Air-stable, recyclable, and time-efficient diphenylphosphinite cellulose-supported palladium nanoparticles as a catalyst for Suzuki–Miyaura reactions

A diphenylphosphinite cellulose palladium complex (Cell–OPPh2–Pd0) was found to be a highly efficient heterogeneous catalyst for the Suzuki–Miyaura reaction. The products were obtained in good to excellent yield under mild reaction conditions. Moreover, the catalyst could be easily recovered by simple filtration and reused for at least 6 cycles without losing its activity.

Herein, we report the synthesis of a novel diphenylphosphinite cellulose palladium complex (Cell-OPPh 2 -Pd 0 ) by a simple procedure from diphenylphosphinite funtionalized cellulose (Cell-OPPh 2 ) and PdCl 2 in ethanol solution [35][36][37][38][39][40][41][42][43][44].This class of supported palladium catalysts would solve the basic problems of homogeneous catalysts, i.e., the separation and recycling of the catalysts.This palladium complex catalyst also has the advantage of avoiding contamination of the products by residue ligand and metal.Moreover, it showed excellent catalytic activity in the Suzuki-Miyaura coupling reaction of various aryl haildes bearing electron-withdrawing and/or electron-donating groups.

Results and Discussion
Synthesis and characterization of Cell-OPPh 2 and Cell-OPPh 2 -Pd 0 It is well known that nanopalladium shows unique reactivity in various organic reactions.However, it is very difficult to use the nanopalladium as practical catalyst because of its tendency to agglomerate and its sensitivity to air and moisture.The Cell-OPPh 2 -Pd 0 catalyst was prepared with diphenylphosphinite cellulose and palladium dichloride in ethanol (Scheme 1).The as-prepared Cell-OPPh 2 -Pd 0 catalyst was characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analyses (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The Pd loading was determined to be 0.33 mmol/g by ICP-AES.The IR spectra of Cell-OPPh 2 and Cell-OPPh 2 -Pd 0 catalysts were recorded.The IR spectrum of Cell-OPPh 2 contained absorption bands at 1029.9 cm −1 for C-O-P bond.This band is negatively shifted to 1028.8 cm −1 in the Cell-OPPh 2 -Pd 0 , indi-  cating coordination of the phosphine atom with the palladium and further confirming the formation of a palladium complex on the surface of the polymer.Figure 1 shows the XRD pattern of the cellulose-supported palladium catalyst and the Cell-OPPh 2 matrix.The Cell-OPPh 2 displays a broad diffraction with 2θ ranging from 5° to 35°, suggesting the amorphous structure of the polymeric scaffold.Besides the broad diffraction ascribed to the polymeric scaffold, other three diffraction peaks at 2θ of 40.0°, 46.1°, and 67.5° for the diffraction of the (111), ( 200) and ( 220) lattice planes of the face-centered cubic crystalline structure of the Pd nanoparticles are correspondingly clearly seen from the XRD pattern of Cell-OPPh 2 -Pd 0 catalyst.
The thermal stability of Cell-OPPh 2 -Pd 0 has a great effect on its catalytic activity and recyclability because the Suzuki-Miyaura reaction usually requires heating.TGA of the  catalyst systems demonstrated high thermal stability with decomposition starting at around 250 °C under a nitrogen atmosphere.An initial weight loss of around 2.5% was observed up to 100 °C, likely due to the release of adsorbed water (Figure 2).
The morphology of Cell-OPPh 2 -Pd 0 and Cell-OPPh 2 was studied by SEM and TEM.A clear change in morphology is observed after anchoring palladium onto the polymer support (Figure 3).The TEM image of the Cell-OPPh 2 -Pd 0 catalyst shows that the average size of the nanopalladium particles is in the range of 4-15 nm (Figure 4a).The TEM image of the used catalyst indicates that the size and morphology of the nanopalladium has suffered slightly from agglomeration in the recovered catalyst after being reused six times (Figure 4b).

