Chiral Bis(dihydrooxazolyl)pyridineruthenium Complexes oftrans-Cyclooctene andtrans-Cycloheptene

Nishiyama, Hisao; Naitoh, Tomonari; Motoyama, Yukihiro; Aoki, Katsuyuki

doi:10.1002/(sici)1521-3765(19991203)5:12<3509::aid-chem3509>3.0.co;2-2

Cited by 21 publications

(10 citation statements)

References 4 publications

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“…Unfortunately, our attempts to replace the ethylene ligand in catalyst 4 with alternative olefins, such as trans ‐2‐butene or trans ‐cyclooctene, were not successful, presumably because of the steric hindrance imparted by both the indenyl‐pybox and bromide ligands 14. However, we were able to synthesize complexes 12 and 13 in which the olefin was replaced by carbon monoxide and triphenylphosphine, respectively (Table 3).…”

Section: Methodsmentioning

confidence: 97%

Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

2008

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C À H bond amination has emerged as a powerful tool for the synthesis of complex nitrogen-containing molecules. Following the early discoveries by Breslow and Gellman, [1] Du Bois and co-workers revolutionized this area of chemistry by developing protocols for practical, efficient, and predictable reactions for oxidative C À H amination.[2] Dirhodium(II) tetracarboxylate catalysts were shown to be particularly effective. Manganese-and ruthenium-porphyrin complexes, [3] silver complexes, [4] and preoxidized nitrogen sources have also been developed as catalysts to carry out this important reaction. [5] Despite these recent advances, general methods for both enantioselective and intermolecular C À H amination remain elusive. Although chiral dirhodium(II) complexes have been developed as catalysts for highly enantioselective metallocarbene reactions, [6] their application to CÀH amination chemistry is yet to produce the same spectacular results. [7] To date, the most effective protocol for asymmetric CÀH amination requires the combination of enantioenriched sulfoxamines as chiral auxiliaries and a chiral dirhodium(II) catalyst.[8] To address the challenge of catalytic asymmetric CÀH amination, we chose to study ruthenium(II)-pybox (pybox = pyridine bisoxazoline) complexes (Scheme 1). Despite reports that show complex 1 exhibited limited reactivity and selectivity in C À H amination reactions, [3f] we felt that the modular nature of the ligand, and the fact that the anionic ligands were independent of the chiral pybox ligand, offered us an opportunity which had not been possible by using either dirhodium(II)-or porphyrin-based catalyst systems.Ruthenium(II)-pybox complexes 1-4 were readily prepared by using the method developed by Nishiyama et al. (Scheme 1).[9] In our initial study, the challenging test substrate sulfamate ester 5[10] was treated with 1.1 equivalents of the oxidant bis(acetoxy)iodobenzene and 5 mol % catalyst 2 to give the desired product of CÀH insertion 6, albeit in modest yield and enantiomeric excess (Table 1, entries 1-3).Based on the study by Fiori and Du Bois, which demonstrates that rhodium-catalyzed C À H amination involves formation of an electrophilic metallonitrene and a build up of positive charge on the carbon center during the insertion process, we rationalized that a cationic catalyst would be more reactive than its neutral analogue.[2e] Halide abstraction from the Scheme 1. Synthesis of ruthenium(II)-pybox complexes. [b] ee [%] [b]

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Section: Methodsmentioning

confidence: 97%

Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

2008

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“…[1] When two or more chiral ligands are bound to the same ligand, diastereomers may be formed, and the selectivity for one or other diastereomer may be studied. [2Ϫ7] The bis(oxazolinyl)pyridine (pybox) family of ligands (Scheme 1) were first developed by Nishiyama and co-workers for enantioselective homogeneous catalysis, [8,9] and have also been used to generate chiral Lewis acid catalysts. [10] We have recently used these ligands for the enantioselective synthesis of helical complexes.…”

