Palladium bulky substituents

The new arsino(phosphino)methanes with bulky substituents at the two donor centers can behave both as chelating and bridging ligands toward palladium(II). Besides neutral and mono- as well as di-nuclear cationic compounds, in which these ligands are bonded in a chelating fashion, a di-nuclear complex of the A-frame type could also be generated (see Scheme 5).396... 

The structure catalyst prepared using this ligand is shown in Fig. 11 and suggests that the high molecular weights can be attributed to the bulky substituent blocking at least one of the axial positions on the palladium center. 

In 2001, a palladium-catalyzed asymmetric hydrosilylation of 4-substituted-but-l-en-3-ynes (146) was reported by Hayashi and co-workers [115]. It was found that a monodentate bulky chiral phosphine, (S)-(R)-bisPPFOMe, was effective for the asymmetric synthesis of the axially chiral allenes 147 and up to 90% ee was achieved (Scheme 3.75). The bulky substituent at the 4-position in 146 is essential for the selective formation of the allene 147 the reaction of nC6H13C=CCH=CH2 gave a complex mixture of hydrosilylation products which consisted of <20% of the allenylsilane. 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydro-naphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8. 

The Suzuki-Miyaura reactions with relatively inert arylchloride are known to require palladium complexes possessing highly electron-rich ligands which favor the oxidative addition of the arylchloride into Pd(0)-complex (Scheme 11) [67-69]. Herrmann et al. showed that the utilization of NHC ligands with bulky substituents... 

The palladium catalysed ring closure of the A-(2 -bromobcnzyl)-naphthylamine derivative shown in 5.18. could in principle occur either in the 2-position or in the 8-position of the naphthalene ring giving rise to the formation of a six or a seven membered ring respectively. Surprisingly the latter case is favoured in most cases, unless there is a bulky substituent in position 7 of the naphthalene ring. This finding, where preference of the observed reaction path is explained by the intramolecular coordination of the benzylamino function to the palladium, has been successfully exploited in the preparation of a series of naphthobenzazepines.17... 

Since its discovery by Tsuji [15,16] and catalytic expansion by Hata [17] and Atkins [18], allylic substitution has become the most popular palladium-catalyzed method for carbon-carbon bond formation along with crosscoupling reactions. However, the first report using NHC in this transformation only appeared recently [19]. An imidazolium salt with a bulky substituent on the nitrogen atoms, IPr HC1, was found to be a suitable ligand for allylic substitution with soft nucleophiles (Scheme 2). Pd2(dba)3 as palladium source and Cs2C03 as base completed the catalyst system. 

Palladium-catalyzed hydrostannylation is stereoselective, giving syn-adduct. Regioselectivity is not always perfect, however. Exclusive formation of (-B)-l-stannylalkene is obtained only when 1-alkyne with a bulky substituent is employed (Scheme 34) [147-150]. Nicolaou also reported a stereo- and regio-selective hydrostannylation (Scheme 35) [151]. Use of a combination of tri-butyltin chloride and poly(methylhydro)siloxane in the presence of TBAF is also an impressive method in this class [152]. 

One of the more extraordinary recent developments in nickel and palladium polyalkene catalysis has been the development of a-diimines with bulky substituents as ligands in nickel and palladium complexes. When bulky aryl groups are used (R = isopropyl), these catalysts polymerize ethylene with high activities to high molecular weight highly branched... 

Enantioselective intramolecular bis-silylation was accomplished by use of the optically active re rt-alkyl isonitrile ligand 75 on palladium (Eq.38) [71]. The bulky substituents of 76 on the silicon atom proximal to the ether oxygen are crucial to attain good enantioselectivity. 

In 2006, Ackermann and coworkers pointed out that bulky substituents on the nitrogen atoms of secondary chlorophosphanes facilitate the efficient palladium-catalyzed a-arylation of ketones bearing sp -hybridized C-H bonds in the a-position with electron-rich aryl chlorides (Scheme 8.23). The active catalyst for these reactions was generated from Pd(dba)2 and the bulky sterically hindered ligand shown in Scheme 8.23 [55]. 

In 2008, Shimizu and coworkers developed a novel approach of palladium-catalyzed intramolecular coupling of readily available 2-(arylsilyl)aryl triflates (Scheme 4.32) [47]. The key to this reaction was the installation of bulky substituents on silicon and the use of Et2NH as a base. Symmetrical and asymmetrical 9-silafluorenes, substituted with electron-donating and/or electron-withdrawing groups, such as NMc2, OMe, CFg, and CN, were synthesized from the... 

It is obvious that the chirality will be eroded if the p-hydride elimination occurs at the chiral a-position of tosyUiydrazones. The selectivity of p-hydride elimination in this reaction can be interpreted as follows. Alkylpalladium complex, the intermediate generated after migratory insertion of palladium carbene, is favorable to afford 1,1-disubstituted chiral olefin (Fig. 18, path a) because of the less steric interactions in the transition state at p-hydride elimination. Apparently the alternative syn p-hydride elimination will lead to the erosion of chiral center. However, the latter pathway is not preferred because it leads to the eclipse of the bulky substituent with the methyl group of the substrate (Fig. 17, path b). 

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