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Ruthenium-Catalyzed Alkyne trans-Hydrometalation: Mechanistic Insights and Preparative Implications.

Authors
  • Roşca, Dragoş-Adrian1
  • Radkowski, Karin1
  • Wolf, Larry M1
  • Wagh, Minal1
  • Goddard, Richard1
  • Thiel, Walter1
  • Fürstner, Alois1
  • 1 Max-Planck-Institut für Kohlenforschung , D-45470 Mülheim/Ruhr, Germany. , (Germany)
Type
Published Article
Journal
Journal of the American Chemical Society
Publisher
American Chemical Society
Publication Date
Feb 15, 2017
Volume
139
Issue
6
Pages
2443–2455
Identifiers
DOI: 10.1021/jacs.6b12517
PMID: 28169542
Source
Medline
License
Unknown

Abstract

[Cp*RuCl]4 (1) has previously been shown to be the precatalyst of choice for stereochemically unorthodox trans-hydrometalations of internal alkynes. Experimental and computational data now prove that the alkyne primarily acts as a four-electron donor ligand to the catalytically active metal fragment [Cp*RuCl] but switches to adopt a two-electron donor character once the reagent R3MH (M = Si, Ge, Sn) enters the ligand sphere. In the stereodetermining step the resulting loaded complex evolves via an inner-sphere mechanism into a ruthenacyclopropene which swiftly transforms into the product. In accord with the low computed barriers, spectral and preparative data show that the reaction is not only possible but sometimes even favored at low temperatures. Importantly, such trans-hydrometalations are distinguished by excellent levels of regioselectivity when unsymmetrical alkynes are used that carry an -OH or -NHR group in vicinity of the triple bond. A nascent hydrogen bridge between the protic substituent and the polarized [Ru-Cl] unit imposes directionality onto the ligand sphere of the relevant intermediates, which ultimately accounts for the selective delivery of the R3M- group to the acetylene C-atom proximal to the steering substituent. The interligand hydrogen bonding also allows site-selectivity to be harnessed in reactions of polyunsaturated compounds, since propargylic substrates bind more tightly than ordinary alkynes; even the electronically coupled triple bonds of conjugated 1,3-diynes can be faithfully discriminated as long as one of them is propargylic. Finally, properly positioned protic sites lead to a substantially increased substrate scope in that they render even 1,3-enynes, arylalkynes, and electron-rich alkynylated heterocycles amenable to trans-hydrometalation which are otherwise catalyst poisons.

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