaturated sulfoxides.

Carbohydrate-based oxepines: ring expanded glycals for the synthesis of septanose saccharides

A ring-closing metathesis (RCM) approach to a family of carbohydrate-based oxepines is described. A variety of readily accessible, protected monosaccharide derived dienes were used to demonstrate the utility of the synthetic sequence and to investigate how factors such as rigidification and deoxygenation mediate RCM using the Grubbs or Schrock catalyst. The seven-membered cyclic enol ethers are ring expanded glycals to be used in the synthesis of septanose carbohydrates.

A general approach to medium ring alkynes by using metathesis of cobalt hexacarbonyl containing dienes.

The assembly of medium sized rings (7-9) was achieved by using the metathesis of dienes linked by a cobalt hexacarbonyl complexed alkyne with either Grubbs' or Schrock's catalysts. The products of metathesis were subjected to transformations involving the dicobalt hexacarbonyl complexes, for example, decomplexation to liberate cyclic alkynes or Pauson-Khand reaction.

Asymmetric Tandem Additions to Chiral 2,3‐Dihydronaphthyloxazolines: Synthesis of the Triptoquinone/Triptinin A Ring System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Asymmetric Tandem Additions to Chiral 2,3-Dihydronaphthyloxazolines: Synthesis of the Triptoquinone/Triptinin A Ring System

A study to reach the diterpenoid (+)-triptoquinone A (3) or its analog (+)-triptinin A (4) via an asymmetric tandem addition to naphthyloxazolines is described. The tandem addition to the chiral dihydronophthalene 6 resulted in a 70% yield of a single diastereomer 10. Further manipulation gave the natural products’ tricyclic ring system, compounds 20 and 29 via ring-closing metathesis in 90% yield, using the Schrock catalyst. Final assault to the target compounds 3 or 4 fell short due to the failure to either reduce a neopentyl hydroxymethyl group to a methyl or to install the conjugated carboxylic acid present in 3 or 4.

Cross-metathesis of vinyl aromatic heterocycles with 1-octene in the presence of a Schrock catalyst

Metathesis of 2-vinyl aromatic heterocycles such as furan, thiophene, pyrrole and pyridine in the presence of a molybdenum-based Schrock catalyst has been investigated from a synthetic point of view. The self-metathesis of 2-vinyl aromatic heterocycles was not successful. However, in cross-metathesis of thiophene, furan and styrene with 1-octene, the cross-metathesis product, heterodimer, was readily obtained selectively, together with only small amounts of the two corresponding self-metathesis products. The origin of the surprisingly high selectivity of heterodimer formation was elucidated through metallacyclobutane intermediate mechanism, observations of carbenes by in situ 1 H NMR, and reaction products.

Cross-metathesis of vinyl aromatic heterocycles: comparison of Grubbs catalyst and Schrock catalyst

Metathesis of 2-vinyl aromatic heterocycles such as furan and thiophene has been investigated in the presence of a ruthenium-based Grubbs catalyst from a synthetic standpoint. The self-metathesis of 2-vinyl aromatic heterocycles was not successful. However, the cross-metathesis of these aromatic heterocycles with 1-octene occurred efficiently, but the selectivity of cross-metathesis product was very low, below 50%. The origin of the low selectivity of heterodimer formation was elucidated through metallacyclobutane intermediate mechanism, observations of carbenes by in situ 1 H NMR, and the reaction products. The effect of oxygen on the reaction behavior was also examined. Furthermore, the data obtained on the Grubbs catalyst were compared with those on a molybdenum-based Schrock catalyst.

Novel Synthesis of π-Conjugated Molecules by Cross-Metathesis of Vinylarene and Vinylferrocene with a Schrock Catalyst

Novel Synthesis of π-Conjugated Molecules by Cross-Metathesis of Vinylarene and Vinylferrocene with a Schrock Catalyst