Tsuji-Trost alkylation

A silicon-position-dependent 6-mdb-trig cyclization during Tsuji-Trost alkylation has been reported. It could favour either the production of an open chain diene by a direct elimination on the palladium intermediate from (8) or a cyclohexenyl ring via a 6-endo-trig process from (9) (Scheme 4). 

To obtain other compounds with the cyclobutene motive, we synthesized the 1,8-enynes II-24a-b in moderate yields via allylic Tsuji-Trost alkylation (Scheme 3.17) [Ref. 173 in Chap. 1]. 

A combination of a Tsuji-Trost and a Michael addition was used for the synthesis of (+)-dihydroerythramine 6/1-269, as reported by Desmaele and coworkers [128]. The Pd-catalyzed reaction of the allylic acetate 6/1-267 with the nitromethylarene 6/1-266 in the presence of Cs2C03 as base led to the domino product 6/1-268 as a 4 1 mixture of two diastereomers in 79% yield. Further manipulation of 6/l-268a yielded the desired dihydroerythramine 6/1-269 (Scheme 6/1.70). Interestingly, using the corresponding allylic carbonate without additional base gave the mono-alkylated product only. 

The Tsuji-Trost reaction is the Pd(0)-catalyzed allylation of a nucleophile [48-51]. The NH group in imidazole can take part as a nucleophile in the Tsuji-Trost reaction, whose applications are found in both nucleoside and carbohydrate chemistry. Starting from cyclopentadiene and paraformaldehyde, cyclopentenyl allylic acetate 64 was prepared in diastereomerically-enriched form via a Prins reaction [52], Treating 64 with imidazole under Pd(0) catalysis provided the N-alkylated imidazole 65. 

For further details of this reaction, the reader is referred to Chapter 9. The catalytic allylation with nucleophiles via the formation of Ti-allyl metal intermediates has produced synthetically useful compounds, with the palladium-catalyzed reactions being known as Tsuji-Trost reactions [31]. The reactivity of Ti-allyl-iridium complexes has been widely studied [32] for example, in 1997, Takeuchi idenhfied a [lrCl(cod)]2 catalyst which, when combined with P(OPh)3, promoted the allylic alkylation of allylic esters 74 with sodium diethyl malonate 75 to give branched... 

Beside the Friedel-Crafts-type alkylation of arenes, the direct functionalization of 2,4-pentanediones is of great interest in Lewis acid catalysis. Although Pd-catalyzed Tsuji-Trost type allylations of 1,3-diketones are known, direct benzylation procedures catalyzed by Lewis acids are less explored [40-43]. Based on the previously described Friedel-Crafts alkylation of arenes and heteroarenes, the Rueping group developed a Bi(OTf)3-catalyzed benzylation of 2,4-pentanediones. Alcohols such as benzyl, allyl or cinnamyl alcohols were used as the electrophilic component to yield important 2-alkylated 1,3-dicarbonyl compounds. Initially, different Bi(III) salts were screened. In contrast... 

Although not discussed in this chapter, the Tsuji-Trost reaction159 is undoubtedly the most extensively investigated Pd-catalyzed allylation with allyl electrophiles. There have also been some uncatalyzed and Cu-catalyzed reactions of allyl electrophiles with alkyl metals and metal cyanides. On the other hand, the Pd- or Ni-catalyzed reactions of allyl electrophiles with organometals containing allyl-, benzyl-, propargyl- and other alkylmetals do not appear to have been extensively investigated. 

The preparation of a-lithio aldehydes, o -lithio ketones, and related compounds and their applications to organic synthesis has been reviewed.10 The Tsuji-Trost allylic alkylation with ketone enolates has been highlighted.11... 

Palladium(0)-catalyzed allylation of nucleophiles (the Tsuji-Trost reaction) is a versatile synthetic method that has gained immense popularity in recent years. Rarely applied to ambident nucleophilic aromatic heterocycles before 1991, the Tsuji-Trost reaction has been extensively used in the chemistry of these compounds since 1991. Two factors have played decisive roles in this increased interest in the Pd(0)-catalyzed allylation of such heterocyclic rings one is that, unlike other alkylation procedures, the Pd(0)-catalyzed allylation can sometimes give the product of thermodynamic control when applied to ambident nucleophiles and the second is that the Tsuji-Trost allylation has become one of the standard methods for synthesizing carbanucleosides, which are important antiviral compounds (93MI1, 93MI2). Of course, the double bond of an allylic system can be modified in different directions, thus adding versatility to the Tsuji-Trost reaction. 

We include in Sections I,A and I,B some general features of the Tsuji-Trost reaction with comments on kinetic versus thermodynamic control in allylations and in alkylations in general. Then we review in Sections II, III, and IV all cases known to the authors of the application of the Tsuji-Trost reaction to ambident nucleophilic aromatic heterocycles. This leaves out of the review the allylation of such heterocyclic ambident nucleophiles as 2-piperidone and the like. By aromatic, we mean any heterocycle for which a tautomeric or mesomeric formula can be written that is aromatic in the normal structural sense of having 4n + 2n- electrons cyclically conjugated. 