Section: Introductionmentioning

confidence: 99%

Diastereoselectivity of Octahedral Cobalt(II) Pybox Complexes

Provent

Bérnardinelli

Williams

et al. 2001

Euro J of Inorganic Chem

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“…[52][53][54][55][56][57] Several studies had shown that metal complexes of transcycloheptene can be isolated. [16,[149][150][151] Jendralla described the preparation of AgOTf and AgClO 4 complexes of 3-methoxytrans-cycloheptene and 6-Methoxy-(Z),4(E)-cycloheptadiene through Ag-mediated ring opening of a nitrosourea derivative of bicyclo[4.1.0]heptane. [16,150] In unspecified yields, the AgClO 4 · 3-methoxy-trans-cycloheptene complex was combined with a number of dienes to give the products of metal decomplexation and [4 + 2] cycloaddition.…”

Section: Flow Photochemical Synthesis Of Trans-cycloheptenes and Silamentioning

confidence: 99%

“…Several studies had shown that metal complexes of trans‐ cycloheptene can be isolated . Jendralla described the preparation of AgOTf and AgClO 4 complexes of 3‐methoxy‐ trans ‐cycloheptene and 6‐Methoxy‐( Z ),4( E )‐cycloheptadiene through Ag‐mediated ring opening of a nitrosourea derivative of bicyclo[4.1.0]heptane .…”

Section: Figurementioning

confidence: 99%

Flow Photochemical Syntheses of trans‐Cyclooctenes and trans‐Cycloheptenes Driven by Metal Complexation

Pigga

Fox

2019

Israel Journal of Chemistry

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trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisomerization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silicagel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the transisomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.The unusual reactivity and well-defined chiral structure of trans-cycloalkenes has made them attractive targets for synthesis for nearly 70 years. [1][2][3] For example, trans-cyclooctene possesses planar chirality and displays a high barrier to racemization (E a = 35.6 kcal/mol), [4] and the most stable "crown" conformer has an alternating sequence of equatorial and axial hydrogens that is akin to chair cyclohexane. [5][6] The double bond of trans-cyclooctene is twisted severely in the crown conformation, [7] and as a consequence the HOMO of trans-cyclooctene is relatively high in energy. [8] trans-Cycloheptene also has a rigid structure with a distorted alkene. [7] The double bonds of medium-ring trans-alkenes are twisted. [7] trans-Cycloalkenes display unusual reactivity in HOMOalkene controlled cycloaddition reactions with dienes, [9] 1,3dipoles [8] and ketenes. [10] Strained trans-cycloalkenes also serve as ligands for transition metals. [11][12][13][14][15][16] This reactivity profile has made trans-cycloalkanes interesting targets for applications in synthesis and biology.The broadest applications of trans-cycloalkenes are in the field of bioorthogonal chemistry. The inverse-electron demand Diels-Alder reaction between tetrazines and strained alkenes has a rich history in physical organic and synthetic chemistry (Figure 1). [17][18][19] In 2008, three groups described the bioorthogonal reactions of tetrazines with strained alkenes. [20][21][22] The variant introduced by our group that used trans-cyclooctene is marked by exceptionally rapid kinetics, with rate constants that can exceed 10 6 M À 1 s À 1 in the fastest cases. [23][24] With the advances including the development of fluorogenic tetrazines [25] and reactions in live cells [26] and animals, [27] the tetrazine ligation has become a widely used tool for applications that span chemical biology, biomedical imaging, and materials science. [28][29][30][31][32][33][34][35][36][37] This account describes the development of general, flow photochemical methods for the preparative synthesis of transcycloalkene derivatives and enabled applications in synthesis and bioorthogonal chemistry. Selectiv...

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Chiral Bis(dihydrooxazolyl)pyridineruthenium Complexes oftrans-Cyclooctene andtrans-Cycloheptene

Cited by 21 publications

References 4 publications

Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

Diastereoselectivity of Octahedral Cobalt(II) Pybox Complexes

Flow Photochemical Syntheses of trans‐Cyclooctenes and trans‐Cycloheptenes Driven by Metal Complexation

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