Liquid Phase Alkylation of Anisole and Phenol Catalyzed by Niobium Phosphate

A atividade catalítica do fosfato de nióbio foi avaliada na reação de alquilação em fase líquida do anisol com 1-dodeceno, 1-octeno, 2-octanol e 1-octen-3-ol e na reação do fenol com 1-octen3-ol. Melhores resultados foram alcançados na alquilação do anisol e do fenol com 1-octen-3-ol que produziu principalmente produtos monoalquilados. Na reação com o fenol e 1-octen-3-ol os principais produtos formados foram os octenil-fenóis (C-alquilação) e o fenil-octenil éter (O-alquilação). A reação favorece a formação de C-alquilados em relação aos O-alquilados.


Introduction
The alkylation of phenol with alcohols and olefins is an important process used in industrial scale.2][3][4][5] Both homogeneous and heterogeneous catalysts have been applied to the phenol alkylation.Homogenous acid catalysts such as HF, H 2 SO 4 , AlCl 3 , or BF 3 are commonly used in Friedel-Crafts reaction, 6 but the toxic aqueous waste resulting from catalyst remains problematic.On the other hand, utilization of the ecofriendly heterogenous catalysts such as macroporous cation-exchanged resins (Amberlyst-15), 7 zeolites, 8,9 SAPO-11, 10 mesoporous materials 11 have advanced in recent years.
Conventionally, the alkylation of phenol is carried out by the reaction of phenol with 1-dodecene in presence of cation-exchange resin.Although cation-exchange resin catalysts are environmentally friendly, they have the disvantage of low stability at high temperatures.Therefore, considerable efforts have been made for the development of suitable heterogeneous catalysts.The catalysts reported for this reaction include zeolites 8,9 and others solid acids like zirconia, 1 but its poor stability limits its applicability.
2][13] Information about the use of niobium compounds for the alkylation of phenol with allylic alcohol is almost unexistent in the available literature.

General alkylation procedures
The reaction was carried out in a round-bottom 50 mL 2-necked flask provided with a condenser, and a septum for sample removal.The reaction mixture was magnetically stirred at atmospheric pressure, and the bath temperature was kept at 110 o C. The molar ratio anisole/ alkylating agent was 15/1 and for phenol/alcohol was 10/1 (150 mmol, 16.2 g anisole/10 mmol, alikylating agent; 200 mmol phenol,18.8g / 20 mmol, 2.56 g allylic alcohol) and the catalyst amount was 250 mg.The catalyst, niobium phosphate from CBMM, Companhia Brasileira de Metalurgia e Mineração, was calcined at 400 o C or 500 o C in an oven, under static air for 2 h before use.
Samples of the reaction mixture were periodically withdrawn and analyzed by high-resolution gas chromatography (n-dodecane was used as internal standard).

Analytical procedure
The variations of the substrate, alkylating agents and product contents were followed using a VARIAN model 3800 gas-chromatograph equipped with a hydrogen flame ionization detector system and capillary column VA-5, 30 m, 0.32 mm ID, 1µm df.The temperature was programmed from 80 o C to 280 o C at 20 o C min -1 with H 2 , 2 mL min -1 , as carrier gas.
The identification of the products obtained previously was carried out by gas-chromatography mass spectrometry analysis (CG-MS) on a HP 6890, utilizing a DB-5 (30m) fused silica column in the same temperature conditions with He as carrier gas.