Suzuki-Miyaura cross-coupling reactions
To explore the efficiency of the diphenylphosphinite cellulosesupported nanopalladium catalyst, it was initially used in the Suzuki-Miyaura cross-coupling reaction, which is a versatile and a well studied method for the generation of C sp2 -C sp2 bonds in organic synthesis.The influence of base, solvent and amount of catalyst on Suzuki-Miyaura cross-coupling were carefully examined with the reaction of 4-iodoanisole and Scheme 3: Reaction of aryl halides with arylboronic acids.phenylboronic acid chosen as the model reaction (Scheme 2).The results are summarized in Table 1, Table 2 and Table 3, respectively.
We found that using K 2 CO 3 as the base in 95% ethanol at 78 °C gave the coupled product, biphenyl 3e, in 93% yield after 15 min (Table 1, entry 1).The other inorganic bases such as Na 2 CO 3 , Cs 2 CO 3 , NaOH, K 3 PO 4 •3H 2 O, CH 3 ONa, KOH and Na 3 PO 4 •12H 2 O were not as effective as K 2 CO 3 , and only afforded the coupling products in moderate to low yield (Table 1, entries 2-8).Among the screened bases, K 2 CO 3 proved to be the best and was thus chosen as the base in the Suzuki-Miyaura reaction.From Table 2, we found that using 95% ethanol as the solvent gave the highest yield, i.e., 93% (Table 2, entries 1-8).The results in Table 3 showed that the loading of catalyst has an effect on the yield.When 0.15 mmol % of the catalyst was used, the yield of the product reached 89%.On increasing the amount of catalyst from 0.3 to 0.5 mmol %, the reaction yield rose from 91% to 94%.Further increasing the amount of catalyst had apparently no significant effect on the reaction yield.Therefore, 0.5 mmol % of the catalyst was enough to push the reaction to completion (Table 3, entry 3).
To examine the scope for this coupling reaction, a variety of substituted aryl halides were coupled with different arylboronic acids in 95% ethanol in the presence of a catalytic amount of Cell-OPPh 2 -Pd 0 (0.5 mmol % Pd) with K 2 CO 3 as base (Scheme 3).The typical experimental results are summarized in Table 4.
The data presented in Table 4 show that the Cell-OPPh 2 -Pd 0 catalyst was highly effective for both aryl bromides and aryl iodides.Most of these reactions proceeded rapidly and were complete within 20 min.The catalytic performance was excellent for substrates with electron-withdrawing groups (Table 4, entries 2, 3, 19, 20 and 22) and was only slightly lower for substrates with electron-donating groups (Table 4, entries 4 and 17), except for 4-iodoanisole and 4-bromoanisole (Table 4,  entries 5 and 18).As expected, the reactivity of aryl bromides was slightly lower than that of the corresponding aryl iodides and in these cases a prolonged time was required.The reaction of aryl chlorides with arylboronic acids was sluggish and gave only small amounts of products in acceptable times.
The reusability of the supported catalyst is a very important theme from the standpoint of green chemistry and for its suitability for commercial applications.Finally, we explored the reusability of the Cell-OPPh 2 -Pd 0 catalyst again with the reaction of 4-iodoanisole with phenylboronic acid as the model reaction.After the first run, the catalyst was filtered and extensively washed with ethanol and dried in vacuo.Then the catalyst was reused directly under the same conditions mentioned above.The results are shown in Figure 5.
It can be seen from the results that the catalyst could be reused up to six times whilst still retaining good catalytic activity.Characterization of the reused catalysts by TEM showed that the slight agglomeration of the palladium nanoparticles had no apparently effect on its catalytic performance (Figure 4).

Conclusion
Nanopalladium immobilized on the surface of Cell-OPPh 2 has high catalytic activity in Suzuki-Miyaura cross-coupling reactions in 95% aqueous ethanol under atmospheric conditions.The catalyst can be easily separated and recovered from the reaction mixture by filtration and reused up to six times without any noticeable loss of activity.This simple procedure, coupled with the easy recovery and reusability of the catalyst is expected to contribute to the development of chemical processes and products.

Preparation of diphenylphosphinite cellulose (Cell-OPPh 2 )
A mixture of cellulose (3.0 g) and dry pyridine (120 cm 3 ) in a round-bottomed flask was vigorous stirred at 80-90 °C for 30 min.After being cooled to room temperature, diphenylchlorophosphine (12 cm 3 ) was added to the mixture and the reaction mixture stirred at room temperature for 5 d.The reaction mixture was filtered and the solid obtained washed with a large volume of ethanol and dried under vacuum at 60 °C to give white Cell-OPPh 2 .

General procedure for the Suzuki-Miyaura crosscoupling reaction
In a typical experiment, the Cell-OPPh 2 -Pd 0 catalyst (0.005 mmol of Pd) was added to a mixture of aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), and K 2 CO 3 (2.0 mmol) in 95% ethanol (5 cm 3 ), and the reaction mixture was stirred and heated under reflux.After the reaction was judged to be complete by TLC analysis, the catalyst was removed by filtration, washed with ethanol (3 × 4 cm 3 ) and dried under vacuum for the next run.The organic fractions were then concentrated on a rotary evaporator to obtain the desired biaryl in excellent yield.The crude products were further purified by recrystallization.All of the products (3a-3r) are known compounds and their data was identical to that reported in literature.The data of some products are as follows: 3b: Mp 113-114 °C.

Figure 4 :Scheme 2 :
Figure 4: TEM image of the fresh Cell-OPPh 2 -Pd 0 catalyst (a) and the recovered catalyst after being reused six times (b) at the same magnification.

Table 1 :
Effect of base on the Suzuki-Miyaura cross-coupling reaction a .

Table 2 :
Effect of solvent on the cross-coupling reaction a .

Table 3 :
Effect of the amount of Cell-OPPh 2 -Pd 0 catalyst on the cross-coupling reaction a .

Table 4 :
The coupling reactions of aryl halides with various arylboronic acids a .