High-Performance Materials from Bio-based Feedstocks. Группа авторов
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series of different steroids was successfully functionalised in yields of 88–96%, with a lower yield of 60% being obtained with a steroidal epoxide of the opposite configuration to that shown in Figure 3.7. Similar yields were obtained when methylthioacetonitrile (MeSCH2CN) was used, with the product being the MeSCH2C(O)N‐product. With MeSO2CH2CN as the amide, oxazoline was formed by trapping of the intermediate carbocation (formed by attack of the nitrile on the protonated epoxide) by the vicinal hydroxy group.
Figure 3.6 Conversion of xylose to furfural and extraction of furfural.
Figure 3.7 Functionalisation of steroids via Starbon acid‐catalysed Ritter reaction.
3.2.3.1.4 Sulphonated Starbon in Acylations and Alkylations
Luque et al. have also published research on a range of different Starbon acids, with sulphonic and carboxylic groups attached, as well as ZnCl2 and BF3 adsorbed onto the surface [29]. They characterised the nature of the acidity by pyridine titration and found that the sulphonic acid systems had both Brønsted and Lewis acidity. In contrast, the two Lewis acid‐doped materials had a greater Lewis acidity (as expected) but an unexpectedly low quantity of acid sites. The various materials were tested in the acetylation of 5‐acetyl methyl salicylate, and were found to be very active in the O‐acetylation, with Friedel Crafts acylation being absent despite the presence of Lewis acidic sites (Figure 3.8a). Alkylation of phenol with cyclohexene was also investigated (Figure 3.8b). While O‐alkylation was very dominant, especially after short reaction times, C alkylation (mainly in the ortho position) increased upon prolonged reaction. The reasons for this are not discussed in the paper, but similar behaviour is seen with some other catalysts. This may be due to a change in the nature of the catalyst with time, or more likely, to a slow Fries rearrangement reaction following a rapid O‐alkylation.
Further alkylations (of toluene and xylenes) with benzyl chloride have also been successfully carried out using Starbon‐400‐SO3H under microwave conditions. High yields (70–95%) were achieved after 15 minutes for toluene and p‐xylene, while longer times were required for m‐xylene and, surprisingly, anisole [17].
Figure 3.8 Friedel Crafts reactions catalysed by a range of Starbon acids. (a) O‐Acylation of a phenol and (b) O‐alkylation of a phenol.
3.2.3.1.5 Supported Metal Complexes
Only a very few examples of supported metal complexes and their catalytic activity have been reported, perhaps surprising, given the large numbers that exist with silica as support. Interestingly, little has been done to extend the well‐established organo‐silica chemistry beyond the initial studies by Doi et al. [30] who used expanded starch (i.e. non‐pyrolysed Starbon) as a support. Two approaches that successfully attach complex catalytic species to the surface of Starbon are available, and are discussed next.
Matharu et al. [23] published details of an Fe‐NHC catalytic system (NHC = N‐heterocyclic carbene), which they tethered to the surface of a starch‐based Starbon‐350 and also to the surface of the precursor‐expanded starch (Figure 3.9). They utilised a multistage ligand synthesis, anchored the ligand to the surface, and then finally attached the metal species [23].
The synthesis of the heterocyclic ligand containing amine functionality as an anchor was carried out. Separately, Starbon surface was functionalised with a succinimyl carbonate group, following an adapted literature procedure, where the toxic dimethyl formamide (DMF) solvent was replaced with propylene carbonate, a safer alternative to dipolar aprotics [31]. Finally, the ligand was bound to the functionalised Starbon surface. Anchoring of the ligand system was achieved by reaction of the functionalised Starbon with the amine pendant on the ligand moiety. The degree of substitution achieved for the succinimidyl carbonate grafting was 0.33, approximating to 1 in 10 hydroxyls being substituted (in the case of the expanded starch, this is likely to be somewhat higher for the Starbon‐350, although the complexity of the structure is much greater with a wider range of functionalities). Given the extensive H‐bonding and steric hindrance pertinent to the majority of the hydroxyls in such polysaccharides, this is a reasonably significant degree of substitution, and led to metal centre loadings of 0.26 and 0.3 mmol g−1, well within the range of loadings achieved for highly porous silicas.
Catalytic activity was very promising in the dehydration of fructose to 5‐hydroxymethyl‐2‐furaldehyde (HMF), with the expanded starch catalyst slightly outperforming the Starbon‐350 material (86% vs. 81% yield after 0.5 hour at 100 °C). Reuse was also very good, with consistent performance over 5 runs, and no discernible leaching of iron. Given the simpler route to the expanded starch material, it is clear that this is the catalyst of choice here.
While Matharu’s approach utilised the abundant hydroxyl groups on the surface of the material, Silva and co‐workers have described the synthesis of a chiral bis‐oxazoline catalyst on the surface of a Starbon‐700 material utilising the abundant unsaturation present in the higher temperature materials [24]. Given the low O content of these higher temperature materials, the typical functionalisation routes involving surface hydroxyls (i.e. reaction with silane esters such as (RO)3SiR′ [30] and the succinimidyl carbonate route described earlier) are unsuitable. To circumvent this problem, Carneiro et al. [24] utilised a bromine functionalisation, reacting bromine with double bonds on the surface, to give a 1,2‐dibromo‐functionalised surface (Figure 3.10). As the ligand to be attached is a diol, the formation of pairs of anchor sites close together is particularly attractive.
Figure 3.9 Synthesis of an N‐heterocyclic carbine‐based catalyst on the Starbon surface.
The resultant materials were used in the kinetic resolution of 1,2‐diphenylethane‐1,2‐diol. While activity was high, the enantiomeric excess of the process was significantly lower than that obtained by a solution‐phase process (Figure 3.11). This was attributed to a low concentration of catalytic groups on the surface of the catalyst, and the high conversion suggests non‐chiral‐active sites may also be present.
3.2.3.1.6 Photocatalytic Processes
A further aspect of Starbon catalysis relates to the photocatalytic decomposition of water pollutants. Colmenares and colleagues have recently published work [32] that illustrates the excellent activity of Starbon‐based titanium dioxide photocatalysts