Tsuji-Trost allylation reactions offer multiple pathways to tetrahydrofuran synthesis including C-C bond-formation steps. A palladium-catalyzed sequence of allylic alkylation and Hiyama cross-coupling provides a convenient synthesis of 4-(styryl)-lactones (Scheme 67) <2006SL2231>. 

Figure 3.85 The use of the mixed NHC/phosphane ligand system in the Tsuji-Trost allylic alkylation reaction.

The catalyst is not only active in the Tsuji-Trost reaction (allylic alkylation), but also in the corresponding amination reaction. This was shown by the reaction of (ii)-l,3-diphenylprop-... 

An ingenious extension of the Tsuji-Trost reaction was the cornerstone of Oppolzer s enantioselective synthesis of a heteroyohimbine alkaloid, (-t-j-B-isorauniticine (267) [117]. Substrate 263 was prepared from a commercially available glycinate equivalent by Malkylation, installation of the sultam chiral auxiliary followed by a sultam-directed C-alkylation. As illustrated in Scheme 48, the crucial double cyclization was accomplished by the treatment of 263 with Pd(dba), Bu,P, in the presence of carbon monoxide (1 atm) in acetic acid to give enone 264 and two other stereoisomers in a 67 22 11 ratio. In this case, an allyl carbonate, rather than an allyl acetate, was used as the allyl precursor. Since carbonate is an irreversible leaving group, formation of the n-allylpalladium complex occurs readily. In the presence of Pd(0), the allylic carbonate is converted into a n-allylpalladium complex with concurrent release of CO, and... 

Palladium Catalysts Palladium catalysts are effective and powerful for C—H bond functionalization. Carbene precursors and directing groups are commonly used strategies. Generally, sp3 C—H bond activation is more difficult than sp2 C—H bond activation due to instability of potential alkylpalladium intermediates. By choosing specific substrates, such as these with allylic C—H bonds, palladium catalytic systems have been successful. Both intramolecular and intermolecular allylic alkylation have been developed (Scheme 11.3) [18]. This methodology has presented another alternative way to achieve the traditional Tsuji-Trost reactions. 

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. 

Recent results have appeared related to the use of a new bio-support for SAPC. The authors report the first example of a palladium complex containing TPPTS supported on cellulose, a natural polysaccharide. The cellulose powder, which presents a specific surface area of 1.35 m and 3 wt% water content with [Pd(TPPTS)3] formed in situ, is an efficient support for the Trost-Tsuji allylic alkylation reaction [29]. 

The allylic alcohol products from Morita-Baylis-Hillman reactions were shown to participate in a DMAP-mediated Tsuji-Trost-type reaction with /3-diketones or /3-ketoesters, forming the C-allylation product without requiring the use of palladium. Previously, it was shown that allylic alcohols combined with /8-ketoesters and DMAP afforded the transesterification products, in which the allylic alcohol displaced the ester substituent. The difference between these diverging reaction pathways is likely due to the electron-withdrawing group on the allylic alcohol in the MBH adducts vs. just alkyl substituents in the latter case. 

The Pd-catalysed allylation of carbon nucleophiles with allylic compounds via Jt-aUylpaUadium complexes is called the Tsuji-Trost reaction [32]. Typically, an allyl acetate or carbonate (54) reacts with a Pd-catalyst resulting in displacement of the leaving group to generate a Jt-allylpalladium complex (55) that can undergo substitution by a nucleophile (56) (Scheme 4.14). In 1965, Tsuji reported the reaction of ti-aUylpaUadium chloride with nucleophiles such as enamines and anions of diethyl malonate and ethyl acetoacetate. A catalytic variant was soon reported thereafter in the synthesis of allylic amines [33]. In 1973, Trost described the alkylation of alkyl-substituted 7i-aUylpalladium complexes with methyl methylsulfonylacetate... 

The Tsuji-Trost protocol has been successfiilly employed for the allyUc alkylation of preformed lactone enolates. It has been demonstrated that this Pd-catalysed reaction can be carried out in an enantio- and diastereo-selective manner. The use of additives, such as LiCl, was found to be crucial for reaching high levels of product selectivity. Among the five pathways investigated by DFT methods, the reaction between an (allyl)Pd(BINAP) (2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) complex and a LiCl-lithium enolate adduct was predicted to be the most likely route for C-C bond formation. 

SCHEME 5.11 (a) Tsuji-Trost asymmetric allyUc alkylation applied to the total synthesis of vinyl or isopro-penyl chiral dihydrophenanthrenes. (h) Stereoselective synthesis of the (-)-cedril A precursor 36b. 

Thanks to the fundamental studies of Tsuji, Trost, and others, palladium-catalyzed allylic substitution has become a versatile, widely used process in organic synthesis [40]. The search for efficient enantioselective catalysts for this class of reactions is an important goal of current research in this field [41]. It has been shown that chiral phosphine ligands can induce substantial enantiomeric excesses in Pd-catalyzed reactions of racemic or achiral allylic substrates with nucleophiles [42]. Recently, promising results have also been obtained with chiral bidentate nitrogen ligands [43]. We have found that palladium complexes of neutral aza-semicorrin or methylene-bis(oxazoline) ligands are effective catalysts for the enantioselective allylic alkylation of l,3-diphenyl-2-propenyl acetate or related substrates with dimethyl malonate (Schemes 18 [25,30] and 19 [44]). 

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. 

---