P 2 O 5 /SiO 2 an efficient, green and heterogeneous catalytic system for the solvent-free synthesis of N -sulfonyl imines

Silica-supported P 2 O 5 (P 2 O 5 /SiO 2 ) was used as an efficient, green and cheap catalytic system for the synthesis of N -sulfonyl imines via condensation of sulfonamides with several aldehydes under solvent-free conditions. The reactions proceeded rapidly at 110 °C and the desired products were obtained in high to excellent yields


Introduction
Imines bearing electron-withdrawing N-substituents are useful intermediates in organic synthesis. 1Among them, N-sulfonyl imines are the centre of attention for organic chemists because the sulfonyl moiety has proven to be a powerful activating group of the C=N bond in these compounds.As a consequence, N-sulfonyl imines have been widely used in organic synthesis. 2In addition, they are excellent substrates in nucleophilic additions, 3 reductions, 4 aza Diels-Alder reactions, 5 aziridine 6 and oxaziridine synthesis 7 as well as ene reactions. 8Several kinds of synthetic routes toward N-sulfonyl imines have been developed namely via the Lewis acid catalyzed reactions of sulfonamides with aldehyde precursors, 9 rearrangement of oxime Osulfinates, 10 tellurium mediated reaction of aldehydes with chloramines T by utilization of in situ generated N,N'-ditosyltellurodiimide, 11 application of N-sulfinyl sulfonamides instead of sulfonamides to generate sulfonyl imine in situ via a [2+2] cycloaddition and extrusion of sulfur dioxide, 12 generation of sulfonamidosulfones and basic elimination, 13 and catalyzed isomerization or rearrangement of N-sulfonyl aziridines. 14t is worth noting that the methods that have been established for the preparation of Nsulfonyl imines are associated with one or more of the following drawbacks: (i) long reaction times, (ii) unsatisfactory yields, (iii) formation of toxic byproducts, and (iv) the use of expensive and hazardous reagents.Furthermore, some other procedures need cumbersome experimental and multi-step procedure.Therefore, it seems highly desirable to find a one step, and inexpensive protocol for the synthesis of N-sulfonyl imines.The aim of this work is to report a simple protocol that makes these valuable building blocks readily available.
As it is known Green Chemistry (a simple definition) is recommended with the corresponding reference.In this sense, heterogeneous organic reactions have many advantages, such as ease of handling, low corrosion, minimum execution time, environmentally safe disposal and waste minimization by developing cleaner synthesis routes. 15Another important goal in green chemistry is represented by the elimination of volatile organic solvents, in fact, solventfree conditions that makes synthesis simpler, saves energy, and prevents solvent waste, hazards, and toxicity. 16Consequently, it is important to note that the combination of heterogeneous catalysis with the use of solventless conditions represent a suitable way toward the so-called ideal synthesis. 17ilica-supported phosphorus pentoxide (P 2 O 5 /SiO 2 ) is an inexpensive, heterogeneous and commercially available catalytic system which has been used in several transformations, such as condensation of indoles with carbonyl compounds, 18 oxidation of sulfides to the corresponding sulfoxides, 19 nitration of aromatic compounds, 20 conversion of aldehydes to nitriles, 21 acylation of amines, 22 deprotection of 1,1-diacetates, 23 Fries rearrangement, 24 and acetalization of carbonylic compounds. 25onsidering the above subjects and also along with our previous studies on green organic synthesis, 9a,18,26 herein, is reported an efficient, green and simple method for the production of a set of eighteen N-sulfonyl imines from benzene, p-toluene sulfonamides and aromatic aldehydes in the presence of P 2 O 5 /SiO 2 under solvent-free conditions, Scheme 1.

Results and Discussion
In order to optimize the reaction conditions, the condensation of 4-methylbenzenesulfonamide with benzaldehyde was studied as a model reaction to provide compound 3a (Scheme 1).At a first scope, the reaction was examined in the presence of catalytic amount of P 2 O 5 at 110 °C under solvent-free conditions; however, the product was obtained with low yield even after prolonging the reaction time (Table 1, entry 1).In these conditions, a sticky reaction mixture was obtained.The yield was remarkably increased when P 2 O 5 supported on silica gel (P 2 O 5 /SiO 2 ) was applied as catalyst (Table 1, entry 2).The influence of other supports on the reaction was also investigated (Table 1), as it can be seen, silica gel promoted higher yields.Therefore, silica gel was the support of choice for all reactions.It is also worth mentioning that the model reaction was also carried out in the presence of silica gel without P 2 O 5 under solventless conditions at 110 °C, however compound 3a was afforded in trace yield within 6 hours.To compare the efficiency as well as capacity of the solvent-free conditions with respect to solution conditions, the model reaction was examined in the presence of silica-supported P 2 O 5 in several solvents; the corresponding results are depicted in Table 2.As it can be seen, the solventfree method is more efficient.ARKAT USA, Inc.
The efficiency and applicability of this new procedure was compared with some reported methods for the preparation of N-sulfonyl imines (Table 3).As Table 3 demonstrates, our method afforded the better results.
In order to establish the generality of this method, the benzene and p-toluene sulfonamides were condensed with various aldehydes to furnish N-sulfonyl imines in high to excellent yields in relatively short reaction times; the respective results are displayed in Table 4.The influence of both electron-releasing and electron-withdrawing substituents on the aromatic ring of aldehydes upon results of the reaction was investigated.The results showed that electron-releasing substituents had no significant effect on the reaction times and the yields (Table 4, entries 2-4 and 14).However, the presence of electron-withdrawing substituents on the aromatic ring of aryl aldehydes increased the reaction times and decreased the yields (Table 4, entries 5-7 and 15).Moreover, the presence of a halogen on the aromatic ring of aldehydes had negligible effect on the reaction yields; however, these substituents increased the reaction times (Table 4, entries 8-10, 16 and 17).
.b  In this reaction, bis-N-sulfonyl imine was obtained in very low yield.
All chemicals were purchased from Merck or Fluka chemical companies.Silica gel 60, 0.063-0.200mm (70-230 mesh ASTM) was used as support.All known compounds were identified by comparison of their melting points and 1 H NMR data with the authentic samples.The 1 H NMR (250 MHz) spectra were run on a Bruker Avanced DPX-250, FT-NMR spectrometer.Mass spectra were recorded on a Shimadzu GC MS-QP 1000 EX apparatus.Microanalyses were performed on a Perkin-Elmer 240-B microanalyzer.Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.Preparation of P 2 O 5 /SiO 2 catalytic system.A mixture of SiO 2 (2 g) and P 2 O 5 (1 mmol, 0.142 g) was ground vigorously to give P 2 O 5 /SiO 2 catalytic system as a white powder (2.142 g).

Table 1 .
The effect of different supports on condensation of 4-methylbenzenesulfonamide with benzaldehyde in the presence of P 2 O 5 a Isolated pure product.b Without support.

Table 2 .
Comparative the reaction of 4-methylbenzenesulfonamide with benzaldehyde in the presence of P 2 O 5 /SiO 2 using solution conditions versus the solvent-free method a Isolated pure product.b The Solvent-free conditions.

Table 3 .
The comparative synthesis of compound 3a using the reported methods versus the present method a Isolated pure product.