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

arylsilanolates;biaryls;mechanism;palladium

Smith, Russell C.

关键词: arylsilanolates;    biaryls;    mechanism;    palladium;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16830/2_Smith_Russell.pdf?sequence=3&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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.