Results and Discussion
The commercial niobium phosphate present resonable value for the surface area, 138 m 2 g -1 .The data obtained by XRD analysis showed that the niobium phosphate used in this work were amorphous.Py-FTIR characterization of these catalyst indicated the presence of Brfnsted (BAS) and Lewis acid sites (LAS) NbP-Com has almost the same amount of total Brfnsted and Lewis sites, 163.3 and 160 µmol g -1 respectively as shown in previous works. 17he catalytic activity of niobium phosphate was evaluated in the reaction of anisole with 1-dodecene, 1-octene, 2-octanol and 1-octen-3-ol; and in the reaction of phenol with 1-octen-3-ol.The results are presented in table 1.
The alkylation of anisole with 2-octanol did not proceed to any appreciable extent at the end of 7 h (entry 3).The conversion of 1-dodecene was higher than the 1-octene one after 4h, 65 and 22%, respectively (entry 1,2).This results is due to, probably, the reaction temperature that is higher for dodecene (150 o C) than for 1-octene (110 o C).Best results were achieved with 1-octen-3-ol and the conversions were total for the anisole 15 and phenol (entry 4,5).Ours results was superior to those present in the literature. 18e catalytic properties of niobium oxide were evaluated in the alkylation reaction of dodecene with benzene.At 80 o C and a benzene /dodecene molar ratio 10/1 the conversion was 1.2% after 30 min. 18he alkylation of anisole with 1-octene and 1-dodecene over niobium phosphate resulted mainly in the formation of monoalkylated products (Table 2, entry 1 and 2).
Alcohols are preferable alkylating agents rather than alkyl halides and olefins because hydrogen halides are not co-produced and no polymerization takes place.Using allylic alcohols allows formation of 1-aryl-2-alkenes. 15,20his process was shown to be of interest for producing biodegradable alkyl aromatic compounds.
After choosing allylic alcohol as the best alkylating agent we studied the alkylation of phenol aimaing to produce alkylating products (Scheme1).
The conversion of 1-octen-3-ol as function of time for the alkylation of anisole and phenol over niobium phosphate is presented in Figure 1.The reaction between phenol and 1-octen-3-ol produced mainly carbon alkylated (C-alkylated ) and oxygen alkylated (O-alkylated ) products (Figure 2).These appear to be formed in parallel and constitute isomeric (1-and 2-) octenyl-phenols and octenyl-phenyl ethers.The products distributions were presented in Figure 2.
In the alkylation of phenol the O-alkylation requires weak acid sites and low temperatures (range of 50-80 o C) while C-alkylation ocurs on stronger acid sites and higher temperatures. 2In this work the reaction temperature was superior to 110 o C and the mainly product in the phenol alkylation reaction were the C-alkylated product (65%) in accordance with the literature. 2 Dialkylation of the phenol to produce dioctenyl-phenol was limited to < 10%.The selectivity for 1-phenyl-(o,p-hydroxy)-2-octene was 65% and for 3-phenyl-(o,p-hydroxy)-1-octene, 35%.BAS (NbOH and POH) and LAS (coordinatively unsaturated Nb +5 sites) are probably responsible for alkylation reaction.However, we believe that BAS are the more important sites for the alkylation reactions because are they capable of generating a carbenium intermediate from octene, dodecene and allylic alcohol.The carbenium ion formed on acid sites attacks the aromatic compound forming the monoalkylated producs.The Py-FTIR experiments show that the commercial phosphates 15 present a high density of Brønsted acid sites with predominantly weak or moderate strenght.This acidity characteristics could be responsible for the high alkylation activity of this catalyst.The alkylation of m-cresol with t-butanol were evaluated and the authors suggested that the strong acidic sites present in the catalysts are responsible for the C-alkylation. 13A systematical theoretical study using ab initio calculation 21 and ab initio density functional theory (DFT) 12 were reported in the literature.They concluded that O-alkylation to form the phenolic ether is the product most energetically favorable in neutral conditions and an ionic rearrangement mechanism describes intramolecular migrations of the alkyl group from the phenolic ether to form C-alkylphenols.The intermediate from the C-alkylation is more stable and as a result, the O-alkylated products disappear gradually. 12,21The liquid phase alkylation of phenol with 1-octen-3-ol over niobium phosphate an ionic rearrangement of the O-alkylated products to C-alkylated products can occur.A mechanistic study are in progress in our laboratory to clarify this point.
In this work, the reaction was carried out in solvent-free liquid phase conditions, with 100% conversion of one of the reactants.Thus the niobium phosphate is also a very good environment-friendly option against the conventional solid and liquid acid catalyst.

Conclusions
The liquid phase alkylation of the anisole with 1-octene and 1-dodecene to produce octyl-and dodecyl-anisole was shown high selectivity in monoalkylation products.No dialkyl-anisole was formed under the reaction conditions.The alkylation of phenol with 1-octen-3-ol can be successfully carried out in the presence of niobium phosphate as a catalyst.This process displays good regioselectivity with respect to the 1 position of the allylic alkylating reagent and the linear product is predominantly formed.

Table 1 .
Alkylation of anisole and phenol catalyzed by niobium phosphate

Table 2 .
Products distribution in alkylation reaction