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Palladium-catalyzed C-P bond formation: Mechanistic studies on the ligand substitution and the reductive elimination. An intramolecular catalysis by the acetate group in PdII complexes
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
2008 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 27, no 22, 5876-5888 p.Article in journal (Refereed) Published
Abstract [en]

Ligand substitution and reductive elimination of the palladium-catalyzed C−P bond forming cross-coupling were investigated in depth. It was found that for PhPdII(PPh3)2X (X = I, Br, Cl) complexes, a step commonly referred to as ligand substitution commenced with coordination of an H-phosphonate diester, followed by its deprotonation to form an equilibrium mixture of penta- and tetracoordinate palladiumphosphonate intermediates, from which reductive elimination of the product (diethyl phenylphosphonate) occurred. For the acetate counterpart, PhPdII(PPh3)2(OAc), the incorporation of a phosphonate moiety to the complex was preceded by a rate-determining removal of the supporting phosphine ligand, facilitated by an intramolecular catalysis by the acetate group. Both the reaction steps, i.e., formation of palladiumphosphonate intermediates and reductive elimination, were significantly faster for the acetate versus halides containing PdII complexes investigated. Similar observations were found to be true also for bidentate ligand complexes [(dppp)PdII(Ph)X]; however, in this instance, a single palladiumphosphonate intermediate, (dppp)PdII(Ph)(PO(OEt)2), could be observed by 31P NMR spectroscopy. The synthetic and kinetic studies on the cross-coupling reaction of diethyl H-phosphonate with phenyl halides permitted us to elucidate a crucial catalytic role of an acetate group in PdII complexes and to propose two distinctive catalytic cycles, which complemented traditional Pd0/PdII schemes, for the palladium-mediated C−P bond formation.

Place, publisher, year, edition, pages
2008. Vol. 27, no 22, 5876-5888 p.
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-17638DOI: 10.1021/om800641nISI: 000260791400017OAI: oai:DiVA.org:su-17638DiVA: diva2:184159
Available from: 2009-01-19 Created: 2009-01-19 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Synthesis of C(sp2)-P bonds by palladium-catalyzed reactions: Mechanistic investigations and synthetic studies
Open this publication in new window or tab >>Synthesis of C(sp2)-P bonds by palladium-catalyzed reactions: Mechanistic investigations and synthetic studies
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on synthetic and mechanistic aspects of palladium-catalyzed C(sp2)-P bond-forming reactions, with the aim to develop mild and efficient methods for the synthesis of biologically active phosphorus compounds, e.g. DNA analogs.

The first part of the thesis is devoted to detailed mechanistic investigations of the palladium-catalyzed C-P cross-coupling reaction, in order to fully understand the underlying chemistry and by rational design of the reaction conditions, improve the overall efficiency of the process and broaden its applicability. In particular influence of palladium coordination by different anions on the rate of ligand substitution and reductive elimination steps of the reaction was studied. It was found that coordination of acetate ion results in unprecedented acceleration of both of the mechanistic steps, what leads to remarkable shortening of the overall reaction times. In-depth kinetic investigations enabled to ascribe the observed effects to ability of the acetate ion to act as a bidentate ligand for palladium. This causes considerable alternation of the reaction mechanism, comparing to the reaction involving halide-containing complexes, and results in significant rate increase.

Based on the above mechanistic studies an efficient method for the synthesis of arylphosphonates, using substoichiometric amounts of inorganic acetate additive and reduced amount of catalyst, was developed.

In the next part of the thesis, efforts to further enhance the palladium-catalyzed cross-coupling efficiency by using a microwave-assisted synthesis are described. These explorations resulted in a successful development of two protocols, one for a cross-coupling of H-phosphonates and the other for H,H-phosphinates, under the microwave heating conditions. Application of this energy source resulted in extremely short reaction times, measured in minutes.

The final chapter of this thesis deals with studies on palladium-catalyzed SN2’ propargylic substitution reaction with phosphorus nucleophiles, which leads to allene products. Efficient procedure for the synthesis of allenylphosphonates and related compounds was developed. The method enables full control of stereochemistry in the allene moiety and at the asymmetric phosphorus center. Some conclusions on the mechanism of the reaction were also drawn.

Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University, 2011. 95 p.
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-56467 (URN)978-91-7447-298-1 (ISBN)
Public defence
2011-06-09, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 7: In press.Available from: 2011-05-12 Created: 2011-04-18 Last updated: 2011-05-13Bibliographically approved

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