Phosphoramidites: marvellous ligands in catalytic asymmetric conjugate addition.
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Frequently Asked Questions (14)
Q2. What catalysts can be used to convert acyclic enones?
11The ability of organozinc reagents to undergo transmetalation permits their conversion in more reactive organometallic reagents RMLn, which has been demonstrated with Ni,12 Cu,13 Pd,14 and Ti.15 Alkyl transfer from diorganozinc reagents to enones can also be effected by nickel catalysts.
Q3. What is the name of the ligands used in stoichiometric?
Trivalent phosphorus compounds represent another class of ligands that have been used for a considerable time in stoichiometric conjugate additions.
Q4. What are the widely used methods for the construction of carbon-carbon bonds?
Among the most widely used methods for the construction of carbon-carbon bonds are conjugate addition reactions of carbon nucleophiles to R,â-unsaturated compounds.3
Q5. What is the enantioselective effect of the catalyst?
The finding that the copper-phosphoramidite catalyst tolerated functionalized dialkylzinc reagents constitutes the foundation for novel catalytic enantioselective annulation methods.
Q6. What is the prominent protocol in the construction of carbocyclic compounds?
The Hajos-Parrish asymmetric versionof the Robinson annulation is one of the most prominent protocols in the construction of carbocyclic compounds (Scheme 10a), frequenty used in the synthesis of steroids and terpenes.
Q7. Why is the enone complex 5 a bimetallic complex?
Due to the high levels of stereocontrol reached in this catalytic cycle, 5 might well be formulated as a bimetallic complex, leading to a fixed conformation of the enone.
Q8. Why have phosphoramidites not been used in asymmetric catalysis?
In contrast to the large number of phosphines and phosphites frequently employed as ligands in asymmetric catalysis, phosphoramidites have not been used, presumably because of the sensitivity toward hydrolysis attributed to this class of compounds.
Q9. What is the way to achieve stereocenters in cyclohexanones?
The first catalytic regio- and enantioselective threecomponent coupling of organozinc reagents was, indeed, achieved, affording trans-2,3-disubstituted cyclohexanones with ee’s exceeding 90% in all cases examined (Scheme 9).10
Q10. What is the enantioselective version of acyclic en?
Enantioselective versions of these alkyltransfer reactions to acyclic enones, in particular chalcones, have been reported, employing several chiral ligands including diamines, pyridine methanols, proline amides, and amino alcohols.16 A notable accomplishment is the 90% enantiomeric excess (ee) reached by Soai in the conjugate addition of Et2Zn to acyclic enones using a nickel-ephedrine-based catalyst.
Q11. What is the optimum ratio of ligand to copper?
The optimum ligand-to-copper ratio of 2, the nearly identical selectivities with mono- and bidentate phosphoramidites (except for cyclopentenones),23,25b and the observation of nonlinear effects25a strongly point to the presence of two ligands in the active catalyst.
Q12. What is the X-ray structure of the CuI complex of phosphoramidite?
The X-ray structure of the CuI complex of phosphoramidite 2 guided us, as it shows that the obvious positions for ligand modification are the amine moiety and the 3,3′- positions of the binaphthyl part of the ligand.
Q13. What is the example of a tandem addition of cyclohexenones?
The tandem addition (1.2 mol % catalyst) of Me2Zn and propanal to cyclohexenone, providing diketone 20 (after oxidation) in 81% yield with 97% ee, is an illustrative example.
Q14. What is the simplest way to explore the structure of phosphoramidites?
Structural variations of the phosphoramidites can be easily explored by a modular variation of the amine and binaphthol moieties.