Catalysis of allylic substitutions by Pd complexes of oxazolines containing an additional P, S, or Se Center. X-ray crystal structures and solution structures of chiral π-allyl palladium complexes of phosphinoaryloxazolines

Sprinz, Jürgen; Kiefer, Matthias; Helmchen, Günter; Reggelin, Michael; Huttner, G.; Walter, Olaf; Zsolnai, László

doi:10.1016/s0040-4039(00)76748-7

Cited by 342 publications

(162 citation statements)

References 11 publications

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“…The proposal that nucleophilic attack occurs in an S N 2Ј fashion with inversion is consistent with the well documented mechanism of palladium -allyl chemistry (73)(74)(75)(76) and the proposed mechanism of rhodium-catalyzed reactions of allylic carbonates put forward by Evans (12). The proposal that the alkoxide moiety directs the nucleophilic attack by shifting the position of the metal on the allyl fragment has precedent in the chemistry of palladium where neighboring groups have been found to exert this type of directing influence (77,78).…”

Section: Discussionsupporting

confidence: 79%

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: Discussionsupporting

confidence: 79%

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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“…These include chiral PÀN ligands, as was demonstrated by Pfaltz and Helmchen, who employed chiral phosphinooxazoline ligands possessing C 1 symmetry. [5,[11][12][13][14][15][16][17][18] The difference in donor properties of the coordinating phosphorus and nitrogen atoms offered the possibili-Abstract: A series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pdcatalysed allylic alkylation reaction. Reaction of disodium diethyl 2-methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite ligands proceeds smoothly at room temperature.…”

Section: Introductionmentioning

confidence: 99%

Bulky Monodentate Phosphoramidites in Palladium‐Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity

Boele

Kamer

Lutz³

et al. 2004

Chemistry A European J

112

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A series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pd‐catalysed allylic alkylation reaction. Reaction of disodium diethyl 2‐methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite ligands proceeds smoothly at room temperature. The regioselectivities observed depend strongly on the leaving group and the geometry of the allylic starting compounds. Mono‐coordination occurs when these ligands are ligated in [Pd(allyl)(X)] complexes (allyl=C3H5, 1‐CH3C3H4, 1‐C6H5C3H4, 1,3‐(C6H5)2C3H3; X=Cl, OAc). The solid‐state structure determined by X‐ray diffraction of [Pd(C3H5)(1)(Cl)] reveals a non‐symmetric coordination of the allyl moiety, caused by the stronger trans influence of the phosphoramidite ligand relative to X−. In all of these complexes, the syn,trans isomer is the major species present in solution. Because of fast isomerisation and high reactivity of the syn,cis complex, the major product formed upon alkylation is the linear product, especially for monosubstituted phenylallyl substrates in the presence of halide counterions. In the case of biphenol‐ and BINOL‐based phosphoramidites, however, a strong memory effect is observed when 1‐phenyl‐2‐propenyl acetate is employed as the substrate. In this case, nucleophilic attack competes effectively with the isomerisation of the transient cinnamylpalladium complexes. The asymmetric allylic alkylation of 1,3‐diphenyl‐2‐propenyl acetate afforded the chiral product in up to 93 % ee. Substrates with smaller substituents gave lower enantioselectivities. The observed stereoselectivity is explained in terms of a preferential rotation mechanism, in which the product is formed by attack on one of the isomers of the intermediate [Pd{1,3‐(C6H5)2C3H3}(L)(OAc)] complex.

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“…This process would give rise to a (P,L)-Pd-(p-allyl) complex (L unspecified nonphosphane ligand or vacant site) and it is therefore instructive to consider (P,N)-Pd-(p-allyl) complexes. Crystal structures [44] of these complexes indicate that the C ± Pd bond to the allylic terminus trans to P is longer than that trans to N. Arguments have been put forward that the carbon trans to P should be attacked more rapidly by nucleophiles [12,45] and, by reverse arguments, that ionisation should occur with the nucleofuge trans to P. [27a] Mismatched ionisation of (R)-1 by a non-chelate complex of type 'Pd(R,R)-3' should thus occur to give a (P,L)-Pd-[h 3 -allyl] complex with a-C trans to P and nucleophilic attack should also be a-C and trans to P. A memory effect with a-C selectivity would then be observed provided endo-exo rotamer equilibration or re-coordination of the second phosphine (13-membered chelate formation) is slow (Figure 10). Ligand 3 as a ' 'PP' ' versus ' 'P' ', ' 'PO' ' or ' 'PL' ' ligand: Monophosphane derivatives of (R,R)-3 (one Ph 2 P replaced by Ph 2 C or H) generate Pd catalysts that induce lower and reversed enantioselectivity in the allylic alkylation of (AE )-1 and 19.…”

mentioning

confidence: 99%

Memory Effects in Pd-Catalysed Allylic Alkylation: Stereochemical Labelling through Isotopic Desymmetrization

Lloyd‐Jones

Stephen

1998

Chem. Eur. J.

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Catalysis of allylic substitutions by Pd complexes of oxazolines containing an additional P, S, or Se Center. X-ray crystal structures and solution structures of chiral π-allyl palladium complexes of phosphinoaryloxazolines

Cited by 342 publications

References 11 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

Bulky Monodentate Phosphoramidites in Palladium‐Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity

Memory Effects in Pd-Catalysed Allylic Alkylation: Stereochemical Labelling through Isotopic Desymmetrization

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