First highly enantioselective allylic alkylations catalysedby platinum complexes

Blacker, A. John; Clark, Matthew L.; Williams, Jonathan M. J.; Loft, Michael S.

doi:10.1039/a901555h

Cited by 31 publications

(9 citation statements)

References 10 publications

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“…The 4'/4'' ratio was determined by 1 H NMR spectroscopy. But-3-en-2-yl acetate (2 c) reacted with the primary alkyl halide 1 b (Table 3, entry 4) but not the secondary alkyl halide 1 g. More sterically hindered acetates, such as prenyl acetate (2 d), reacted with 1 b and provided the cross-coupling product in a good yield (Table 3, entry 5), but again no coupling product was observed with secondary alkyl bromide 1 g. Excellent yields were obtained using (E)-cinnamyl acetate (2 e; Table 3, entries [6][7][8][9]. The alkylation reaction of 1-bromo-4-chlorobutane resulted in a selective attack at the bromide site, thus affording (E)-7-chlorohept-1-en-1-ylbenzene (4 h) in good yield (Table 3, entry 8).…”

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confidence: 77%

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confidence: 99%

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Cobalt‐Catalyzed Reductive Allylation of Alkyl Halides with Allylic Acetates or Carbonates

Qian¹,

Auffrant²,

Felouat³

et al. 2011

Angew Chem Int Ed

128

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Transition-metal-catalyzed allylic alkylations, using a broad range of metal complexes, have been intensively studied because of their potential applications in the synthesis of new olefinic compounds in particular for total synthesis.[1] Soft nucleophiles are usually used in Pd-, [1] Mo-, [2] Ir-, [3] Ru-, [4] Rh-, [5] Pt-, [6] and even Fe-catalyzed [7] allylic substitutions. Ni, [8] Co, [9] and Cu [10] catalysts allow the use of hard nucleophiles such as alkylzinc or Grignard reagents, but limited functional group compatibility and/or poor regioselectivity can be observed if the system is not designed carefully. To avoid handling the air-and moisture-sensitive organomagnesium and organozinc reagents, straightforward procedures, which do not require organometallic reagents, are highly desirable and many have now been developed. [11] To the best of our knowledge, direct transition-metal-catalyzed alkyl-allyl cross-couplings using in situ generated catalytic organometallic reagents are still unknown. However, a few years ago, we reported a related Co-catalyzed coupling reaction of aryl halides with allylic acetates; [12] these reactions in the presence of an appropriate reducing reagent, gave allylaromatic compounds. Such allylic carboxylates, whilst less reactive than allyl halides, are much more environmentally friendly.Given our previous experience with the direct Cocatalyzed functionalization, including alkylation, [11c] of aryl halides [13] we were interested to take the chemistry further, and herein we report a new and general method for direct reductive cross-coupling of allylic acetates with alkyl halides using a CoBr 2 /Mn system with an acetonitrile/pyridine solvent mixture. The approach accommodates a variety of simple and functionalized alkyl halides and substituted allylic compounds and is experimentally straightforward. Indeed it uses off-theshelf reagents without any particular precautions against air and moisture. First, we investigated the use of the readily available yet poorly reactive ethyl 4-bromobutanoate with a simple allyl acetate as the electrophile. The major challenge here lies in promoting cross-coupling rather than the formation of reduction and homocoupling products.

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confidence: 99%

Cobalt‐Catalyzed Reductive Allylation of Alkyl Halides with Allylic Acetates or Carbonates

Qian¹,

Auffrant²,

Felouat³

et al. 2011

Angew Chem Int Ed

128

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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%

“…34 and references therein and ref. 35), Co (36), Ir (37)(38)(39)(40)(41)(42)(43), Fe (44), Ru (45)(46)(47)(48)(49), and W (50).…”

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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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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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“…Herein we describe the first highly enantioselective allylic alkylations catalysed by platinum complexes. [14] In addition, the scope of platinum-catalysed allylic substitutions has been explored by studying a variety of pro-catalysts and substrates.…”

Section: Introductionmentioning

confidence: 99%

Platinum-Catalysed Allylic Alkylation: Reactivity, Enantioselectivity, and Regioselectivity

Blacker

Clarke

Loft³

et al. 2000

Chem. Eur. J.

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The use of platinum complexes as catalysts for allylic substitution has been studied. A variety of different complexes catalyse the reaction, and several substrates have been tested. In the alkylation of mono(alkyl)-substituted allylic acetates, regioselectivity is highly dependent on ligand choice. By using tricyclohexylphosphine as the ligand, almost complete formation of branched products is observed. The development of a highly enantioselective (ca. 80-90% ee) reaction that makes use of chiral diphenylphosphinooxazoline ligands (abbreviated as (S)-PN) is also described. The enantioselectivity is highly dependent on the ratio of ligand to platinum (when the ratio ligand/Pt is greater than 1:1, the ee drops off dramatically). This is in contrast to palladium and is interpreted in terms of differing coordination chemistry for the two metals ((S)-PN is hemilabile when complexed to platinum) and should be of significance to future systems that utilise heterobidentate ligands. The crystal structures of two isoelectronic platinum and palladium complexes [[(S)-PN]MCl2] are also described.

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First highly enantioselective allylic alkylations catalysedby platinum complexes

Cited by 31 publications

References 10 publications

Cobalt‐Catalyzed Reductive Allylation of Alkyl Halides with Allylic Acetates or Carbonates

Cobalt‐Catalyzed Reductive Allylation of Alkyl Halides with Allylic Acetates or Carbonates

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

Platinum-Catalysed Allylic Alkylation: Reactivity, Enantioselectivity, and Regioselectivity

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