Heteroatom-substituted secondary phosphine oxide ligands

Nickel catalysis has also been used in the formation of biaryls, such as (51), by substitution of the methoxy group in 1-methoxynaphthalene by tolylmagnesium bromide. It is also reported that the reaction of aryl or heteroaryl tosylates with phenylmag-nesium bromide to give biaryl derivatives is catalysed by palladium complexed with heteroatom-substituted secondary phosphine oxide ligands. 

Ackermann HASPO Ligands Ackermann presented two new ligand families, named heteroatom-substituted secondary phosphine oxides (HASPO) and the corresponding phosphine chlorides. The development of the former ligand class is related to studies of Li (DuPont), who described the use of dialfeylphosphine oxides with electronically distinct properties as active and easily accessible ligands for palladium-catalyzed aminations (Figure 3.10) [107]. 

Air-stable sterically congested phosphine oxides such as 48 are also excellent Ug-ands for the nickel-catalyzed cross-coupling of aryl fluorides [43] (Scheme 5.8). The association of nickel salts with these so-caUed heteroatom-substituted secondary phosphine oxide (HASPO) ligands leads to species reactive enough to activate the generally inert arene-fluorine bond (BDE Ph-F = 126kcalmol ). This improved reactivity has been explained by the formation of a bimetallic species 52 (Scheme 5.8), which facilitates the oxidative insertion step that proceeds via transition state TSl (Figure 5.1) [44]. 

Replacement of one C-substituent in phosphines by an alkoxy or aryloxy group produces esters of phosphinous acid (phosphinites). Further substitution of alkyl or aryl by oxy groups gives first the diesters of phosphonous acid (phosphonites) and finally triesters of phosphorous acid (phosphites). Secondary phosphine oxides (SPOs) or heteroatom-substituted phosphine oxides (HASPOs), which are derived from the corresponding free acids by tautomerism, have been only occasionally investigated as ligands. However, they play a pivotal role as hydrolysis products of esters. In this role, they may exert an impact on the catalytic reaction. 

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