Alkene ligand accelerating effects

A stoichiometric procedure for the osmium-mediated, enantioselective aminohydrox-ylation of traws-alkenes RCH=CHR (R = Ph, Et, Pr1) has been developed employing chiral complexes between tert-butylirnidoosmium (BufN=0s03) and derivatives of cinchona alkaloids. The success of the reaction is dependent on a ligand acceleration effect corresponding diols are the by-products. The e.e. varies between 40 and 90%486,487. 

Thus, in cw-vic-dihydroxylations of alkenes with 0s04 tertiary amines, like pyridine, have ligand acceleration effects (this term was introduced in Section 3.4.6, using the Sharpless epoxidation as an example). 

A kinetic study was undertaken to look at ligand acceleration effects in the AD reaction with various alkene substitution patterns. The effect is most dramatic on trisubstituted alkenes and some idea of relative reactivity can be obtained (Figure 3.4) [164]. Table 3.1 summarizes the types of simple alkenes that can be used in an AD reaction. 

Yang12 has effected an intramolecular asymmetric carbonyl-ene reaction between an alkene and an a-keto ester. Reaction optimization studies were performed by changing the Lewis acid, solvent, and chiral ligand. Ligand-accelerated catalysis was observed for Sc(OTf)3, Cu(OTf)2, and Zn(OTf)2 (Equation (6)). The resulting optically active m-l-hydroxyl-2-allyl esters provide an entry into multiple natural products. 

It was proposed that NMO oxidized one of the carbon monoxide ligands to carbon dioxide to open up a coordination site for the alkene. Alternatively (or additionally), NMO may act to scavange CO to make the dissociation of the CO ligand irreversible. Subsequent to this work, a number of V-oxides, including polymer-bound V-oxides, have been shown to accelerate the PKR. More than one equivalent V-oxide is usually required to observe the accelerating effects. Thus polymer-bound V-oxide offers the advantage of simplifying the work-up. Trimethylamino-V-oxide (TMANO) and NMO are the two most common V-oxides used to accelerate the PKR. 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

One of the most important transformations catalysed by palladium is the Heck reaction. Oxidative addition of palladium(O) into an unsaturated halide (or tri-flate), followed by reaction with an alkene, leads to overall substitution of a vinylic (or allylic) hydrogen atom with the unsaturated group. For example, formation of cinnamic acid derivatives from aromatic halides and acrylic acid or acrylate esters is possible (1.209). Unsaturated iodides react faster than the corresponding bromides and do not require a phosphine ligand. With an aryl bromide, the ligand tri-o-tolylphosphine is effective (1.210). The addition of a metal halide or tetra-alkylammonium halide can promote the Heck reaction. Acceleration of the coupling can also be achieved in the presence of silver(I) or thallium(I) salts, or by using electron-rich phosphines such as tri-tert-butylphosphine. ... 

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