Transition metal catalysts conjugate addition

Additions to conjugated systems are further facilitated by some of the features discussed previously e.g., additions to dienes" " and enynes" in the presence of transition-metal catalysts and additions to enynes containing a suitably placed intramolecular function such as hydroxyl. Although uncatalyzed intermolecular additions to alkynylsilanes are not yet demonstrated, additions are seen in the presence of a transition-metal catalysts" and intramolecular additions to alkynylsilane functions by suitably placed internal organomagnesium-halide functions are facile. ... 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

The first application of NMR diffusion measurements to determine the aggregation state of a transition metal catalyst concerned the chiral, tetranuclear Cu(i) catalysts 130-132, used in the conjugate addition reactions of anions to a,p-unsatu-rated cyclic ketones. Compounds 130-132 react wdth isonitriles to form 133-135, and do not degrade to lower molecular weight species (see Eq. (20)) [109]. 

Organometallic reagents can be used direcdy or with a transition metal catalyst in tandem vicinal difunctionalization reactions. Relatively basic reagents find greater use with substrate molecules that are not enolizable so that competitive a-deprotonation is avoided and with substrates that have been activated towards conjugate addition by an electron-withdrawing a-substituent. 

The C-H bond cleavage of active methylene compounds with a transition metal catalyst is another method for the functionalization of these C-H bonds. To date, several reactions have been developed. In particular, the asymmetric version of this type of catalytic reaction provides a new route to the enantioselective construction of quaternary carbon centers. One of the most attractive research subjects is the catalytic addition of active methylene C-H bonds to acetylenes, allenes, conjugate ene-ynes, and nitrile C-N triple bonds. The mthenium-catalyzed reaction active methylene compounds with carbonyl compounds involving aldehyde, ketones, and a,y3-unsatu-rated ketones and esters is described in this section. 

Silyl Anion Equivalent. Silylboronic ester 1 reacts as a silyl anion equivalent in the presence of transition metal catalysts. Cyclic and acyclic a,/3-unsaturated carbonyl compounds serve as good acceptors of the silyl groups in conjugate addition of 1 catalyzed by rhodium and copper complexes, giving /3-silylcarbonyl compounds (eq 30). The silylation takes place with high enan-tioselectivity when Rh/(5)-BINAP or Cu/chiral NHC catalysts are used. Three-component coupling of 1, a,/3-unsaturated carbonyl compounds, and aldehydes affords 8-hydroxyketone stereoselec-tively in the presence of a copper catalyst (eq 31). The copper enolate 32 is presumed as an intermediate of the reaction. 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

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