Preparative and mechanistic aspects of palladium-catalyzed cross-coupling reactions of alkali-metal arylsilanolates

Smith, Russell C.

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https://hdl.handle.net/2142/16830 Copy

Description

Title

Preparative and mechanistic aspects of palladium-catalyzed cross-coupling reactions of alkali-metal arylsilanolates

Author(s)

Smith, Russell C.

Issue Date

2010-08-20T17:59:12Z

Director of Research (if dissertation) or Advisor (if thesis)

Denmark, Scott E.

Doctoral Committee Chair(s)

Denmark, Scott E.

Committee Member(s)

• Hartwig, John F.
• Hergenrother, Paul J.
• Girolami, Gregory S.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• arylsilanolates
• biaryls
• mechanism
• palladium

Abstract

The development of alkali-metal arylsilanolates as notable organometallic reagents for the preparation of biaryls has been realized through in depth mechanistic and preparative studies. The alkali-metal salts (potassium and sodium) of a large number of arylsilanols undergo smooth cross-coupling with a wide range of aromatic bromides and chlorides. The critical feature for the success of these coupling reactions and their considerable scope is the use of bis(tri-tert-butylphosphine)palladium. Additionally, these studies have identified the use of phosphine oxides (e.g. triphenylphosphine oxide) as useful ligands for palladium-catalyzed cross-coupling reactions. This serendipitous discovery was founded on kinetic experiments which revealed a significant rate enhancement employing these species as ligands for palladium. Later, these ligands were implemented in the reaction of arylsilanolates for the preparation of biaryl products. The mechanism of palladium-catalyzed cross-coupling reactions of alkali-metal organosilanolates has also been investigated. Under catalysis by (t-Bu3P)2Pd, the coupling with aryl bromides displays the following rate equation: rate = kobs[R3SiOK]0[ArylBr]0, with kobs = k[(t-Bu3P)2Pd]0.98. An independent study of the individual steps of the catalytic cycle has revealed a dual mechanistic pathway. The transmetalation can occur by a thermal process via an 8-Si-4 intermediate without the need for anionic activation. Additionally, arylsilanolates can serve as activators for transmetalation via a hypervalent 10-Si-5 siliconate intermediate. The independent isolation of the putative arylpalladium(II) arylsilanolate intermediate has confirmed the generation of the Pd-O-Si linkage prior to transmetalation. The unique opportunity to isolate a pretransmetalation intermediate has allowed the transmetalation step to be further refined through a detailed Hammett analysis which has demonstrated that increased electron-density about the arylsilanolate nucleophile increases the rate of coupling. Furthermore, a number of alkenyl- and arylsilanolate pretransmetalation species have been isolated with various ligands in attempts to identify the critical features of the cross-coupling of this class of organonucleophiles.

Graduation Semester

2010-08

Permalink

http://hdl.handle.net/2142/16830

Copyright and License Information

Copyright 2010 Russell C. Smith

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