Palladium complexes monophosphine ligands

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

A catalytic asymmetric [4+2]-cydoaddition of a vinylallene with butadiene has been achieved successfully, in which a palladium complex modified by a ferrocene-derived chiral monophosphine ligand proved to be a superior catalyst transferring chirality to the product (Scheme 16.80) [90],... 

The Beller group has contributed to the development of effective ancillary ligands for use in the palladium-catalyzed monoarylation of ammonia. In a pair of publications it was demonstrated that appropriately constructed imidazole-derived monophosphine ligands (including L8) are capable of supporting active complexes for the monoarylation of ammonia, albeit under somewhat forcing conditions (>120°C lObarN ). 

Asymmetric hydrosilylations of terminal alkenes, 1-arylalkenes, norbomenes and dihydrofurans with HSiCIj have been successfully performed by Hayashi and coworkers [914, 915, 916, 1340, 1341]. These reactions take place at 40°C when catalyzed by chiral palladium complexes, and the most efficient ligand is monophosphine 3.51 (R = Me) (Figure 7.19). The regioselectivity of the hydrosilylation of terminal olefins is opposite to that usually observed after treatment with H2O2/KF, secondary alcohols are obtained as major products [752, 855, 1340], The regioisomeric primary alcohols are typically formed in only about 10% yield in these reactions. 

B. MECHANISM OF THE CROSS-COUPLING CATALYZED BY PALLADIUM(O) COMPLEXES LIGATED BY MONOPHOSPHINE LIGANDS (L)... 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

Palladium complexes also catalyze hydrosilylation, and particular emphasis has been placed on the use palladium catalysts for asymmetric hydrosilylation. The most selective of these catalysts contains a binaphthyl monophosphine ligand. - Finally, lanthanides have also been used for hydrosilylation. Lanthanide-metallocene catalysts can be highly active for the hydrosilylation of olefins, and lanthanides bearing chiral ligands catalyze asymmetric hydrosilylation with measurable enantiomeric excess. ... 

Thus, most of such catalytic systems designed to improve efficiency in the case of the cross-coupling of C(sp )-organozincs with C(sp )-organic halides are similarly applicable with primary alkylzincs. These include the above-mentioned palladium complexes of PtBuj [188, 189, 233-236] biphenyl-based monophosphine ligands of the SPhos (148) family [131-133] or the use of PEPPSI precatalysts [136-138, 237, 238]. 

Palladium complexes of monophosphine hgands such as PCyj or NHC ligands (PEPPSI-IPr catalyst 70) are efficient catalysts for the more challenging alkylations... 

The main steps of the mechanism proposed by Heck have been further on confirmed. New ligands (diphosphines, carbenes, bulky monophosphines, polyphosphines) and new precatalysts (P,C-palladacycles) were introduced all along the last 50 years. Mechanistic investigations revealed that depending on the experimental conditions, the catalytic cycle may involve intermediate palladium complexes whose structure differs from the original ones proposed by Heck. 

Various palladium(O) monophosphine complexes of 1,6-diene have been prepared from tmedaPd(CHj)2, PRj, and the corresponding 1,6-dienes. These molecularly defined Pd complexes catalyzed the Suzuki coupling of aryl chlorides with phenyl-boronic acid more efficiently than traditional Pd(Il)-PRj pre-catalysts. Best results were achieved with the 1,6-diene complex containing Buchwald s ligand (26). With only 0.05 mol% of the catalyst, good to excellent yields of biaryls were obtained from activated (2-chlorobenzonitrile, 97%), non-activated (4-chlorotoluene, 82%, chlorobenzene, 87%) and deactivated (4-chloroanisole, 72%) aryl chlorides (Equation 54) [66]. 

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