Asymmetric Tsuji-Trost reaction

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

The formation of chromane derivatives has also been realised in the palladium catalyzed intramolecular nucleophilic substitution of allyl carbonates (Tsuji-Trost reaction). In most cases the reaction is accompanied by the formation of a new centre of chirality. Using Trost s chiral ligand the ring closure was carried out in an enantioselective manner. The asymmetric allylation of the phenol derivative shown in 4.20. was achieved both in good yield and with excellent selectivity.23... 

These two mechanistic modes have an impact on the development of asymmetric variants of the Tsuji-Trost Reaction. 

Pd-catalyzed allylic substitutions such as the Tsuji-Trost reaction have been investigated widely, essentially in their asymmetric version [44]. This represents a valuable tool in organic synthesis since the catalyst can accommodate various functionalities on the substrate and it is possible to tune the coordination sphere through the electronic and steric effects of the ligands. Those which contain a sulfur atom are based on an oxazoline backbone, and an ee as high as 96 % has been... 

Use of the Tsuji-Trost Reaction for Asymmetric Creation of Quaternary Centers... 

The enantioselectivity associated with quaternary allylation is connected with scenario 5 above (one of the five points associated in the catalytic cycles shown by Schemes 12.10a and b where chirality could be induced), which is where enantioselection of one of two faces of the nucleophile (the enolate ion) occurs. Theoretical studies of the transformation using the PHOX ligand have shown support for an inner sphere mechanism, where nucleophilic attack of the enolate onto the rf-allyl ligand occurs from the Pd-bound enolate and not from an external nucleophile.74 These studies have not been able to definitively determine the step that defines the enantioselectivity of the reaction, and it is not clear how these results would carry over to reactions involving the Trost ligands. At this time, selection of which ligand one should use not only to induce enantioselectivity but also to predict the sense of absolute configuration of any asymmetric Tsuji-Trost allylation is mostly based on empirical results. Work continues on this... 

The asymmetric alkylation of allylic systems by means of palladium catalysis, the so-called Tsuji-Trost reaction, is one of the most investigated asymmetric catalytic reactions [34,35]. It is therefore no surprise that it has also caused interest in the area of ACTC ligands. 

Allylic alkylation, also known as the Tsuji-Trost reaction, operates via a unique mechanism that exploits the electrophihcity of 7t-allyl Pd complexes. It is a versatile transformation in asymmetric synthesis, and new catalysts are generally tested in this benchmark reaction. The investigation of functionalised NHC ligands containing electronically dissimilar groups has met limited success. Actually, allylic alkylation is one of the rare transformations in which phosphines still outperform NHCs. 

In 1977, Trost published the first example of an asymmetric variant of the Tsuji-Trost reaction, termed the asymmetric allylic alkylation reaction (AAA). Much of the subsequent development of the AAA reaction can be attributed to the dedicated work of Trost and co-workers.There was a substantial time lag however, in the development of processes where high enantioselectivities were realized in a predictable fashion. This was due, in part, to the fact that chiral, asymmetrically pure ligands must create a chiral environment on the opposite face of the allyl fragment to the metal centre (a stereoelectronic requirement, vide infra)P This obviously represents a significant design challenge in the production of effective ligand systems. 

The protocols for the utilization of ketone-derived silyl enol ethers in Tsuji-Trost reactions were preceded by a report of Morimoto and coworkers on the enantioselective allylation of sUyl ketene acetals 88. Without external activation, they reacted with the allylic substrate 19d in the presence of the palladium complex derived from the amidine ligand 89 to give y,5-unsaturated esters 90 in moderate chemical yield but high enantiomeric excess (Scheme 5.29) [46]. Presumably, the pivalate anion hberated during the oxidative addition functions as an activator of the silyl ketene acetal. The protocol is remarkable in view of the fact that asymmetric allylic alkylations of carboxylic esters are rare. Interestingly, the asymmetric induction originates from a ligand with an uncomplicated structure. The protocol seems however rather restricted with respect to the substitution pattern of allylic component and sUyl ketene acetal. 

The Tsuji-Trost reaction is the palladinum-catalyzed substitution of allylic leaving groups by carbon nucleophiles. The nucleophile can be carbon-, nitrogen-, or oxygen- based compounds such as alcohols, enolates, phenols, and enamines, and the leaving group can be a halide or an acetate. This emerged as a powerful procedure for the formation of C—C, C—O and C—N bonds. The reaction, also known as Trost allylation or allylic alkylation, was named after Jiro Tsuij, who first reported the method in 1965 [42], and Barry Trost, who introduced an asymmetric version in 1973 [43]. 

These two mechanistic modes have an impact on the development of asymmetric variants of the Tsuji-Trost reaction. For a discussion, see a review by Trost and Van Vranken. ... 

In the laboratory of B.M. Trost, the second generation asymmetric synthesis of the potent glycosidase inhibitor (-)-cyclophellitol was completed using a Tsuji-Trost allylation as the key step. The synthetic plan called for the conversion of the a-nitrosulfone allylation product to the corresponding carboxylic acid or ester. Numerous oxidative Nef reaction conditions were tested, but most of them caused extensive decomposition of the starting material or no reaction at all. Luckily, the nitrosulfone could be efficiently oxidized with dimethyidioxirane under basic conditions (TMG) to afford the desired carboxylic acid in high yield. 

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