Structural and reactivity studies of new organophosphorus amides

Abstract

The bicyclic substrates 1-oxo-2,8-diaryl-2,5,8-triaza-1<font face="symbol">l</font face>5-phosphabicyclo[3.3.0]-octane 3 were studied before. The molecular rearrangement of the alcoholysis product of 3 (the eight-membered ring compound 1-oxo-1-ethoxy-2,8-diaryl-2,5,8-triaza-<font face="symbol">l</font face>5-phosphacyclooctane) to the five-membered ring isomer (the 1,3,2 <font face="symbol">l</font face>5-diazaphospholidine system) is reasonably well understood for the N,N’-diaryl substituted substrates. It was decided to expand the studies of the bicyclic system 3 to other derivatives with aliphatic substituents on the nitrogen atoms (R=PhCH2, Me, Et), as well as the thiophosphoryl and phosphine analogues of 3. Differences between the N-aryl and N-alkyl substrates were observed in the acid and base catalyzed solvolysis of the bicyclic substrate. The reactivity in the rearrangement of the solvolysis product, from an eight-membered ring to the five-membered ring isomer was also different. The N,N’-dialkyl substituted compounds rearranged much slower to the corresponding five-membered ring compounds than the N,N’-diaryl analogues. The thiophosphoryl analoque of 3a (R=Ph), 1-thio-2,8-diphenyl-2,5,8-triaza-1<font face="symbol">l</font>5–phosphabicyclo[3.3.0]octane 11a was prepared by reacting bis-(2-phenylamino-ethyl)amine with P(S)Cl3 in the presence of a base. The alcoholysis product of 11a, observed in 31P NMR (<font face="symbol">d</font>P 76), was the eight-membered ring compound 15. This compound then rearranged to the five-membered ring isomer 16 during GC-MS analysis. This rearrangement is analogous to the rearrangement observed for the corresponding phosphoryl derivatives. Both the thiophosphoryl bicyclic 11a and the phosphoryl bicyclic 3a compounds were detected in the mass spectrum of compound 15. This could be explained in terms of the thiono (P(S)OR) to thiolo (P(O)SR) rearrangement. The NMR spectra of the phosphoryl and thiophosporyl bicyclic compounds 3a and 11a proved to have distinct differences in the aliphatic region as far as coupling constants are concerned. From the crystal structures it was clear that the two halves (two five-membered rings) of 3a and 11a had remarkably different dihedral angles. The NMR data represented an average of the two rings, therefore they appear identical on the NMR-scale. Comparing the dihedral angles as determined from NMR data, by utilization of the Karplus equation, with the dihedral angles obtained from X-ray diffraction data was only approximate. There was very little correlation between the experimental and the calculated dihedral angles for compounds 3a and 11a. An average value of the dihedral angles, resulting from NMR data, was not in agreement with the crystal structures. The MM+ force field of HyperChem® was adapted to perform molecular mechanical calculations in an effort to enhance the conceptualization of the properties and the behaviour of these new heterocyclic compounds. The calculated energies of the eight-membered ring and five-membered ring isomers, for all the different derivatives, confirmed that the rearrangement is thermodynamically controlled. The five-membered ring isomer in each case had lower total strain energy than the eight-membered ring isomer. The thiono and thiolo isomers had comparable potential energies. The thiono isomer of the N-Benzyl derivative had a slightly lower potential energy than the thiolo isomer, for both the eight- and five-membered ring isomers. The calculated energies for both the thiono and the thiolo isomers suggested that the five-membered ring isomer was thermodynamically more stable than the eight-membered ring compound.