Dendritic compound of triphenylene-2,6,10-trione ketal-tri-{2,2-di-[(N-methyl-N-(4-pyridinyl) amino) methyl]-1,3-propanediol}: an easily recyclable catalyst for Morita-Baylis-Hillman reactions

A novel Dendritic Compound (2) of triphenylene2,6,10-trione ketal-tri-{2,2-di-[(N-methyl-N-(4-pyridinyl) amino)methyl]-1,3-propanediol} was conveniently synthesized by aromatization of cyclohexanedione mono-ketal, ketal-exchange reaction with 2,2-dibromomethyl-1,3-propanediol and nuclophilic substitution with N-methylaminopyridine as nuclophilic reagent. The Morita-Baylis-Hillman reaction of various aryl aldehydes with methyl vinyl ketone and acrylonitrile in (DMF/cyclohexane, 1/1, v/v) has been investigated by using Dendritic Compound (2) as catalyst. The corresponding Morita-BaylisHillman adducts was obtained in good yields by using the recycled and reactivated dendritic catalyst.


INTRODUCTION
The Morita-Baylis-Hillman reaction possesses cheap and easy availability of starting materials, easing performance, atom economy, forming chemo-specific functional groups in the product, providing an avenue for introducing asymmetry, and fitting for simulation on the solid phase as a prelude for combinatorial synthesis represent some of the reasons, which have led to an exponential increase in the synthetic utility of this reaction [1,2,3,4,5,6,7,8,9,10].The Morita-Baylis-Hillman reaction can be promoted by using organic bases.However, almost all the Morita-Baylis-Hillman reactions reported so far use small molecular homogeneous catalyst, and it impedes the reusing of the catalyst.In addition, this reaction could be accomplished in a perfect atom-economic way if a recyclable Lewis base was employed as the promoter.Recently, Corma et al. [11] developed a heterogeneous catalyst system by using an insoluble Merrifield type resin-supported 4-(N-benzyl-N-methyl amino)pyridine as reusable catalyst for the Morita-Baylis-Hillman coupling of aromatic aldehydes and unsaturated ketones.Shi [12] reported the use of soluble polymer-supported Lewis bases, such as PEG4600-(PPh2)2 and linear poly (DMAP) in the Morita-Baylis-Hillman reactions of Ntosylimines with unsaturated ketones.
Yang [13] employed the dendritic Lewis base as the catalyst together with a binary solvent system (DMFcyclohexane, 1:1, v/v) ,which could become homogeneous when heated up to 60 • C and then could be readily separated by cooling the system to room temperature.

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JBiSE Scheme 1 The Compound (1) was prepared by one-pot method.Through the exchange reaction after the aromatization, the five-membered ketal transformed to the more stable six-membered ketal.It omitted the hydrolysis of the ketal and separation steps.The reaction was in the Lewis acid, so the dehydration was reduced.
Because Compound (1) contains six bromine atoms, the amount of the alkaline must be controlled seriously in case of more side-reaction such as HBr elimination.The content of 30%NaH should be tested before use.Even so, they were forming some cross linked macromolecules.These were obviously increased in the NaOH/H 2 O system.
The advantages of grafted DMAP to dendritic compound are as follows: for the macromolecular compound can be dissolved in most of the organic solvent instead of water, so it can be isolated by adding water after reaction and be used after activation by alkaline; for it has large cavities and dissolves in organic phase, it is helpful for contacting the reactant and catalyst and thus the reaction rate is higher than that of the polymer carried DMAP.So it combines the advantages of both forms of DMAP.
In order to determine the activity of this dendritic catalyst, we chose the coupling of 4-nitrobenzaldehyde with methyl vinyl ketone (MVK) as the model reaction, which is a paradigmatic example of the Morita-Baylis-Hillman reaction.In all cases only the normal Morita-Baylis-Hillman product was found [20,21,22].The results were summarized in Table 1.As seen from the table, the highest yield was achieved when the molar ratio of aldehyde:MVK: dendritic-DMAP being 1:3:0.2,making it in the condition that the reacting time reach 48 h (Table 1, Entry 5).
The dendritic catalyst could be easily recovered by cooling the reaction mixtures to room temperature and adding water at the end of the reaction.In order to regain the catalyst activity, the deactivated catalyst was treated with 2M NaOH at 45 • C for 2 h and subsequent was applied to the reaction under the same conditions, giving good yields.(see Table 1: Entries (9-13)) To demonstrate the applicability of dendritic DMAP as a homogeneous catalyst for the Morita-Baylise-Hillman reaction, various aldehydes were tested as reactants reacting with MVK or acrylonitrile and the results are shown in Table 2.It was found that the aryl aldehydes had strongly electron-withdrawing groups on the benzene rings such as dinitrobenzaldehyde and nitrilbenzaldehydes reacting with MVK and acrylonitrile gave the corresponding Morita-Baylis-Hillman adducts with high yields (1a,2a, 4a,7a,10a, 1b,2b, 4b,7b,10b).On the contrary, other aryl aldehydes, in particular with electron-donating substituents, provided low yields.

CONCLUSIONS
In summary, we have described a new catalytic system by using dendritic derivative of 4-(N,N-dimethylamino) pyridine as catalyst for the Morita-Baylis-Hillman reactions of aryl aldehydes with MVK and acrylonitrile.The recyclability and applicability of this catalytic system has been well demonstrated.

General Remarks
Aryl aldehydes and acrylonitrile were stored under a nitrogen atmosphere before using.Other commercially supplied reagents were used as 4supplied without further purification.Organic solvents were dried by standard methods when necessary.1H NMR spectra were recorded on an INOVA400 spectrometer for solution in CDCl3 with TMS as internal standard.Varian MAT 44S MS; Cavlo Eba 1106 elemental analysis instrument; WRR melting point apparatus (Shanghai Precision Instrument Co. Ltd.).

Table 2 .
The Baylis-Hillman reactions of aryl aldehydes with active alkene in the presence of Cat 1.