Crystallographic proof of double Walden inversion in nucleophilic substitution reactions of macrocyclic cyclotriphosphazene derivatives

Using X-ray crystallography it is demonstrated unambiguously that a double Walden inversion reaction occurs for successive nucleophilic substitution of the mono-spiropropanoxyamino derivative of the cis-ansa-macrocyclic cyclophosphazene compound N3P3[O(CH2)(3)NH][O(CH2CH2O)(4)]Cl-2 (3). The spiropropanoxyamino moiety enables groups above and below the plane of the N3P3 ring to be distinguished. The first nucleophilic substitution of compound 3 with X- (e.g., X = 2-naphthoxy) gives N3P3[O(CH2)(3)NH][O(CH2CH2O)(4)]XCl (4a), which has a trans-ansa-macrocyclic ring as a result of the inversion reaction, and then subsequent reaction of 4a with the same nucleophile gives N3P3[C(CH2)(3)NH][O(CH2CH2O)(4)]X-2 (5a), in which the macrocyclic ring is cis-ansa again, but it is now on the opposite side of the N3P3 ring from that of the starting material 3 as a result of the second inversion reaction. Structures stereo chemically analogous to compound 5a were also obtained upon reaction with other monofunctional nucleophiles, such as phenol, pyrrolidine and aniline, to give compounds 5b, 5c and 5d, respectively, and with the difunctional nucleophile 2,2,3,3-tetrafluorobutanediol to give the di-ansa derivative 6. Compound 3 was also sequentially treated with two different mononucleophiles - phenol and aniline - to give the unsymmetrically disubstituted compound 7, in which the macrocyclic ring is also cis-ansa again and on the opposite side of the N3P3 ring from that of the starting material, as a result of the double Walden inversion reactions. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).