Heteroatom-substituted secondary phosphine oxides

The use of aryl tosylates as electrophiles is attractive, since they can be synthesized from readily available phenols with less expensive reagents than those required for the preparation of the corresponding triflates. More importantly, tosylates are more stable towards hydrolysis than are triflates. However, this greater stability renders tosylates less reactive in transition metal-catalyzed coupling reactions. As a result, protocols for traditional cross-coupling reactions of these electrophiles were only recently developed [1], In contrast, catalytic direct arylations with aryl tosylates were not reported previously. However, a ruthenium complex derived from heteroatom substituted secondary phosphine oxide (HASPO) preligand 72 [81] allowed for direct arylations with both electron-deficient, as well... 

An alternative approach to ensure selective monoarylations of ketimines, including those without meta-substituents, was based on the development of a novel catalytic system. Significant progress was achieved with air-stable (heteroatom-substituted) secondary phosphine oxides (HA)SPOs, since these preligands gave rise to arylation reactions also with less reactive, yet inexpensive, aryl chlorides as electrophiles. Here, the sterically hindered derivative (l-Ad)2P(0)H (54) was found... 

Following Zhang and He s work, Ackermann and Barfuesser developed a protocol using a Pd-complex derived from air-stable heteroatom-substituted secondary phosphine oxides (HASPO) for the selective C3-arylation of a variety of functionalized NH-free indoles with bromoarenes (09SL808). Optimized conditions gave good-to-high yields and allowed the use of sterically hindered substrates (Scheme 32). 

Heteroatom-substituted secondary phosphine oxide (HASPO) preligands, such as H-phosphonates and their derivatives, display significantly different steric and electronic properties. These preligands (e.g. compound 12), as well as the corresponding phosphine chlorides, were found to be highly effective for Suzuki reactions of aryl chlorides (Equation 2.31) [53]. 

Ackermann has shown that palladium complexes derived from heteroatom-substituted secondary phosphine oxides are efficient for couplings of siloxanes and activated aryl chlorides (Equation 2.53) [53]. 

For a Stille coupling of 4-chloronitrobenzene using a heteroatom substituted secondary phosphine oxide, see Ackermann, L., Gsdirei, C.J., Althammer, A. and Riederer, M. (2006) Chem. Commun., 1419-21. 

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]. 

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. 

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]. 

SPO = secondary phosphine oxide HASPO = heteroatom substituted phosphine oxide... 

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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