Phosphazenes reactions with isocyanates

D. Pseudohalogeno-derivatives.—Little work has been carried out in this area. Isocyanates of cyclic phosphazenes, previously unknown, are thought to be formed in the reaction of NgPaBrg with AgOCN in nitro-methane. They were detected by i.r. spectroscopy, and underwent ready polymerization, which precluded their isolation. On the other hand, isothiocyanates, [NP(NCS)2] (n = 3 or 4), are well known and a detailed study of their spectra has been reported. The azide, N3Pa(N3)8, has been the subject of an i.r. study which suggests that the molecule has Z)3A symmetry. 

N-vinylic phosphazenes react with isocyanates to yield N-vinylic carbodiim-ides. The reaction of isothiocyanates and an N = P moiety has been used to prepare oxazole derivatives The reactivity of the — N = P moiety in synthetic procedures has also been demonstrated by the aza-Wittig reaction with aldehydes " ... 

A-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 -I- 21-cycloaddition-cycloreversion sequence. " The presence of an alkyl substituent in position 3 of A-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed." Other Aza-Wittig reactions include the reaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia, useful carbodiimides for the selective synthesis of pyrimidones." Also the iminophosphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on temperature and aldehyde, trans imines or mixtures of trans and cis imines." " The... 

The aza-Wittig reaction of phosphazenes with heterocumulenes such as isocyanates (X = NR) and ketenes (X = CR2) has been extensively used for the formation of carbodi-imides and ketenimines, respectively (Scheme 15.9). 

This methodology has also been used for the preparation of several alkaloids (Scheme 15.14). Syntheses of the marine alkaloids, isonaamine A (67a), dorimidazole A (67b) and pieclathridine A (67c), have been described by aza-Wittig reactions of phosphazenes 63 with tosyl isocyanate, followed by addition of amine 65 to the carbodiimide intermediate 64 and subsequent intramolecular cyclization of compound 66. 

Blends of polyurethane with [NP(0C6H4C02H)2] have been prepared under conditions which enable reactions of the carboxylic group with the isocyanate group. Loadings of 20 wt% phosphazene or more induce a significant increase in flame resistance.This finding corresponds with the... 

Usually, carbodiimides obtained by an aza-Wittig reaction of A-vinyUc phosphazenes with isocyanates cannot be isolated. Therefore, the very reactive carbodiimides can be used as synthetic intermediates of polyheterocyclic natural products by domino processes involving aza-Wittig/intramolecuIar cycUzation (AW-IC). In the synthesis of variolin B (58), the formation of the annulated 2-aminopyrimidine ring 57 is achieved from phosphazene 56 by a tandem aza-Wittig/carbodiimide-mediated intramolecular cyclization process (Scheme 15.12). 

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