We consider that biotechnological approaches utilizing the enzymatic machinery described above should now be explored to produce allyl and propenyl phenols, diverting (to a predetermined extent) some of the carbon flow, for example, from lignin toward these metabolites, while maintaining the resulting plants’ capacities to grow and function within acceptable boundaries. The resulting cellulosic biomass could thus become more amenable to pulp/paper manufacture and/or biofuel production due to the lower lignin contents. Alternatively, oilseed metabolism could be manipulated to produce these allyl/propenyl phenol substances in larger amounts. Upon processing and purification, these compounds could potentially find uses as biofuels, biofuel precursors, flavors/fragrances, and/or intermediate chemicals (e.g., as monomers for synthetic polymers).
Driven by consumer preferences toward truly ‘‘natural’’ food products, food additives, and pharmaceuticals, several biocatalytic processes for production of plant-derived metabolites using microbes and plant cell cultures have been developed and patented in
Much larger anticipated potential markets for the allyl/propenyl phenols include the industrial polymers and biofuel/biodiesel. Regarding polymer applications, the expected worldwide production of polystyrenes alone was approximately 25 million metric tons in 2006, representing sales of US$ 31 billion. Allyl/ propenyl phenols can be converted into functionalized polystyrene derivatives, and an increased supply creates the potential for their massive usage as intermediate (monomer) chemicals in industrial polymers. Currently, existing applications include eugenol- (33) based polymers, which are widely used in dentistry in zinc oxide impression pastes applied as surgical dressings and temporary cements (Skinner, 1940; Weinberg et al., 1972), as well as specialty modifying (i.e., coating) agents in analytical electrodes (Ciszewski and Milczarek, 1998, 1999, 2001, 2003; Rahim et al., 2004).
The functionalized β-methylstyrenes anethole (38) and isoeugenol (39) can be converted into polymers of several thousand dalton (Bywater, 1963). However, the potential of such conversions has been studied only to a limited extent relative to their vinyl analogues (i.e., styrene and derivatives thereof), in part due to limited supply/availability and because propenylbenzenes do not apparently undergo as efficient free radical polymerization reactions as styrenes—even though their electronic configuration is such that a radical intermediate can also be stabilized by the aromatic ring (Alexander et al., 1981). The most efficient polymerization initiators described thus far for propenylbenzene derivatives are Lewis acids, particularly AlCl3, SnCl4, and BF3 (Alexander et al., 1981; Cerrai et al., 1969a,b; Secci and Mameli, 1956).
In general, the steric factors on the monomers define, to a large extent, both polymerization rates and molecular weights of the resulting polymers, with anethole (38), for example, being more reactive than isoeugenol (39) (Alexander et al., 1981). Polymerization reactions can proceed through a conventional 1,2-chain formation, similar to styrene, with the propagating species being a Lewis acidinduced carbocation that is added to the double bond of another monomer. This results in a polymer backbone composed of the carbons 7 and 8 of the original monomers. The molecular weights of the resulting polymers are higher at lower temperatures and, in the case of anethole (38) when polymerized by SnCl4, can vary from a few thousand up to about 75,000 Da depending on both temperature and dilution levels (Bywater, 1963; Cerrai et al., 1969a; Secci and Mameli, 1956). For isoeugenol (39), the phenolic oxygen moiety also participates in the polymerization reactions, thereby increasing the structural complexity of the resulting polymer(s) so formed (Fig. 13.20A) (Evliya and Olcay, 1974). Additionally, allylphenols, such as methylchavicol (32) and eugenol (33), can form mixed polymers, resulting from the partial rearrangement of the side-chain double bond upon carbocation formation prior to attachment to the polymer chain (Cihaner et al., 2001; Kennedy, 1964) (Fig. 13.20B).
In terms of their potential uses as biofuels, it is noteworthy that ~100 billion gallons of gasoline fuel were consumed in the United States in 2005. In addition, the annual consumption of diesel fuel in 2000, including highway diesel, farms, electric power, railroad, fuel oil (residential, commercial, and heating), and kerosene, totaled approximately 57.1 billion gallons. As a measure of the potential scale of production of biodiesel (from vegetable oils, consisting mainly of fatty acid esters), Peterson (1995) recently estimated that if all harvested cropland (~363 million acres) in the United States was dedicated exclusively to rapeseed (oilseed) production, then approximately 36.3 billion gallons (assuming 100 gallon/acre) of vegetable oil could be obtained annually. [Note that at present there are approximately 27 billion gallons of vegetable oil produced worldwide annually (Peterson, 1995)]. Yields for other plant species, modified to concurrently synthesize chavicol (31), methylchavicol (32), or eugenol (33), now need to be determined to establish to what extent these productivity numbers can be increased through (for instance) whole plant utilization.
Additionally, if one considers the annual pulp and paper production in the United States (~120 million metric tons/year, 1997 figures), the potential also exists to divert some part of the production of lignins/heartwood lignans and other phenylpropanoid derivatives in commercially important woody plant species away from their natural biosynthetic pathways, that is, to afford allyl/propenyl phenols, etc. In principle, the lignin/lignan substances currently produced annually as by-products of pulp/paper industries (more than 50 million tons) could instead be converted to approximately 15 billion gallons allyl/propenyl phenols per annum, if fully converted. Nevertheless, any reduction in carbon flow to lignin, or reductions/changes in heartwood-forming constituents, could represent a significant increase in biofuel production.
© 2018 Biocyclopedia | All rights reserved.