Palladium ligands

In the same study, these authors also described the synthesis of other S/N ligands derived from various amino acids, such as proline, valine and cysteine. These dithioether and azathioether ligands were further tested as potential palladium ligands for the reduction of acetophenone, but no significant induction was observed in each case of ligand (Scheme 8.26). 

The Stille coupling of a-iodo enones is sluggish under standard conditions. Significant rate enhancement was observed for the Stille reaction of 2-chloro-5-tributylstannylpyridine and a-iodo enone 76 using triphenylarsine as the soft palladium ligand and Cul as the co-catalyst [63], Oxygenated functionalities did not affect the efficiency of the reaction provided both Ph3As and Cul were added. Additional manipulations of 77 resulted in the synthesis of (+)-epibatidine (78). 

Asymmetric hydrogenation of imines derived from trifluoropyruvate, in the presence of a chiral complex of palladium (ligand = (R)-BINAP), affords ethyl (/f)-trifluoroalaninate with ca. 90% The ee values strongly depend on the solvent,... 

Farina, V. Krishnan, B. Large Rate Accelerations in the Stille Reaction with Tri-2-furylphosphine and Triphenylarsine as Palladium Ligands Mechanistic and Synthetic Implications, J. Am. Chem. Soc. 1991,113, 9585-9595. 

Farina, V. and Krishnan, B. (1991) Large rate accelerations in the stille reaction with tri-2-furylphosphine and triph-enylarsine as palladium ligands mechanistic and synthetic implications. Journal of the American Chemical Society, 113(25), 9585-95. 

While alkyl halides are typically employed as an electrophile for this transformation, Takemoto developed palladium-catalyzed asymmetric allylic alkylation of 1 using allylic acetates and chiral phase-transfer catalyst 4k, as depicted in Scheme 2.5 [ 2 3 ]. The choice of triphenyl phosphite [(PhO)3P] as an achiral palladium ligand was crucial to achieve high enantioselectivity. 

Takemoto and coworkers extended their palladium-catalyzed asymmetric allylic alkylation strategy using allyl acetate and chiral phase-transfer catalyst to the quaternization of 13 [23b]. A correct choice of the achiral palladium ligand, (PhO P, was again crucial to achieve high enantioselectivity and hence, without chiral phosphine ligand on palladium, the desired allylation product 15 was obtained with 83% ee after hydrolysis of the imine moiety with aqueous citric acid and subsequent benzoylation (Scheme 2.12). 

Quinone methides have been generated by reaction of their transition metal complexes.127 The T)2-methylene-coordinated complex 77 forms stable solutions in water and methanol. NMR spectroscopy showed that 77 and dibenzy-lideneacetone (DBA) in methanol undergo rapid conversion to 79 (Scheme 37). This is consistent with the reaction of DBA with the palladium ligand at 77 to... 

The scope of the Heck and related coupling reactions was substantially broadened by the development, in the last few years, of palladium/ligand combinations which are effective with the cheap and readily available but less reactive aryl chlorides [86, 87] rather than the corresponding bromides or iodides. The process still generates one equivalent of chloride, however. Of interest in this context, therefore, is the report of a halide-free Heck reaction which employs an aromatic carboxylic anhydride as the arylating agent and requires no base or phosphine ligands [89]. 

An extremely useful variant of Suzuki reaction uses B-alkyl substrates with aryl or vinyl halides. The B-alkyls are prepared by hydroboration of an alkene with 9-BBN-H. With appropriate palladium ligands, reductive... 

Some of the most significant improvements in catalyst development were the discoveries of alternative palladium ligands made by Farina in the early 1990s. In particular, tri(2-ftuyl)phosphine (15) and triphenylarsine proved to be far superior to triphenylphosphine when used as ligands in palladium-catalyzed Kosugi-Stille reactions. Perhaps... 

The introduction of alkyl groups at the a-carbon of amino acids has been accomplished most efficiently by formation of imine esters. For example, the benzaldehyde imine of ethyl glycinate can be deproton-ated and alkylated (equation 39). Other imines also have been used. Optical activity has been introduced by using chiral palladium ligands during the alkylation step, ° chiral alcohols to form the ester, and chiral ketones to form the imine. Alkylation of 2-pyrrole acetate esters has been accomplished in a similar fashion. ... 

Alternatively, an achiral amine can be used as the palladium ligand and an optically active secondary amine can be added successively (auxiliary-induced stereoselectivity). The third possibility, which afforded the highest enantiomeric excess on ( )-2-butene, is to use the optically active amine both as ligand and nucleophile, hence combining reagent- and auxiliary-induced diastereoselectivity (double asymmetric induction). 

No reaction occurs in the absence of triphenylphosphine. Bis(diphcnyl)phosphinoethanc can also be used as the palladium ligand. 

As was demonstrated with the Suzuki reaction [57], aqueous reaction conditions could be applied to the Sonogashira reaction and have also been investigated, but required changing the nature of the palladium ligand. Alkynylpyridine 324 was prepar ed by crosscoupling 26 with 323. 

A convenient and effective palladium-catalysed Suzuki-Miyaura cross coupling reaction of deactivated aryl chlorides with phenylboronic acid utilised the 1,3-diazepinium salts 107 as in situ precursors of the palladium ligands. These salts were prepared from the diamines 106 and cyclocondensation with triethyl orthoformate <05SL2394>. 

The generally accepted mechanism for Pd-catalyzed allylic desulfonylations is illustrated in Scheme 1. The first step is coordination of the Pd(0) catalyst to the allylic sulfone. Oxidative addition or internal SN2-type nucleophilic attack of the electron-rich palladium at the allylic position generates a neutral Pd(II) r 3-allyl complex, which leads to a more reactive cationic complex that is finally reduced. The equilibrium between the neutral and the more reactive cationic complexes depends on the nature and concentration of the palladium ligands as well as the counter anions present in solution. 

The Pd-catalyzed, LiHBEt3-mediated reductive desulfonylation of allylic sulfones is also used in the ligand-controlled stereoselective synthesis of dienes191 where it is possible to control the geometry of the diene by a proper selection of the palladium ligand as shown by the distribution of products 1 and 2 (Eq. 115). The method described herein is also applicable to the so-called integrated chemical processes , which allow the preparation of a wide variety of alkenes by combining alkylation of allylic sulfones and reductive desulfonylation in one pot.192,193... 

Triaiylmonophosphines bearing an oxazoline ring 3.52 (X = PPl, R1 = H) have recently been introduced by Helmchen, Pfaltz, Williams and their coworkers [919, 920, 921, 1064], These are very efficient palladium ligands for asymmetric aliylation of methyl malonate and acetylacetone by 5.50 (R = H) at room tempera-... 

---