Introduction
The catalyst bis(tri-tert-butylphosphine)palladium(0) [Pd(t-Bu3P)2, 1, CAS: 53199-31-8] is a colorless, air-sensitive solid. It must be manipulated in a glove box or under inert gas. [Pd(t-Bu3P)2] (1) contains bulky, electron-rich tertiary phosphine ligands [t-Bu3P]. In a palladium-catalyzed cross-coupling reaction, they promote the oxidative addition as they can stabilize higher oxidation states. Reductive elimination is also facilitated because of the bulky ligands. Thus 1 has been shown to be superior in transition-metal-catalyzed cross-coupling reactions compared to the classical [Pd(Ph3P)4] catalyst. [Pd(t-Bu3P)2] is not only efficient for typical cross-coupling reactions, such as Stille, Negishi, Suzuki, Heck, Sonogashira, or Buchwald–Hartwig aminations, with electrophiles R-X (X = Cl, Br, I, OTf, SO2Cl and others), but also for cross-coupling of organolithium reagents,[1] alkenylgermanes,[2] alkali-metal silanolates,[3] triorgano-indium reagents[4] and others. Moreover, it has been used for arylations of hydro-siloxanes,[5] decarboxylative cross-coupling reactions,[6] carbonylations and amino-carbonylations,[7] carboiodinations,[8] C-H functionalizations,[9] cyanations,[10] methylenation of olefins[11] and annulation reactions.[12] In recent years, 1 has become one of the best new-generation catalysts and plays an important role in organic synthesis.
[Pd(t-Bu3P)2] is commercially available and can also be prepared by treating [Pd(η5-C5H5)(η3-C3H5)] with the ligand [t-Bu3P] in n-hexane at room temperature for 3 h.[13] The pale red crude product can be recrystallized from n-hexane at –20 °C to give pure colorless crystals.
Figure 1Bis(tri-tert-butylphosphine)palladium
Table 1 The Use of Bis(tri-tert-butylphosphine)palladium(0) [Pd(t-Bu3P)2]
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[Pd(t-Bu3P)2]-Catalyzed Cross-Coupling of Organolithium Reagents
Feringa and coworkers reported [Pd(t-Bu3P)2]-catalyzed cross-coupling reactions between alkyllithium reagents and a variety of aryl- and alkenylbromides under mild conditions.[1] Those cross-coupling reactions are highly selective, avoiding lithium–halogen exchange and homocoupling side reactions. The authors also extended the cross-coupling reactions to (hetero)aryllithium reagents by using the in situ prepared catalyst [Pd2(dba)3] and [t-Bu3P] as ligand.
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[Pd(t-Bu3P)2]-Catalyzed Cross-Coupling of Alkali-Metal Silanolates
A broadly applicable protocol for the [Pd(t-Bu3P)2]-catalyzed cross-coupling of a wide range of alkali metal arylsilanolates with various aryl halides was developed.[3] This method also applied to the cross-coupling of heteroarylsilanolates.
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[Pd(t-Bu3P)2]-Catalyzed Arylation of Hydrosiloxanes
Symmetrical and unsymmetrical siloxanes were synthesized by [Pd(t-Bu3P)2]-catalyzed arylation of hydrosiloxanes.[5] This method was a one-pot process and showed high functional group tolerance. It was also exploited to perform triple arylations.
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[Pd(t-Bu3P)2]-Catalyzed Decarboxylative Cross-Coupling Reaction
Forgione, and Biloudeau and coworkers developed a procedure for highly selective Pd-catalyzed decarboxylative cross-coupling reactions between heteroaromatic carboxylic acids and various aryl halides in the presence of a reactive C-H group.[6] This process provides a valuable alternative for other cross-coupling reactions, in cases where appropriate cross-coupling partners are not commercially available and hard to be synthesized.
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[Pd(t-Bu3P)2]-Catalyzed Carbonylation and Aminocarbonylation
Traditional methods to synthesize acid chloride involve toxic reagents, such as PCl3, thionyl chloride and oxalyl chloride. Quesnel and Arndtsen described a new method to construct acid chlorides via the [Pd(t-Bu3P)2]-catalyzed carbonylation of aryl iodides under mild conditions.[7] The decisive step of the process was reductive elimination of [(t-Bu3P)(CO)Pd(COAr)Cl], which was facilitated by the combination of the bulky, electron-rich [t-Bu3P], the phosphine chloride and CO coordination. This method was exploited to perform traditional aminocarbonylation of aryl iodides under exceptionally mild conditions (ambient temperature and pressure).
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[Pd(t-Bu3P)2]-Catalyzed Carboiodination
Various functionalized chromans and isochromans were prepared via the intramolecular [Pd(t-Bu3P)2]-catalyzed carboiodination of alkenyl aryl iodides in the presence of an amine base Et3N.[8] Those cyclizations had a broad functional group tolerance and showed high diastereo-selectivities, which was thought to originate from the minimization of axial–axial interactions in the carbopalladation step.
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[Pd(t-Bu3P)2]-Catalyzed C-H Functionalization
Tamba and coworkers described a facile [Pd(t-Bu3P)2]-catalyzed C-H arylation of heteroarene compounds with aryl bromides and aryl chlorides in the presence of LiOt-Bu as a base.[9]
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