Gennari have developed conditions that give Z-alkenes with excellent stereoselectivity.
Several other bases have been found effective. Rathke extended this to lithium or magnesium halides with triethylamine. Masamune and Roush have developed mild conditions using lithium chloride and DBU. Since many substrates are not stable to sodium hydride, several procedures have been developed using milder bases. The stereoselectivity of the Horner–Wadsworth–Emmons reaction of ketones is poor to modest. In case ( Z)-alkenes from aromatic aldehydes are needed, the Still–Gennari modification (see below) can be used. This video comprise of detailed study of preparation, applications, physical and chemical properties, uses of WITTIG REAGENT.syntheticreagents WITTIGREAGE. Īromatic aldehydes produce almost exclusively ( E)-alkenes. The steric bulk of the phosphonate and electron-withdrawing groups plays a critical role in the reaction of α-branched phosphonates with aliphatic aldehydes. Once the reaction is completed in 10-15 min, a 1D 1H NMR can be acquired for benzaldehyde, the Wittig reagent, the crude product and the precipitate formed over the course of the reaction. In a separate study, it was found that bulky phosphonate and bulky electron-withdrawing groups enhance E-alkene selectivity. It uses a pre-prepared stabilized Wittig reagent and doesn’t require anhydrous conditions or caustic bases.
DRAWING A WITTIG REAGENT IN CHEMDOODLE MAC
Available immediately, ChemDoodle has all of the features of vastly more expensive applications, and it works on Windows, Mac OSX, and Linux. While each effect was small, they had a cumulative effect making it possible to modify the stereochemical outcome without modifying the structure of the phosphonate. ChemDoodle is a powerful tool for drawing chemical structures, diagrams and figures. ChemDoodle Web Components allow the wielder to present publication quality 2D and 3D graphics and animations for chemical structures, reactions and spectra.
Thompson and Heathcock have performed a systematic study of the reaction of methyl 2-(dimethylphosphono)acetate with various aldehydes. The ChemDoodle Web Components library is a pure JavaScript chemical graphics and cheminformatics library derived from the ChemDoodle application and produced by iChemLabs. In general, the more equilibration amongst intermediates, the higher the selectivity for ( E)-alkene formation. The Horner–Wadsworth–Emmons reaction favours the formation of ( E)-alkenes. However, these α-hydroxyphosphonates can be transformed to alkenes by reaction with diisopropylcarbodiimide. In the absence of an electron-withdrawing group, the final product is the α-hydroxyphosphonate 3a and 3b. The electron-withdrawing group (EWG) alpha to the phosphonate is necessary for the final elimination to occur. This makes the Wittig reaction an especially attractive method for preparing many alkenes.The ratio of alkene isomers 5 and 6 is dependent upon the stereochemical outcome of the initial carbanion addition and upon the ability of the intermediates to equilibrate. In other words, the position of the double bond is always unambiguous in the Wittig reaction. The newly formed double bond always joins the carbonyl carbon with the negatively charged carbon of the Wittig reagent. In a two-step method, treatment of cyclohexanone with CH₃MgBr forms a 3° alcohol after protonation.ĭehydration of the alcohol with H₂SO₄ forms a mixture of alkenes, in which the desired disubstituted alkene is the minor product.īy contrast, reaction of cyclohexanone with Ph₃P=CH₂ affords the desired alkene as the only product. Whereas other methods of alkene synthesis often give a mixture of constitutional isomers, the Wittig reaction always gives a single constitutional isomer.įor example, two methods can be used to convert cyclohexanone into alkene B (methylenecyclohexane):Ī two-step method consisting of Grignard addition followed by dehydration, or a one-step Wittig reaction. You always know the location of the double bond.
0 kommentar(er)
