172222-68-3Relevant academic research and scientific papers
Stereochemistry of the [4 + 2] cycloaddition of diarylselenoketones with conjugated dienes
Wilker, Stefanie,Erker, Gerhard
, p. 10922 - 10930 (2007/10/03)
The ylides Ph3P=CAr1Ar2 1a-g (Ar = C6H5, p-C6H4Cl, p-C6H4F, m-C6H4CF3, p-C6H4OCH3, p-C6H4-CH3) were treated with elemental selenium (～80°C) to give the corresponding selenoketones Se=CAr1Ar2 2 by Staudinger-chalcogenation. Their reaction with trans,trans-2,4-hexadiene proceeds completely stereospecifically to yield the 2,2-diaryl-3,6-dihydro-cis-3,6-dimethyl-2H-selenapyrans 3. In contrast, the reactions of the selenoketones 2 with cis,trans-2,4-hexadiene proceeds stereoselectively, also giving the dihydro-cis-dimethyl-2H-selenapyrans 3 as the major products, now admixed with small amounts of the dihydro-trans-dimethyl-2H-selenapyran isomers 4. The [4 + 2] cycloaddition of 2 with cis,trans-2,4-hexadiene proceeds stereospecifically, however, when carried out at a pressure of 12 kbar, now yielding 4 as the major products along with the corresponding tetraarylethenes 8. Along with the results of additional mechanistic studies (determination of solvent and substituent effects) it can be concluded that diarylselenoketones are likely to react by means of a concerted [4 + 2] cycloaddition with very reactive conjugated dienes (such as trans,trans-2,4-hexadiene), whereas a stepwise mechanism, resulting in diene cis/trans-isomerization with subsequent mechanistic "leakage" to the concerted pathway, appears to be preferred when a much less reactive conjugated diene such as cis,trans-2,4-hexadiene is employed. The reaction of the corresponding diarylthioketones 5a-g with trans,trans- and cis,trans-2,4-hexadiene, respectively, shows an analogous behavior.
Dihydroselenapyrans by Cycloaddition of Diaryl Selenoketones
Hock, Regina,Hillenbrand, Stefanie,Erker, Gerhard,Krueger, Carl,Werner, Stefan
, p. 1895 - 1904 (2007/10/02)
The phosphorus ylides Ph3P=CAr1Ar2 5 1=Ph, Ar2=4-CH3C6H4 (c); Ar1=Ar2=Ph (d); 4-ClC6H4 (e); 4-FC6H4 (f); 3-CF3C6H4 (g); Ar1=Ph, Ar2=4-ClC4H4 (h)> were allowed to react with elemental selenium at ca. 75 deg C in toluene in the presence of an excess of 2,3-dimethylbutadiene.The diaryl selenoketones 1 thus generated in situ by means of the "Staudinger chalcogenation" reaction were trapped by the added conjugated diene to give the 2,2-diaryl-3,6-dihydro-4,5-dimethyl-2H-selenapyrans 7 in high yield.Similarly, the ylides 5c-f and 5h were treated with selenium, and the resulting diaryl selenoketones 1 added to 1,3-butadiene to give the corresponding 2,2-diaryl-3,6-dihydro-2H-selenapyrans 8.Selenobenzophenone, synthesized analogously, was employed in a cycloaddition reaction with 2,3-dimethoxybutadiene to yield 3,6-dihydro-4,5-dimethoxy-2,2-diphenyl-2H-selenapyran (9), which was characterized by an X-ray crystal structure analysis.Compound 9 crystallizes in the space group P21/n.In the crystal the dihydro-2H-selenapyran adopts a distorted half-chair conformation. - Key Words: Selenoketones / Selenapyrans, dihydro / Staudinger chalcogenation / Phosphorus ylides / Cycloadddition
