Cyclic ortho esters undergo stereoselective and regioselective reaction with phenols when treated with BF(3) x OEt(2) at low temperatures. Attack of the phenol on the ortho ester occurs at an open carbon para to electron-donating groups on the phenol ("C-addition") or at the phenolic hydroxyl group ("O-addition") depending on the nature of the cation formed from reaction of the ortho ester and BF(3) x OEt(2). Products resulting from O-addition undergo reversion to a mixture of starting phenol, C-addition product, and O-addition product if treated with BF(3) x OEt(2) at room temperature, but C-addition products are stable under the same conditions. X-ray structural analysis of the C-addition compound indicates that its stereochemistry is opposite to that observed in reaction of similar ortho esters with chloride from TMSCl. However, the stereochemistry of the reaction can be rationalized by the ability of the ortho ester to isomerize via an intermediate benzylic cation and examination of the preferred trajectory of attack of the nucleophile on the intermediate oxonium ion.
The reaction of chromium hexacarbonyl with 21 substituted polycyclic aromatic hydrocarbons (PAHs) was studied. The PAHs employed for study contain two to four aromatic benzo rings and include parent PAHs, mono-and dimethylated PAHs, PAHs with one or two rings saturated, and -keto-hexacyclic and phenyl-substituted derivatives. In general, a partially hydrogenated ring substituent, presumably due to its electron-donating character, directs the reaction regioselectivity to the substituted aromatic ring. This suggests that steric hindrance may not be as important a factor as electron-donating ability. Further evidence for this conclusion is that a methyl substituent, also having electron-donating character, exhibits a similar effect, whereas an electron-withdrawing keto functional group directs the reaction away from the substituted aromatic ring. On the basis of the electronic effects of the substituents observed in this study, we propose that total x-electron density, in terms of total x-bond order, of a benzenoid ring in a PAH is a decisive factor in determining the regioselectivity of the coordination reactions between PAHs and chromium hexacarbonyl. In contrast to previous reports that a [Cr(CO)3] group always bonds to the terminal aromatic ring of a PAH, we have found that reactions between chromium hexacarbonyl and two PAHs,
tions were used as the chemical actinometer for which a quantum yield of 0.38 was used43 giving a reproducible light intensity of 2.10 X 1016 quanta sec-1. Analyses were performed on a Hewlett-Packard Model 5750 gas chromatograph using a 10% FS-1265 Diasaport S column at 210-235.® The mole ratioiarea ratio response of the instrument was calibrated for each aroylimine and internal standard used, so that yields of product could be measured accurately. The conversions were run to 10% or less. The mass balance in these runs were generally better than 95%. Emission Studies. The emission spectra were made on an Aminco-Bowman spectrophotofluorometer equipped with a phosphoroscope and transmission attachments. The spectrophotofluorometer was equipped with a 1P21 photomultiplier and a highpressure xenon lamp, as supplied by the manufacturer. The emission spectra were measured in a methanol-ethanol (4:1) or methylcyclohexane glass. The solvent was checked for emission each time a spectra was recorded. No interference due to solvent was found at any time. All compounds having short radiative lifetimes were measured by photographing the decay curve on an oscillograph. The chopper speed was adjusted manually to obtain the decay curve.The logarithmic intensities of the decay curve were plotted vs. time and the slope of the line at a logarithmic value of 2.303 gave the mean lifetime (to).Acknowledgment. The authors gratefully acknowledge the support of this research by the National Science Foundation (Grant PO-37550). Aid in the purchase of the NMR spectrometer used in this work was provided by the NSF via an equipment grant. Thanks are also due to Dr. Muhrli Dharan for some experimental assistance.
Stereoselective and Regioselective Reaction of Cyclic ortho Esters with Phenols.-The reaction of cyclic ortho esters with phenols leads to C-and/or O-addition products depending on the electron density of aryl substituent on the ortho ester. The reaction proceeds with complete control of stereoselectivity. -(BOZELL, JOSEPH J.; MILLER, DARRIN; HAMES,
Aus den Azinen (I) erhält man mit Chlor in Methylenchlorid die Addukte (II), die mit Lithiumalanat bzw. ‐deuterid (IIIa) und (IIIb) zu den Azo‐alkanen (IVa) und (IVb) reduziert werden.
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