Christopher R. Strauss

Microwaves in Green and Sustainable Chemistry

The various roles of microwave-assisted organic chemistry in green and sustainable chemistry are discussed beginning with the strategies, technologies, and methods that were employed routinely at the time the first reports of microwave applications for organic synthesis appeared. Applications of open-vessel microwave chemistry and closed-vessel microwave chemistry are presented, with sections on solvent-free methods, reactions in high-temperature water, technology for transposing reaction conditions.

Accounting for clean, fast and high yielding reactions under microwave conditions

Thermal reactions proceed optimally when they are rapidly heated to the highest tolerable temperature, held there for the shortest possible time and then quenched. This is explained through assessments of reaction kinetics in literature examples and models. Although presently available microwave equipment is better suited to rapid heating than resistance-heated systems, the findings do not depend upon the method of heating. Claims that microwave heated reactions proceed faster and more cleanly than their conventionally heated counterparts are valid only when comparably rapid heating and cooling cannot be obtained by conventional heating. These findings suggest that rigid adherence to the sixth principle of green chemistry, relating to the use of ambient temperature and pressure, may not always afford optimal results.

A critical assessment of electrochemistry in a distillable room temperature ionic liquid, DIMCARB

Ionic liquids are frequently advocated as green media for electrochemical studies. However, they are non-volatile and hence difficult to purify or recover. In this paper the electrochemical behaviour of a ‘distillable’ room temperature ionic liquid, DIMCARB, has been investigated. This ionic liquid is unusual because it is readily prepared, in large quantities and at low cost, by mixing of gaseous carbon dioxide with dimethylamine and also easily recovered by decomposition back into its gaseous components followed by reassociation. Almost ideal reversible voltammetry is observed for the Cc+/0 process (Cc = cobalticinium), which therefore is recommended for reference potential calibration. Another IUPAC recommended reference potential process, Fc+/0 (Fc = ferrocene), is only reversible at fast scan rates and occurs near the positive potential limit available. However, decamethylferrocene (DmFc) is reversibly oxidised and behaves ideally as for the reduction of Cc+. The small diffusion coefficients of 1.2 × 10−7 cm2 s−1 (Cc+) and 5.0 × 10−8 cm2 s−1 (DmFc) at 20 °C are attributed to the relatively high viscosity. The potential window of ca. −1.50 V to +0.50 V vs. SHE indicates that DIMCARB is more suitable for electrochemical studies of reductive rather than oxidative processes. Voltammetric studies in DIMCARB reveal a series of reversible reductive processes for the Keggin [α-SiW12O40]4− polyoxometallate. Comparison of reversible potential data reported in other media indicate that the polarity of DIMCARB is intermediate between that of MeCN and the conventional ionic liquid [BMIM][PF6]. The deposition of metallic Pb also has been studied and reveals that Pb(II) is reduced in a single irreversible 2-electron step to the metallic state via a nucleation/growth mechanism. Overall, these studies show that DIMCARB is highly suitable for electrochemical studies, but that it is a potentially reactive medium.

A Strategic, 'Green' Approach to Organic Chemistry with Microwave Assistance and Predictive Yield Optimization as Core, Enabling Technologies

Since 1988, we have pursued enabling technologies and methods as tools for ‘green’ synthetic chemistry. The developed technologies comprise hardware including catalytic membranes and continuous and batch microwave reactors that have established global markets, as well as interactive, predictive software for optimization of yields and translation of conditions. New methods include ‘green’ reactions such as a catalytic symmetrical etherification, Pd-catalyzed coupling processes and a multi-component cascade for aniline derivatives. Reactions and workup were facilitated through solvent-free conditions, aqueous media at high temperature and dimethylammonium dimethylcarbamate (dimcarb) as a ‘distillable’ protic ionic liquid, as well as by non-extractive techniques for product isolation. The technologies and methods were designed for use alone or in various combinations as desired. Consolidation of individual operations or processes into unit steps was achieved through multi-tasking: media, reactants, catalysts, and conditions were selected to serve several purposes at various stages of a reaction. The tools were used to establish a technology platform comprising structurally diverse oligomers, macrocycles, and rod-like molecules supplementary to those available through phenol-formaldehyde chemistry. Dienone precursors were assembled from versatile building blocks containing complementary ‘male’ or ‘female’ fittings that were connected through inherently ‘green’ Claisen–Schmidt-type reactions. Isoaromatization afforded Horning-crowns, macrocyclic phenolic derivatives that were hybrids of calixarenes and crown ethers. Preliminary studies of organic substrates in salt water, with and without CO2, called into question proposals for disposal of anthropogenic CO2 by deep-sea dispersal.

THE USE OF PALLADIUM ON POROUS GLASS FOR CATALYTIC COUPLING REACTIONS

Palladium on porous glass is a useful heterogeneous catalyst for liquid phase organic coupling reactions which can be carried out in the presence of air, without the need for solubilizing and activating ligands.

A direct, efficient synthesis of unsymmetrically substituted bis(arylidene)alkanones

A direct method to unsymmetrically substituted bis(arylidene)alkanones by sequential, selective condensation reactions is reported. Analytically pure compounds were obtained in high yield using atom-economical reactions and without the need for purification of the final products. Although a two-step process could be conducted in one pot by three different methods, recrystallisation was required.

Tunable thermomorphism and applications of ionic liquid analogues of Girard's reagents

A series of ionic liquids based on Girards reagents was synthesised. Their tunable thermomorphic behaviour with water was demonstrated, and slight modifications in the cationic structure led to drastic changes in their water miscibility. Their phase behaviour, involving monophasic–biphasic transitions, drove a number of practical applications, including scavenging water-soluble dyes and the extraction of metals from water.

Pd-catalysed arylation of propan-1-ol and derivatives: oxidative role of the arylating agent

With excess PhI under Pd catalysis, 1-PrOH was converted to a mixture of 3,3-diphenylpropenal and trans-2,3-diphenylpropenal by a concerted, oxidative sequence that involved two arylative couplings and an olefinic aldehyde that was generated in situ.

Platform technology for dienone and phenol–formaldehyde architectures

Claisen–Schmidt condensations, yielding only water as a by-product, performed on building blocks serving as shape-selective male or female terminals and unions, enable the preparation of diverse molecular structures including novel linear rods and semi-elliptical, rectangular or trapezoidal macrocycles. Isoaromatization affords a corresponding range of phenol-formaldehyde derivatives, in atom economical reactions.

Development of predictive tools for optimizing organic reactions.

Although microwave chemistry and its applications have undergone rapid growth over the last decade, the technology is not yet employed routinely in all synthetic laboratories. A significant obstacle to implementation concerns the empirical work required to adapt established conditions into alternatives employing higher temperatures. We now have established predictive tools to translate reaction conditions from conventional heated (ambient pressure) to elevated temperature (ambient or elevated pressure). We have studied 45 reactions (including published literature examples) and made in excess of 200 estimations for specific yield or conversion, with a high degree of accuracy. Linear regression analysis of estimated vs. experimental conversion or yield was 0.90 (first iteration) and 0.98 (second iteration).