N-Aryl Acylureas as Intermediates in Sequential Self-Repetitive Reactions To Form Poly(amide−imide)s

Abstract

Introduction. Carbodiimides (CDI), easily prepared from isocyanates, 1 have been known to react with carboxylic acids to form a mixture of acid anhydrides (anhydrides), N, N′-substituted ureas (ureas), and N-acyl-N,N′-disubstituted ureas (or N-acylureas). Earlier mechanistic studies by Khorana 2,3 and Silverstein 4,5 showed that two parallel reaction pathways might account for the diversity of products observed. The initial formation of an O-acylisourea intermediate that can either rearrange into N-acylurea or undergo a further substitution reaction with another acid molecule to produce the corresponding ureas and anhydrides as final products (Scheme 1). The earlier literature indicated that aromatic CDIs seem to favor the formation of N-acylureas upon treatment of carboxylic acids, whereas aliphatic CDIs often lead to the formation a mixture of anhydrides and N,N′-disubstituted ureas. 6 Recent evidence has shown that ferrocenecarboxylic acid was able to selectively add onto an aromatic CDI to yield an N-acylurea as the main product. 7 In another report by Lau, 8 the synthesis of di-and trisubstituted N-acylureas on a solid support was also achieved in excellent yields.Although the thermal conversion of an N-acylurea into an isocyanate and an amide has been well documented, 9 the overall transformation from isocyanate to CDI and then from CDI to amide through thermolysis of the N-acylurea has not been fully exploited either as synthetic intermediates or as latent isocyanate sources. The contamination by urea and anhydride as byproducts may prevent a clean isolation of N-acylurea. To the best of our knowledge, high-yield formation of an N-acylurea as an isocyanate precursor or as an isolable intermediate for polyamide synthesis has not been developed previously. Herein, we report the preparation of aryl N-acylureas in high yield, their consequential thermal reactions, and applications in a stepwise synthesis of aryl amides and polyamides.Results and Discussion. In our recent selectivity study of CDI reactions, carboxylic acids were allowed to react separately with two CDI model compounds: dicyclohexylcarbodiimide (DCC) as an aliphatic CDI whereas diphenylcarbodiimide (DPCDI) which was prepared from phenyl isocyanate and a catalytic amount of 1,3-dimethyl-3-phospholene oxide (DMPO) as the aromatic CDI (Scheme 1). In doing so, we found two distinctive product types indicating the existence of different pathways in the CDI reaction (Table 1). Using DCC as starting material to react with benzoic acid 2a or acetic acid 2g, for example, the reaction yielded anhydrides and ureas as the major product. Low yields (38 and 25%; entry 1 and 2) of N-acylurea were observed in the product mixture. Other carboxylic acids also afforded poor yields of N-acylureas when treated with DCC.The selectivity for N-acylurea was enhanced dramatically when DPCDI was used instead of DCC. The migration of the acyl group from the O to N atom in the initial isourea seems to be dominant. For example, treatment of DPCDI with acetic acid, 2a, at ro...

Well-Defined Polyamide Synthesis from Diisocyanates and Diacids Involving Hindered Carbodiimide Intermediates

“…17 In our previous paper in this series, we described a "sequential self-repetitive reaction (SSRR)" for the synthesis of polyamide through condensation of diisocyanate with diacid in the presence of a carbodiimide (CDI) catalyst. 18,19 The standard "SSRR" reaction involves three basic repetitive yet sequential reactions including: (i) condensation of two isocyanates into a CDI in the presence of a CDI catalyst (DMPO) 20 (ii) addition of acid to the CDI to yield N-acylurea and (iii) thermal fragmentation of the Nacylurea to form amide and isocyanate fragments both in half of the original molar amount, (Scheme 1). It was observed that step i and step ii were very facile even at a ambient temperature, whereas step iii could only be effective at a temperature of greater than 140 °C.…”

“…[21][22][23] However, when this three-step cycle repeats under heated conditions with sufficient acid, all of the isocyanate and CDI are converted into amide as the final product in excellent conversion and selectivity. 18,19 By extending this unique methodology, we have devised a new twist to the SSRR process by addition of small amount of a hindered carbodiimide (CDI) such as N,N 0 -bis(2,6-diisopropylphenyl)carbodiimide, iPr-CDI 1, or a hindered isocyanate such as 2,6-diisopropylphenyl isocyanate, iPr-NCO 7. Doing so creates terminally new hindered-CDIs (PiPr-CDI type 10, Scheme 4) on the growing polymer chains.…”

Nonlinear optical, poly(amide-imide)–clay nanocomposites comprising an azobenzene moiety synthesised via sequential self-repetitive reaction

“…In 1 H NMR spectrum of PIM, the peak at 10.4 ppm vanished and a new peak at 10.5 ppm emerged. This single peak at 10.5 ppm was the characteristic absorption of amide proton, indicating the completion of SSRR [29].…”

“…In this research, we report a series of new NLO-active poly(amide-imide)-clay materials prepared via SSRR [19,29]. Difunctional diamino diphenyl methane (DDM) was chosen to be the intercalating agent.…”

Formation of Heterodimeric Resorcin[4]arene Capsules on Surfaces: An X-Ray Photoelectron Spectroscopy Investigation

“…[14] The achieved labelling turned out to be unaffected by this, reaching efficiencies about 0.43, as long as individual labels, such as the used benzoic acid were given to the labelling solution in sufficient surplus. In a case where the carboxyl functionalities were bound to the resorcinarene at explicit positions, on the other hand, an inhibition of capsulation by dehydration urea reaction products could be assumed.…”