Highly Stereo- and Regioselective Allylations Catalyzed by Mo−Pyridylamide Complexes:  Electronic and Steric Effects of the Ligand

Belda, Oscar; Kaiser, Nils‐Fredrik K.; Bremberg, Ulf; Larhed, Mats; Hallberg, Anders; Moberg, Christina

doi:10.1021/jo0006829

Cited by 69 publications

(26 citation statements)

References 19 publications

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“…Nucleophilic attack at the carbon bearing the leaving group has also been documented with Ru (48) and Fe (44) catalysts. On the other hand, Ni (27)(28)(29)(30)(31)(32)(33), Pd (19)(20)(21), and Pt (34,35) all typically produce a mixture of regioisomeric products, whereas Ir (37-43), Mo (22)(23)(24)(25)(26), and W (50) favor attack at the more highly substituted allylic terminus.…”

Section: Discussionmentioning

confidence: 99%

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Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: Catalyst and substrate studies leading to a mechanistic working model

Lautens

Fagnou²

2004

Proc. Natl. Acad. Sci. U.S.A.

103

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Catalyst and substrate studies have been performed on the rhodium-catalyzed asymmetric ring opening reaction. A working model is advanced that involves oxidative insertion with retention to form an organorhodium intermediate that then undergoes nucleophilic attack with inversion. Kinetic and competition experiments have uncovered evidence for a proton transfer step in the catalytic cycle that may activate both the allylrhodium intermediate and the nucleophile. We have also conducted experiments designed to understand which properties of the PPF-P t Bu2 ligand contribute to the high reactivities and enantioselectivities.W e have previously reported that chiral rhodium(I) catalysts induce ring opening oxa-and azabicyclic alkenes with a variety of soft nucleophiles such as alcohols, phenols, amines, anilines, malonates, and carboxylates in high yield and enantioselectivity (1-8). The products are generated by an S N 2Ј nucleophilic displacement of the bridgehead leaving group with inversion providing the 1,2-trans product as one regio-and diastereomer. The formation of the 1,2-trans products is atypical for ring opening reactions of oxabicyclic alkenes, which usually generate the syn-1,2 diastereomers via exo nucleophilic attack (1, 2). Furthermore, regio-and stereochemical results diverge from previously documented rhodium-catalyzed reactions with allylic carbonates (9-18). The observations obtained with oxabicyclic alkenes find little precedent when compared to other allylic functionalizations including those catalyzed by Pd (refs. 19 In this report we describe catalyst and substrate studies and advance a mechanistic working model that rationalizes the stereochemical outcome and other experimental observations (Scheme 1). We have also conducted experiments designed to understand the properties of the PPF-P t Bu 2 ligand that contribute to high reactivities and enantioselectivities.Experimental procedures and characterization data can be found in Supporting Text, which is published as supporting information on the PNAS web site. Ligand Studies: Variations on the Josiphos Template and Other Ligand MotifsSeveral structural elements are present on the PPF scaffold that could contribute to its successful application in these reactions (51-53). PPF has elements of central and planar chirality as well as two phosphine atoms that differ both sterically and electronically. Two methods were used to determine the influence of the ligand on the rate. In cases where a significant difference in rate was observed, percent conversion vs. time, as determined by crude 1 H NMR, was used to quantify the reactivity of the catalyst. In cases where a more precise measurement of rate was desired, kinetic runs were performed.By comparing the reactivity and enantioselectivity of ligands 3 and 4, two trends emerge (Scheme 2). First, by increasing the steric bulk ( t Bu Ͼ Cy) the enantioselectivity is increased from 88% with 3 to 96% enantiomeric excess (ee) with the bulkier t Bu 2 P ligand 4. This increase in enantioselectivity comes at ...

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Section: Discussionmentioning

confidence: 99%

“…19 and 20 and references therein and ref. 21), Mo (22)(23)(24)(25)(26), Ni (27)(28)(29)(30)(31)(32)(33), Pt (ref. 34 and references therein and ref.…”

mentioning

confidence: 99%

Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: Catalyst and substrate studies leading to a mechanistic working model

Lautens

Fagnou²

2004

Proc. Natl. Acad. Sci. U.S.A.

103

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“…Despite these limitations, a number of very impressive enantioselective reactions involving chiral transition-metal complexes have been described. The research groups of Moberg, Hallberg, and Larhed reported on microwave-mediated palladium- [115,116] and molybdenum-catalyzed [117][118][119] asymmetric allylic alkylation reactions involving neutral carbon, nitrogen, and oxygen nucelophiles in 2000. Both processes were carried out under non-inert conditions and yielded the desired products in high chemical yield and with typical ee values of > 98 %.…”

Section: Asymmetric Allylic Alkylationsmentioning

confidence: 99%

Controlled Microwave Heating in Modern Organic Synthesis

2004

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Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of “microwave flash heating” in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

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“…As one part of ligand design, the modification of ligands at proper positions is important when a privileged ligand structure has been found, for the information on the structure-activity relationship obtained during this research process can direct the design of novel catalysts with enhanced efficiency. Besides the tuning of ligand steric effect which affects the selectivity of the catalytic process more directly, tuning of electronic effect has attracted more attention from the science community during recent years [1][2][3][4][5][6][7][8][9][10][11][12][13] . Not only the coordination ability of the ligands and the electronic density on the metal centers can be tuned, but also the inversion of the diastereoselectivity can be observed sometimes [12,13] .…”

Section: Introductionmentioning

confidence: 99%

Modification of diphenylamine-linked bis(oxazoline) ligands: Tuning of electronic effect and rigidity of ligand skeleton

2009

Sci. China Ser. B-Chem.

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The electronic effect of diphenylamine-linked bis(oxazoline) ligands was tuned through introduction of electron-withdrawing bromo and nitro substituents onto the 4 and 4′ position. The variation of the NH bond acidity was determined by the different chemical shifts of NH. The catalytic activity and enantioselectivity of the modified ligands were tested in the asymmetric Friedel-Crafts alkylation of indole with β-nitrostyrene. The effect of ligand skeleton rigidity was also investigated through the synthesis of iminodibenzyl-linked bis(oxazoline) ligands and evaluation of their catalytic activity in Friedel-Crafts alkylation.asymmetric catalysis, electronic effect, bis(oxazoline), Friedel-Crafts alkylation

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Highly Stereo- and Regioselective Allylations Catalyzed by Mo−Pyridylamide Complexes: Electronic and Steric Effects of the Ligand

Cited by 69 publications

References 19 publications

Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: Catalyst and substrate studies leading to a mechanistic working model

Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes: Catalyst and substrate studies leading to a mechanistic working model

Controlled Microwave Heating in Modern Organic Synthesis

Modification of diphenylamine-linked bis(oxazoline) ligands: Tuning of electronic effect and rigidity of ligand skeleton

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