Ring-Closing Metathesis Reactions of Terminal Alkene-Derived Cyclic Phosphazenes

The first examples of ring-closing metathesis (RCM) reactions of a series of terminal alkene-derived cyclic phosphazenes have been carried out. The tetrakis-, hexakis-, and octakis(allyloxy)cyclophosphazenes (NPPh2)(NP(OCH2CHCH2)2)2 (1), N3P3(OCH2CHCH2)6 (2), and N4P4(OCH2CHCH2)8 (3) and the tetrakis(allyloxy)-S-phenylthionylphosphazene (NS(O)Ph)[NP(OCH2CHCH2)2]2 (4) were prepared by the reactions of CH2CHCH2ONa with the cyclophosphazenes (NPPh2)(NPCl2)2, N3P3Cl6, and N4P4Cl8 and the S-phenylthionylphosphazene (NS(O)Ph)(NPCl2)2. The reactions of 1−4 with Grubbs first-generation olefin metathesis catalyst Cl2RuCHPh(PCy3)2 resulted in the selective formation of seven-membered di-, tri-, and tetraspirocyclic phosphazene compounds (NPPh2)[NP(OCH2CHCHCH2O)]2 (5), N3P3(OCH2CHCHCH2O)3 (6), and N4P4(OCH2CHCHCH2O)4 (7) and the dispirocyclic S-phenylthionylphosphazene compound (NS(O)Ph)[NP(OCH2CHCHCH2O)]2 (8). X-ray structural studies of 5−8 indicated that the double bond of the spiro-substituted cycloalkene units is in the cis orientation in these compounds. In contrast to the reactions of 1−4, RCM reactions of the homoallyloxy-derived cyclophosphazene and thionylphosphazene (NPPh2)[NP(OCH2CH2CHCH2)2]2 (9) and (NS(O)Ph)[NP(OCH2CH2CHCH2)2]2 (10) with the same catalyst resulted in the formation of 11-membered diansa compounds NPPh2[NP(OCH2CH2CHCHCH2CH2O)]2 (11) and (NS(O)Ph)[NP(OCH2CH2CHCHCH2CH2O)]2 (13) and the intermolecular doubly bridged ansa−dibino−ansa compounds 12 and 14. The X-ray structural studies of compounds 11 and 13 indicated that the double bonds of the ansa-substituted cycloalkene units are in the trans orientation in these compounds. The geminal bis(homoallyloxy)tetraphenylcyclotriphosphazene [NPPh2]2[NP(OCH2CH2CHCH2)2] (15) upon RCM with Grubbs first- and second-generation catalysts gave the spirocyclic product [NPPh2]2[NP(OCH2CH2CHCHCH2CH2O)] (16) along with the geminal dibino-substituted dimeric compound [NPPh2]2[NP(OCH2CH2CHCHCH2CH2O)2PN][NPPh2]2 (17) as the major product. The dibino compound 17, upon reaction with the Grubbs second-generation catalyst, was found to undergo a unique ring-opening metathesis reaction, opening up the bino bridges and partially converting to the spirocyclic compound 16.