Tethered NHC Ligands for Stereoselective Alkyne Semihydrogenations

Pape, Felix; Teichert, Johannes F.

doi:10.1055/s-0036-1590112

Cited by 15 publications

(7 citation statements)

References 27 publications

(40 reference statements)

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“…Canonical hydrogenations of π-bonds with the aid of transition metal catalysts, as long as they do not involve transfer of H • , are under frontier orbital control. − As a consequence, they invariably proceed via suprafacial delivery of the two hydrogen atoms of H 2 to the substrate. In view of this stringent syn -selective course, it is unsurprising that a considerable number of heterogeneous as well as homogeneous catalysts is known that allow internal alkynes to be semihydrogenated with formation of the corresponding ( Z )-alkenes; among them, Lindlar hydrogenation is arguably the most popular protocol. − Although the individual methods greatly differ in efficiency and practicality as well as in their bias to saturate the olefin products initially formed, the stereoselectivity is generally excellent. If over-reduction prevails or other reducible sites are endangered under hydrogenation conditions, alternative stoichiometric procedures are available to the practitioner that also allow alkynes to be transformed into ( Z )-olefins with appreciable selectivity. , …”

Section: Introductionmentioning

confidence: 99%

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

et al. 2018

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The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with trans-hydrogenation. The computed scenario concurs with para-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct trans-delivery of H, the formation of carbene intermediates by gem-hydrogenation, and their evolution into product and side products alike. Propargylic -OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective trans-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by gem-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions.

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Section: Introductionmentioning

confidence: 99%

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

et al. 2018

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“…Noteworthy, the complex 5 was never evaluated in this reported reaction. Therefore, 5bm behaves similarly to other copper(I)/NHC complexes in this transformation [54][55][56][57][58][59][60]. The catalytic 1,2-reduction of carbonyl compounds is mainstay for copper(I)/NHC complexes [61][62][63][64][65][66][67], which is why we also tested 5bm in these transformations: The 1,2-reduction of benzaldehyde (14) and acetophenone (15) proceeded with good yields (Scheme 3c).…”

Section: Resultsmentioning

confidence: 80%

Mechanochemical solid state synthesis of copper(I)/NHC complexes with K₃PO₄

Remy-Speckmann¹,

Zimmermann²,

Gorai³

et al. 2023

Beilstein J. Org. Chem.

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A protocol for the mechanochemical synthesis of copper(I)/N-heterocyclic carbene complexes using cheap and readily available K3PO4 as base has been developed. This method employing a ball mill is amenable to typical simple copper(I)/NHC complexes but also to a sophisticated copper(I)/N-heterocyclic carbene complex bearing a guanidine moiety. In this way, the present approach circumvents commonly employed silver(I) complexes which are associated with significant and undesired waste formation and the excessive use of solvents. The resulting bifunctional catalyst has been shown to be active in a variety of reduction/hydrogenation transformations employing dihydrogen as terminal reducing agent.

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“…To further demonstrate mechanistic dichotomy of selective single hydride (deuteride) transfer from H 2 (or D 2 ) and “classic” hydrogenations, in which two hydrogen atoms are transferred from H 2 , a combination of both processes each relying on copper(I)–NHC complexes was performed (Scheme ). First, a copper(I)‐catalyzed alkyne semihydrogenation with imidazolinium salt 15 as the ligand precursor was carried out, yielding a dideuterated Z ‐allyl silyl ether (not shown) from propargylic silyl ether 12 . After subsequent transformation to allylic chloride Z ‐ 13 ‐ d 2 , the catalytic hydride (deuteride) transfer process gave dideuterated 14 ‐ d 2 (from the allylic substitution of Z ‐ 13 ‐ d 2 with H 2 ) as well as trideuterated 14 ‐ d 3 (both catalytic processes using D 2 ) with excellent isotope incorporations.…”

Section: Methodsmentioning

confidence: 79%

Catalytic Generation and Chemoselective Transfer of Nucleophilic Hydrides from Dihydrogen

Pape

Brechmann

Teichert

2018

Chemistry A European J

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Copper(I)–N‐heterocyclic‐carbene (NHC) complexes enabled the catalytic generation of nucleophilic hydrides from dihydrogen (H2) and their subsequent transfer to allylic chlorides. The highly chemoselective catalyst displayed no concomitant hydrogenation reactivity; in fact, the terminal double bond formed in the hydride transfer remained intact. Switching to deuterium gas (D2) allowed for regioselective monodeuteration with excellent isotope incorporation.

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Tethered NHC Ligands for Stereoselective Alkyne Semihydrogenations

Cited by 15 publications

References 27 publications

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

Mechanochemical solid state synthesis of copper(I)/NHC complexes with K₃PO₄

Catalytic Generation and Chemoselective Transfer of Nucleophilic Hydrides from Dihydrogen

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Tethered NHC Ligands for Stereoselective Alkyne Semihydrogenations

Cited by 15 publications

References 27 publications

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

Half-Sandwich Ruthenium Carbene Complexes Linktrans-Hydrogenation andgem-Hydrogenation of Internal Alkynes

Mechanochemical solid state synthesis of copper(I)/NHC complexes with K3PO4

Catalytic Generation and Chemoselective Transfer of Nucleophilic Hydrides from Dihydrogen

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Mechanochemical solid state synthesis of copper(I)/NHC complexes with K₃PO₄