Phosphination aryl triflates

Tertiary phosphines. Aryl triflates react with diary Ichlorophosphines by... 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

Catalytic arsination has also been reported.226 The arsine analog of BINAP (BINAs) has been prepared from the reaction of diphenylarsine with the optically pure ditriflate that is generated from binaphthol. Triarylarsines themselves have been used like triphenyl phosphine, in this case as a reagent for the conversion of aryl triflates to mixed aryl diphenylarsines. 

In addition, aryl triflates have proven to be viable substrates for the Pd-catalyzed Csp2—P bond formation reactions [87-90], Intriguingly, phosphorylation can be achieved from the Pd-catalyzed coupling of alkenyl triflate with not only dialkylphosphites, but also with hypophosphorous acid [88]. Thus, phosphinic acid 87 was obtained when triflate 86 was treated with hypophosphorus acid in the presence of Pd(Ph3P)4. Due to the abundance of alkenyl triflates and milder reaction conditions, alkenyl triflates have certain advantages over the corresponding alkenyl halides as substrates for Pd-catalyzed phosphorylations to make alkenyl phosphonates or phosphinates. 

The coupling of aryl triflates with diphenylphosphine (432) catalysed by Ni and Pd complexes is an important synthetic method for optically active bidentate phosphines. Bis-substitution of optically active 2,2/-bis[(trifluoromethanesulfonyl)-oxy]-l,l/-binaphthyl (431) with Ph2PH (432) to give 433 has been achieved using NiCl2(dppe)... 

The initial step of the catalytic cycle is oxidative addition of aryl triflate to a BINAP-coordinated Pd(0) species. Since, in the actual catalytic system, Pd(OAc)2 and (/ )-BINAP are used as the precursors of the Pd(0) species, reduction of Pd(OAc)2 into the BINAP-coordinated Pd(0) species should be operative prior to the catalytic reaction. Although Pd(OAc)2 is the most commonly used precursor of a Pd(0) species in many palladium-catalyzed organic reactions, no direct information has been reported so far on its reduction process. In this study, we confirmed for the first time that the reduction proceeds according to the process involving a combination of tertiary phosphine (BINAP) and water as the reducing reagent (Scheme 8) (Ozawa, F. Kubo, A. Hayashi, T., submitted for publication). 

Palladium(O) complexes containing P(o-C6H4Me)3 as ligand show low reactivity toward aryl triflates [95,96]. Thus, the original catalyst is not effective for the ami-nation of aryl triflates. However, palladium complexes with the chelating phosphines DPPF and BINAP are effective [97,98]. Selected animations of aryl triflates by aniline are shown in Eq. (9), and selected animations of aryl triflates by alkylamines in Eq. (10). 

The phosphine-thiazole ligand 149 has been shown to promote asymmetric intermolecular Heck coupling of 2,3-dihydrofuran with aryl triflates <07ASC2595>. Microwave irradiation of a mixture of 2,3-dihydrofuran (6 equiv.), aryl triflates (1 equiv.) and DIPEA (3 equiv.) in THF in the presence of Pd2(dba)3 (3 mol%) and the ligand 149 (6 mol%) at 120 °C provides 2-aryl-2,5-dihydrofurans 150 in high enantioselectivity. Other phosphine-thiazole ligands have also been evaluated, and ligand 149 proves to be the best in terms of enantioselectivity. 

Catalysts based on bulky phosphine 6 effect the arylation of primary aliphatic amines with aryl triflates at room temperature, Eq. (118) [42a]. Substantial improvement in scope is observed when the reactions are performed at elevated temperatures with mild bases such as K3PO4 triflate cleavage is often less problematic under these conditions. 

Bulky phosphine 6 affords an extremely active catalyst which is capable of effecting the C-N bond formation reaction between aryl triflates and aniline derivatives at room temperature [42a]. However,better yields,less triflate cleavage, and a wider substrate scope are observed when the reactions are performed with K3PO4 at elevated temperatures, Eq. (125). 

Other miscellaneous syntheses that have benefited from the introduction of dppf to the catalytic mixture include the coupling reactions of aryl triflates or halides with ketene trimethylsilyl acetals [165], The catalyst mixture is [Pd(OAc)(i -C4H7)]2/LiOAc/phos-phine. In this case, nucleophilic activation of the Si-O bond of the acetal enables an Si-Pd transmetalation to give a a-complex 52 without necessitating a phosphine... 

Cyanation of aryl halides to form nitriles is generally robust unless catalyzed by Co, Ni or Pd complexes [167 —171], A useful catalytic mixture comprises NiBr2/Zn/ phosphine/KCN. The use of PPh3 is sufficient to achieve good conversion of chlorobenzene to benzonitrile. But for the more resistant o-dichlorobenzene and o-chlorobenzonitrile, dppf is reported to show better conversion and selectivity [172], The choice of phosphine is obviously crucial as P(o-Tol)3 and dppe are distinctly ineffective. The superiority of dppf over many other phosphines is remarkably evident in the cyanation of aryl triflates and halides when catalyzed by phosphine-doped Pd2(dba)3 [173]. 

Most of the Pd-catalyzed syntheses are carried out in the presence of phosphine although there are exceptions when this is not necessary. One such example is the coupling of aryl triflate with methyl a-acetamidoacrylate catalyzed by a mixture containing Pd(OAc)2/dppf/nBu3N/LiF (Scheme 1-24) [184]. A mixture containing Pd(0Ac)2/NaHC03/nBu4NCl is also catalytically active in most cases. 

Hu et al. [104] disclosed that the spiro phosphine oxide ligand (/ )-30a is highly efficient for the Pd-catalyzed Heck reaction of cyclic and heterocyclic olefins with aryl triflates. An asymmetric domino Heck cyclization was established using catalyst Pd/(/ )-30a to form fused carbo- and heterocycles with excellent stereoselectivity (Scheme 42) [105]. This method was used in a short synthesis of (-)-martinellic acid, a primary ingredient in traditional eye medicine in South America. 

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