Pyridinyl catalysts

The feasibility of bonding pyridinyl groups to silicon which contains a hydrolytically sensitive functional group has recently been demonstrated 15-71. 2-Fluoro-3-(dimethylchlorosilyl)pyridine and 3-fluoro-4-(dimethylchloiosilyl)pyridine as well as 2-, 3-, and 4-(dimethylchlorosilyl)pyridine were prepared by the reaction of the corresponding lithiopyridines with excess Me2SiCl2- Hydrolysis of the pyridinyl substituted chlorosilanes gave disiloxanes which were insoluble in water. In the present report we will describe extension of this work to include pyridinyl dichlorosilanes which can be hydrolyzed to polysiloxanes. These polymers can be N-oxidized and the resultant derivatives have been shown to be effective hydrophobic transacylation catalysts. 

Similar results were reported by the Barret group by using stoichiometric amounts of an enantiopure 2-(2-pyridinyl)-2-oxazoline [46], hi 1996, Iseki and Kobayashi achieved a catalytic version of the asymmetric allylation [47], They applied proline-based chiral HMPA derivatives for the allylation. The catalyst 21 proved to be the best one regarding catalyst loading down to 1 mol% (Scheme 16) [48],... 

Widenhoefer and co-workers have developed an effective Pd-catalyzed protocol for the asymmetric cyclization/ hydrosilylation of functionalized 1,6-dienes that employed chiral, non-racemic pyridine-oxazoline ligands." " " Optimization studies probed the effect of both the G(4) substituent of the pyridine-oxazoline ligand (Table 7, entries 1-6) and the nature of the silane (Table 7, entries 6-15) on the yield and enantioselectivity of the cyclization/ hydrosilylation of dimethyl diallylmalonate. These studies revealed that employment of isopropyl-substituted catalyst (N-N)Pd(Me)Gl [N-N = (i )-( )-4-isopropyl-2-(2-pyridinyl)-2-oxazoline] [(i )-43f and a stoichiometric amount of benzhydryldimethylsilane provided the best combination of asymmetric induction and chemical yield, giving the corresponding silylated cyclopentane in 98% yield as a single diastereomer with 93% ee (Table 7, entry 15). 

Comparable levels of selectivity (s = 7.6-24) can also be achieved for the KR of a similar range of aryl alkyl sec-alcohols using Yamada s reasonably readily accessed chiral thiazolidine-2-thione-based DMAP 19 (Fig. 8.4) at between 0 °C and rt over 3 to 12 h [100]. The N-(4/-pyridinyl)-a-methylproline derived catalysts 16, and the solid-supported variant 17 (Fig. 8.4) developed by Campbell [95-98]... 

H-bond interactions between the catalyst and substrate were suggested by Campbell to explain why his N-(4/-pyridinyl)-a-methylproline-derived catalysts 16 and 17 (Fig. 8.4) are more enantioselective for N-acylated 1,2-amino alcohols (s = 9-18.8) than for other classes of substrate [95-98]. Recently, Ishihara has designed the histidine derivative 34 as a minimal artificial acylase for the KR of mono-protected cis- 1,2-diols and N-acylated 1,2-amino alcohols [135]. This catalyst incorporates a sulfonamide linkage specifically to allow the NH group to engage as a H-bond donor with the substrates, and gives impressive levels of selectivity with a range of appropriate substrates (Scheme 8.16). 

The pyridinyl- and 1-oxypyridinyl-substituted silanes and siloxanes were patented as IPTC catalysts in transacylation reactions [178]. In the IPTC nucleophilic substitution reaction of benzoyl chloride with KSCN catalyzed by cyclic and acyclic sulfides such as tetrahydrothiophene and diethyl sulfides, etc., the active ionic intermediate, benzylsulfo-nium ion, formed by benzyl chloride and sulfide in the organic phase, transferred into the aqueous phase to react with thiocyanate ion to produce benzylthiocyanate [179]. In the following discussion, selected IPTC systems are presented, focusing on the kinetic and mechanistic aspects. 

A highly active palladium catalyst for Suzuki coupling reactions can be generated from palladium diacetate and 2-(di-tert-butylphosphino)biphenyl. Potassium fluoride is the preferred base for this system. The coupling of both bromides and chlorides proceeds at room temperature in excellent yields as exemplified by the preparation of 3-phenylthio-phene 46. Cross-coupling between 2-bromothiophene and diethyl(3-pyridmyl)borane gives a pyridinyl 2-substituted thiopene 47. ... 

