Monophosphine ligands formation

Formation of carboxylic acids from ethylene, isobutylene, and 1-octene was observed by in situ 13C solid-state MAS NMR over H-ZSM-5 zeolite at 23-100°C.298 A systematic study with various Pd complexes revealed that styrene is transformed into 2-phenylpropionic acid as the major product when monophosphine ligands were applied, whereas 3-phenylpropionic acid was obtained in the presence of diphosphines 299... 

Significantly better results in addition of non-stabilized nucleophiles have come from hydrogenolysis reactions using formate as a hydride donor as shown in Scheme 8E.46. The racemic cyclic acetate and prochiral linear carbonates were reduced in good enantioselectivities by monophosphine ligands (/ )-MOP (16) and (Zf)-MOP-phen (17), respectively [195]. The chirality of the allylsilane can be efficiently transferred to the carbinol center of the homoallylic alcohol by the subsequent Lewis acid catalyzed carbonyl addition reaction 1196], The analogous... 

In 1974, a mechanism was proposed by Dieck and Heck [8] for reactions catalysed by Pd(OAc)2 associated with monophosphine ligands. Such a mechanism written by Heck as successive reactions [la—c] is presented as the catalytic cycle in Scheme 1.8. After formation of a Pd(0) catalyst from the precursor Pd(OAc)2 by a vaguely defined reduction process, the following steps of the catalytic cycle were proposed ... 

A new Pd(0) -catalyzed carboiodination reaction of alkenes with aryl iodides, which generates a C-C and a C-I bond, involves a rate-determining reductive elimination step to form the C(sp ) - I bond, which is facilitated by bulky monophosphine ligands by preventing the formation of tetracoordinated intermediates [365]. 

In 1983, Migita and coworkers reported the first palladium-catalyzed C—N coupling of organotin derivatives as nitrogen nucleophile using P(o-tol)j as ligand (Scheme 20.6) [6]. The limitation of this method is the formation of low amount of desired anilines over the (3-hydride-eliminated byproduct arene. This could be attributed to the labile nature of the bulky monophosphine ligand. 

Intermolecular C—O bond formation has been less well investigated in the synthesis of benzofurans. However, Anderson et al. reported a route involving such a step using o-chloroalkyne substrates such as 58 (Scheme 24.29, disconnection D-3) [118]. These substrates have been used in the synthesis of indoles (Scheme 24.5), and indazoles (Scheme 24.19), and in this case a combination of potassium hydroxide and a palladium catalyst derived from the bulky monophosphine ligand t-butyl XPhos 59 led to benzofuran derivatives (Scheme 24.32). Zhao et al. went on to develop a copper-catalyzed version of this process utilizing o-iodoalkynes such as 60 [119]. 

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... 

The formation of alkoxo intermediates may be occurring when monophosphines are used, but the stability of the amine complexes favors the deprotonation of coordinated amine. Instead, the alkoxo complexes may be important in catalytic systems involving chelating ligands [51]. Indeed, the DPPF complex [Pd(DPPF)(p-Bu C6H4)(0-f-Bu) reacted with diphenyl amine, aniline, or piperidine, as shown in Eq. (48), to give the product of amine arylation in high yields in each case [51]. Since, no alkali metal is present in this stoichiometric reaction, the palladium amide is formed by a mechanism that cannot involve external deprotonation by alkali metal base. 

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