Strong Electronic Effects on Enantioselectivity in Rhodium‐Catalyzed Hydroborations with Novel Pyrazole‐Containing Ferrocenyl Ligands

Schnyder, Anita; Hintermann, Lukas; Togni, Antonio

doi:10.1002/anie.199509311

Cited by 148 publications

(44 citation statements)

References 31 publications

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“…In the conceptually similar ferrocenyl ligands introduced by Knochel and co-workers [56], the steregenic center is introduced by an oxazaborolidine reduction of a ferrocenyl aryl ketone, whereby the ensuing oxygen functionality serves to control a directed lithiation step in which the heterocyclic ring is introduced. As observed earlier by Togni with a different family of ferrocene-based P,N-ligands [57], the high enantioselectivity in styrene hydroboration is accompanied by lower regioselectivity. A simple atropisomerically chiral P,N-ligand based on 2-(2'-diphenylphosphinophenyl)pyridine has been prepared and resolved, and its rhodium complex is at least as effective as the corresponding QUINAP complex for the asymmetric hydroboration of simple styrenes [58].…”

Section: Phosphinamine and Related Ligandssupporting

confidence: 49%

Rhodium‐Catalyzed Hydroborations and Related Reactions

Brown¹

2005

Modern Rhodium‐Catalyzed Organic Reactions

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The development of metal-catalyzed hydroboration was initiated in a single paper by Männig and Nöth in 1985 [1]. They demonstrated that catecholborane reacted with simple alkenes in the presence of transition-metal catalysts, the most effective being Wilkinson's catalyst, RhCl(PPh 3 ) 3 . Since this reaction of catecholborane with alkenes is normally rather slow, even at elevated temperatures, this observation provided a real opportunity for catalysis. Prior to this work, the only precedent had been the addition of a secondary alkoxyborane to the same rhodium complex, giving a borylrhodium hydride analogous to known silylrhodium hydrides [2]. The synthetic potential of catalytic hydroboration was quickly realized and further extended through Burgess's demonstration of asymmetric catalysis [3], which was later refined and extended by Hayashi [4]. All this early work has been reported in a 1991 review [5], with a later and more extensive review by Belet'skaya and Pelter in 1997 [6]. As these reviews are readily available, the present chapter is confined largely to developments that have taken place since the beginning of 1997. Fig. 2.1 summarizes some of these early milestones. Although metal catalysts other than rhodium have been examined, for example in the palladiumcatalyzed hydroboration of allenes [7], the majority of the interesting developments have been made with rhodium.In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following: (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane; (b) the scope of alkenes for which efficient rate, regio-and enantioselectivity can be achieved is limited, and (c) the "standard" transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereospecific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9].A reaction pathway for catalytic hydroboration was suggested by Evans and co-workers [10], and amplified through computational studies [11]. This involves the rhodium acting through oxidative addition across the B-H bond, and the resulting borylrhodium hydride complexing to the alkene (reversibly). This is followed by a hydride migration, with a subsequent reductive elimination of the alkylboronate ester to complete 33 2

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Section: Phosphinamine and Related Ligandssupporting

confidence: 49%

Rhodium‐Catalyzed Hydroborations and Related Reactions

Brown¹

2005

Modern Rhodium‐Catalyzed Organic Reactions

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“…However, the ligands are limited in terms of regiochemical control, entries 1 and 2. Increasing the size of the pyrazole substituent from methyl to isopropyl resulted in a slightly lower 92% ee, [131] showing there was some dependence of the enantioselectivity on the steric properties of the ligand. However, the single most dominant influence was the electronic nature of the ligand.…”

Section: Planar Chiralitymentioning

confidence: 95%

“…The different electronic properties of the pyrazole and phosphine moieties exert opposite influences and high asymmetries are induced when the combination exists such that nitrogen is a good s-donor (electron-rich pyrazole) and phosphorus a good p-acceptor (electron-poor phosphine). [131] To this end, while essentially conserving the steric bulk, replacement of the pyrazole methyl groups in 105 with the electron-withdrawing trifluoromethyl groups in 106 gave rise to a dramatic decrease in enantioselectivity, although the regioselectivity was largely unaffected, entry 2. However, when the CF 3 groups were placed on the phosphine instead in ligand 107, thereby rendering the phosphine electron-deficient, an exceptional 98.5% ee was obtained, entry 3.…”

Section: Planar Chiralitymentioning

confidence: 98%

“…The possibility that the low enantioselectivities in the case of electron-poor pyrazole ligands 106 and 108 was due to partial dissociation of the ligand from rhodium during catalysis was found to be unlikely and the pronounced effects on enantioselectivity therefore deemed largely electronic in nature. [131] In terms of chiral ligands that incorporate both planar and atom-centred chirality, the ferrocenyl framework is unrivalled in its versatility. Moreover, the synthetic approach towards this ligand class allows for tailoring of the ligand through steric and electronic effects.…”

Section: Planar Chiralitymentioning

confidence: 99%

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The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

Carroll

O’Sullivan

Guiry

2005

Advanced Synthesis & Catalysis

328

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Rhodium-catalysed enantioselective hydroboration of olefins is a valuable synthetic transformation, typically employing a chiral catalyst and an achiral borane source. The pertinent chemo-, regio-and enantioselectivity issues of this reaction are discussed. However, the main emphasis of this review is on the evolution of catalytic asymmetric hydroboration. This has primarily relied upon the development and application of chiral bidentate P,P and P,N ligands which have exhibited varying degrees of success in this transformation.

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“…[81] They were capable of high enantioselectivities in the rhodium-catalysed hydroboration of styrene although the regioselectivity obtained was only moderate, Scheme 14, Table 20. [82] In the allylic amination of 1,3-diphenyl-2-propenyl acetate (15), where the addition of small potentially coordinating anions, such as fluoride or borohydride, was found to enhance the selectivity of the system and enantiomeric excesses of 99.5% were obtained. [83] The general ligand structure has also been used in the preparation of bi-metallic and dendrimer type catalysts for asymmetric synthesis.…”

Section: Pyrazole N Donormentioning

confidence: 99%

The Development of Bidentate P,N Ligands for Asymmetric Catalysis

2004

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In this review, an attempt is made to systemise the ro √ le which bidentate phosphinamine (P,N) ligands play in asymmetric catalysis. The ligands will be classified, not by the reaction to which their metal complexes have been applied, but by the nature of their donor atoms. In this manner the development of ligand architectural design can be more easily monitored. The asymmetric transformations to which metal complexes of these ligands have been applied include among others, palladium-catalysed allylic substitutions, copper-catalysed 1,4-additions to enones and rhodiumcatalysed hydroboration of vinylarenes. Excellent enantioselectivities, regioselectivities and reactivities have been achieved in each of these processes.

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Strong Electronic Effects on Enantioselectivity in Rhodium‐Catalyzed Hydroborations with Novel Pyrazole‐Containing Ferrocenyl Ligands

Abstract: COMMUNICATIONSwith a Bruker IFS-66 FT-IR spectrometer in the range 4000-400 cm-' with a resolution of I cm-'. Description of the matrix isolation equipment: W Sander, J! Org.

Cited by 148 publications

References 31 publications

Rhodium‐Catalyzed Hydroborations and Related Reactions

Rhodium‐Catalyzed Hydroborations and Related Reactions

The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

The Development of Bidentate P,N Ligands for Asymmetric Catalysis

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