Arylphosphinic halides —

Complexes ligated by alkylphosphines had been used rarely as catalysts in cross-coupling chemistry, but several studies suggested that they could catalyze the amination of aryl halides with higher selectivity and activity than catalysts of arylphosphines. Steric hindrance promotes reductive elimination at the expense of /3-hydrogen elimination.54 Therefore, reactions of primary amines and, in... 

In addition, complexes of P(/-Bu)3 have been shown to catalyze the formation of diaryl heteroarylamines from bromothiophenes.224 Aminations of five-membered heterocyclic halides such as furans and thiophenes are limited because their electron-rich character makes oxidative addition of the heteroaryl halide and reductive elimination of amine slower than it is for simple aryl halides. Reactions of diarylamines with 3-bromothiophenes occurred in higher yields than did reactions of 2-bromothiophene, but reactions of substituted bromothiophenes occurred in more variable yields. The yields for reactions of these substrates in the presence of catalysts bearing P(/-Bu)3 as ligand were much higher than those in the presence of catalysts ligated by arylphosphines. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p-dibromobenzene in yields of 70-80% (isolated after oxidation, as the phosphine oxides).143 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the 5rn1 reaction, had the mechanism been recognized at the time. Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett,48 but no extension to the scope of the reaction was made. Rossi and coworkers have developed a procedure for one-pot preparation of triarylphosphines starting from elemental phosphorus (Scheme 6).146 As an example of the synthesis of a symmetrical tri-arylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i-iii only). Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (AiOC) were used. 

In general, alkylphosphines have been used less often than arylphosphines in crosscoupling chemistry. However, several studies pointed to the potential of such ligands in palladium-catalyzed animation of aryl halides. Most remarkable is the milder temperatures for animation of aryl bromides, improved yields with acyclic secondary amines, high turnover numbers for these reactions, and the ability to conduct mild animations of inexpensive aryl chlorides and tosylates. 

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

The palladium-catalysed cross-coupling of aryl halides or vinyl halides with dialkyl phosphonates (31) to yield dialkyl arylphosphonates and dialkyl vinylphosphonates, respectively, was first reported by Hirao and co-workers 69 the halides used most frequently are bromides and the reaction is stereospecific with haloalkenes. Subsequently, analogous reactions of alkyl alkylphosphinates (32), alkyl arylphosphinates (32), alkyl phosphinates (33), and secondary phosphine oxides (34), replacing [P—H] bonds with [P—C] bonds to yield various phosphinates and tertiary phosphine oxides, have been developed (Figure 7.1). Alkyl phosphinates (33) may be mono- or diarylated as desired by the selection of appropriate conditions. Aiyl and vinyl triflates have also found limited... 

Alkyl- or arylphosphinic acids form from reaction of HjO with dialkylphos-phinous halides at 100°C ... 

Methods for the synthesis of C-functionalised arylphosphines based on the direet introduetion of phosphino groups into aryl halides or tosylates, eatalysed by a variety of metals, have eontinued to develop. The reaetions of seeondary phosphines (and seeondary phosphine oxides) with bromo- or iodo-arenes, eatalysed by palladium aeetate or other palladium complexes, have been used... 

Organophosphorus compounds find wide use in the chemical industry as catalysts, intermediates, complexes, and end-use products. Arylphosphines and phosphine oxides are often produced by the reaction of a preformed Grignard reagent with a halophosphine or phosphine oxide. Yields are reduced by the production of unwanted side-reaction products such as biaryls. These unwanted products are reduced when the reaction is conducted under Barbier conditions. When alkyl and aryl halides are reacted with magnesium metal, a trihalophosphine or phosphine oxide, a metal halide or amine catalyst, in THE benzene mixtures, at reflux, good yields of phosphines or phosphine oxides are obtained [74]. For example, triphenylphosphine can be prepared in a 97.2% yield from the reaction of bromobenzene, trichlorophosphine, magnesium metal, aluminum chloride, and sodium chloride in THF-benzene at 70 80 C. 

Recently, extensive synthetic studies on multiply functionalized chiral arylphosphines by Pd-catalyzed arylation of primary and secondary phosphines with aryl halides have been carried out by Stelzer [2]. The synthetic methods can be summarized by the following general schemes. 

A general and efficient method for the microwave-assisted synthesis of mono- and diarylphosphinic acids (579) and (581) from anihnium 7f-phosphinate (580) and aryl halides, using Pd(0) and Xantphos as a supporting ligand, has been developed by Stawihski and Kalek (Scheme A2)P The procedure was highly efficient and provided a rapid access to a broad spectrum of arylphosphinate derivatives. 

The overall mechanism for Pd-catalyzed amination of aryl halides is shown in Scheme 5. Initially, a Pd (0) complex is rapidly formed from Pd(OAc)2 and phosphine ligand in the presence of amine and base. If Pd(dba>2 is used, then either bisphosphine Pd(0) complexes are formed, as with P(t-Bu)3,t or mixed phosphine/dba Pd(0) complexes are formed as with arylphosphines.f In some cases, dba appears to be consumed under the reaction conditions, and simple bis-ligand Pd(0) complexes are formed. The mechanism for reduction of Pd(II) to Pd(0) has been studied," but the process occurs more rapidly in the presence of amine and base than in the absence of these reagents, even when the amine cannot undergo /3-hydrogen elimination. 

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