Allyl carbonates transition metal catalyzed reactions

Fischer-type carbenes can also be modified via transition metal catalyzed reactions. Fischer chromium aminocarbene complexes can be used as nucleophiles in palladium-catalyzed allyUc substitution reactions with aUylic acetates and carbonates, alFording the corresponding allyl-substituted aminocarbenes. For example, reaction of the Uthiated carbene (15) gives (16) in good yield (Scheme 25). ... 

Transition metal catalyzed reactions, providing a formal Alder-Ene product, have a unique mechanism and selectivity when compared to the traditional Alder-Ene reaction product. There has been a debate in explaining the initial step, whether a 7t-allyl complex was formed from the ene reaction components, or merely complexation of each reacting component and metallocycle formation. For the Ru-catalyzed reaction, Trost and coworkers reported that the below description rationalized the observed selectivity. Coordination followed by carbocyclization provides either 110 or 111. Carbometallations normally prefer to attaeh at the less substituted terminus of the alkyne, favoring initial formation of 109 leading to 111. However, steric effects between the two bonding carbons may disfavor such a mode and therefore 108 may be preferred due to steric effects which would form carbocycle 110. So, as R increases in size, one would expect that more... 

As with the silanes, some of the most useful procedures for synthetic applications of stannanes involve electrophilic attack on alkenyl and allylic derivatives. The stannanes are more reactive than the corresponding silanes because there is more anionic character on carbon in the C—Sn bond and it is a weaker bond. There are also a number of synthetic procedures in which organotin compounds act as carbanion donors in transition metal-catalyzed reactions. Organotin compounds are also used in free-radical reactions, as will be discussed in Chapter 10. 

Recent trend in the synthesis of olefinic pheromones is the use of transition metal-catalyzed cross coupling reaction for carbon-carbon bond formation. Scheme 8 summarizes a synthesis of the termite trail marker pheromone, (3Z,6Z)-3,6-dodecadien- l-ol (2) by Oehlschlager [19]. The key-step is the palladium-catalyzed cross-coupling of allylic chloride A and alkenylalane B. 

Ketone and ester enolates have historically proven problematic as nucleophiles for the transition metal-catalyzed allylic alkylation reaction, which can be attributed, at least in part, to their less stabilized and more basic nature. In Hght of these limitations, Tsuji demonstrated the first rhodium-catalyzed allylic alkylation reaction using the trimethly-silyl enol ether derived from cyclohexanone, albeit in modest yield (Eq. 4) [9]. Matsuda and co-workers also examined rhodium-catalyzed allylic alkylation, using trimethylsilyl enol ethers with a wide range of aUyhc carbonates [22]. However, this study was problematic as exemplified by the poor regio- and diastereocontrol, which clearly delineates the limitations in terms of the synthetic utihty of this particular reaction. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Me3SiCN is a convenient, reactive cyanide donor in transition metal-catalyzed processes. The Pd-catalyzed reaction of aryl iodides with Me3SiCN is useful for the synthesis of aryl cyanides.257 Me3SiCN works also as an effective co-catalyst for the Pd-catalyzed cyanation of aryl iodides with KCN.258 Allylic acetates, carbonates, and the related compounds undergo the Pd-catalyzed cyanation with Me3SiCN.259-261 The tandem cyclization-cyanation reaction of 2-bromo-l,6-heptadienes with Me3SiCN proceeds under catalysis by an Ni complex (Equation (68)).262... 

The transition metal-catalyzed allylation of carbon nucleophiles was a widely used method until Grieco and Pearson discovered LPDE-mediated allylic substitutions in 1992. Grieco investigated substitution reactions of cyclic allyl alcohols with silyl ketene acetals such as Si-1 by use of LPDE solution [95]. The concentration of LPDE seems to be important. For example, the use of 2.0 M LPDE resulted in formation of silyl ether 88 with 86 and 87 in the ratio 2 6.4 1. In contrast, 3.0 m LPDE afforded an excellent yield (90 %) of 86 and 87 (5.8 1), and the less hindered side of the allylic unit is alkylated regioselectively. It is of interest to note that this chemistry is also applicable to cyclopropyl carbinol 89 (Sch. 44). 

Besides the transition-metal-catalyzed asymmetric addition reactions to prochiral olefins, the substitution reaction of a carbon nucleophile to allylic esters has been investigated using a variety of chiral transition-metal catalysts. Using the aforementioned sugar diphosphites... 

Transition metal catalyzed allylation by using allylic carbonates is one of the most versatile allylation methods. The Pd-catalyzed reaction is believed to proceed by the following mechanism. Eirst of all, oxidative addition of the C-0 bond to Pd(0) gives (r/ -allyl)(carbonato)palladium(II) complex, which undergoes decarboxylation to give the alkoxo complex [70]. The resulting (t] -allyl)(alkoxo)palladium(II) complex immediately reacts with nucleophiles (HNu)... 

Transition metal-catalyzed allylic substitution reactions with carbon nucleophiles are among the most important carbon-carbon bond formation methods in modem organic synthesis, because of their broad substrate scope under mild reaction conditions. In addition, they are applicable to enantioselective reactions, as well as exhibiting versatility towards the alkene functionality adjacent to the chiral centre for stereoselective derivatization. Tsuji-Trost allylic substitution, involving a (Ti-allyl) metal intermediate, has provided a particular advance in this regard [34, 35]. Most recently, Sawamura et al. [36, 37] have improved the regioselectivity of this reaction with unsymmetrically substituted allylic esters, and thus opened a new approach to sertraline. 

---