A bench-scale process was reported by Merck [114] for the hydrogenation of an ot,p-unsaturated ester (Fig. 17) for the synthesis of a D2 (DP) receptor antagonist and operated on a 5-kg scale. The potential of Ir/P N catalysts was demonstrated in a feasibility study by DSM in collaboration with Pfaltz [115] for the hydrogenation of y-tocotrienyl carried out with the pyridinyl phosphinite depicted in Fig. 16. The hydrogenation of the two prochiral C=C bonds occurs with excellent stereoselectivity to give almost exclusively the (R,R,R) product. The existing stereogenic center does not influence the reaction. 

Indole carboxylic acid 187 was converted via a Barton ester to fused indole 194 (11 examples, 5-79% yield). Barton ester 189 was formed by treatment of indole 187 with S-(l-oxido-2-pyridinyl)-l,l,3,3-tetramethyl thiouronium hexafluorophosphate (188, Garner s HOTT reagent) in the absence of light. Upon refluxing in MeCN, the Barton ester 189 decomposes to give nucleophilic ethyl radical 190, which adds to the unsubstituted carbon of alkyne 191 furnishing vinyl radical 192.This species cyclizes onto the C2 position of the indole to provide 193, which aromatizes to dehver the final product 194. The sequence proceeds without the need for an initiator or metal catalyst (14JOC5903). 

Tridentate bis(oxazolinyl)pyridinyl rhodium and ruthenium pincer complexes are useful as catalysts for hydrosilylations and cyclopropanations. These NNN-type inorganic pincer complexes are not as stable, however, as phosphine or salen-type pincer complexes. On the other hand, an organometallic tridentate bis(oxazolinyl) phenyl NCN-type complex is stable. These optically active NCN-type pincer complexes act as efficient catalysts for enantioselective hetero Diels-Alder reactions of Danishefsky s diene with glyoxylates [26]. 

A Rh(I) catalyst has likewise been found to promote cross-coupling between readily available benzoic acids as arylating agents and arenes bearing N-based directing groups such as pyridinyl, heteroaryl, and iminyl moieties (Scheme 22.34). The process involves the intermediacy of a mixed anhydride formed in situ by the reaction between the benzoic acid and (f-BuCO)jO [49]. 

Carbonylation. Pd(PhCN)2Cl2 can be readily used as the catalyst for the carbonylation of indolyl, pyridinyl, or quinolinyl halides with amines, alcohols, or even water under an atmosphere of CO (eq 1).2... 

Recently, the same type of arylation of benzylamine derivatives equipped with a 2-pyridinyl directing group was carried out with aryl halides but with the ruthenium(II) catalyst precursor ([RuCl2(p-cymene)]2) in the presence of potassium pivalate [19, 20] or directly with Ru(carboxylate)2(p-cymene) [21]. Both catalytic systems operate at 140°C for 24 h in toluene or o-xylene in the presence of 2.5-5 mol% catalyst loading and a carbonate as a base. The advantage of using the well-defined ruthenium(II)bis(carboxylate) complex is that it does not require an excess of carboxylate as it is the case with the in situ generated catalytic system. 

Phosphoramidation of the arene (5) C-H bonds with phosphotyl azides (6) has been realised in the presence of an iridium catalyst. This reaction was compatible with several directing groups such as pyridinyl, pyrazoyl, and quinolinyl, and provided AT-atyl phosphoramidates (7) in moderate to good yields (Scheme 3). ... 

A wide range of vicinal diols and polyols, including 1,2-diols, triols, and tetraols, were oxidized selectively at the secondary alcohol to afford a-hydroxy ketones, using chiral palladium complexes with pyridinyl oxazoline derived ligands as catalysts and benzoquinone as the terminal oxidant. The ligand geometry and environment have a significant influence... 

Scheme 30.9 Strecker reaction supported by pyridinyl-catalysts 39 and 40.

K. Denk, J. Fridgen and W. A. Herrmann. A-heterocyclic carbenes. Part 33. Combining stable NHC and chelating pyridinyl-alcoholato hgands A ruthenium catalyst for applications at elevated temperatures. Adv. Synth. Catal. 344, 2002, 666-670. 

M. Jordaan and H. C. M. Vosloo. Ruthenium catalyst with a chelating pyridinyl-alcoholato ligand for application in hnear aUcene metathesis. Adv. Synth. Catal. 349, 2007,184—192. 

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