Electronic Effects in Asymmetric Catalysis. Synthesis and Structure of Model Rhodium Complexes Containing Ferrocenyl Ligands for Use in the Hydroboration Reaction

Schnyder, Anita; Togni, A.; Wiesli, Ursula

doi:10.1021/om9605995

Cited by 90 publications

(50 citation statements)

References 21 publications

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“…Several structural elements are present on the PPF scaffold that could contribute to its successful application in these reactions (51)(52)(53). PPF has elements of central and planar chirality as well as two phosphine atoms that differ both sterically and electronically.…”

Section: Ligand Studies: Variations On the Josiphos Template And Othementioning

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.

101

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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: Ligand Studies: Variations On the Josiphos Template And Othementioning

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.

101

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“…Furthermore, there is considerable current interest in hybrid P/N ligands and a search in the Cambridge Structure Database revealed that although a vast number of Groups 9-10 complexes with nitrogen and phosphorus ligands have been characterised structurally, only a relatively small number of Pd(II) cations and Rh(I) cations [54][55][56][57][58][59][60][61][62][63][64][65] containing bidentate N(sp 2 )-P(sp 3 ) donors are reported. Since bulky substituents at the aryl groups of N(sp 2 ) donors have been found to strongly influence the catalytic activity and selectivity of Group 10 complexes [66], it is surprising that no Rh(I) complexes of ligands combining tertiary phosphine donors and (sp 2 )N-aryl donors with bulky ortho substituents have been structurally investigated.…”

Section: Introductionmentioning

confidence: 99%

New cationic palladium (II) and rhodium (I) complexes of [Ph2PCH2C(Ph)N(2,6-Me2C6H3)]

Pascu

Coleman

Cowley

et al. 2005

Journal of Organometallic Chemistry

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“…Rhodium complexes (1 mol%) prepared in situ from [Rh(COD) 2 ]BF 4 and the respective ligands (70 -73) were employed in the enantioselective hydroborationoxidation of styrene (28) ( Table 3.3 ) [63,64] .…”

Section: Selectivity Of Metal-catalyzed Hydroboration 75mentioning

confidence: 99%

The Development and Application of Rhodium‐Catalyzed Hydroboration of Alkenes

Coyne

Guiry

2008

Modern Reduction Methods

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3.1 IntroductionHydroboration, the addition of a boron -hydrogen bond across an unsaturated moiety, was fi rst discovered by H. C. Brown in 1956 [1] . Usually the reaction does not require a catalyst and the borane reagent, most commonly diborane (B 2 H 6 ) or a borane adduct (BH 3 · THF), reacts rapidly at room temperature to afford, after oxidation, the anti -Markovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms, as is the case in catecholborane (1,3,2 -benzodioxaborole) the electron density at boron is increased and elevated temperatures are needed for hydroboration to occur [2,3] .The development of a catalytic hydroboration process was aided by the observation of Kono and Ito in 1975 that Wilkinson ' s catalyst [Rh(PPh 3 ) 3 Cl] undergoes oxidative addition when treated with catecholborane (2) , Scheme 3.1 [4] . Subsequently, Westcott et al. reported the isolation of the oxidative addition adduct with the tri -isopropylphosphine derivative and its characterization by X -ray crystallography [5] . However, it was another decade before the idea of developing a rhodiumcatalyzed olefi n hydroboration process came to fruition with the fi rst examples reported by M ä nnig and N ö th [6] .The conversion of an alkene (1) into an organoborane intermediate (3) has made this a valuable synthetic technique, particularly since the development of enantioselective variants [7, 8] . They serve as synthons for numerous functional groups [9] and are often subject to a consecutive carbon -oxygen [10] , carbon -carbon [11 -13] boron -carbon [14] , boron -chlorine [5] , carbon -nitrogen [15] and other bondforming reactions, Scheme 3.2 . These and other approaches toward extending the synthetic scope of catalytic asymmetric hydroboration have been the subject of recent excellent reviews [16 -18] . Many metals including nickel [19] , ruthenium [20] , iridium [21] , lanthanum [22] , titanium [23] and zirconium [24] have been employed in this transformation with varying degrees of success, although rhodium has remained the metal of choice for transition metal hydroboration.

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Electronic Effects in Asymmetric Catalysis. Synthesis and Structure of Model Rhodium Complexes Containing Ferrocenyl Ligands for Use in the Hydroboration Reaction

Cited by 90 publications

References 21 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

New cationic palladium (II) and rhodium (I) complexes of [Ph2PCH2C(Ph)N(2,6-Me2C6H3)]

The Development and Application of Rhodium‐Catalyzed Hydroboration of Alkenes

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