Green Chemistry Metrics, A Review

Attending both the United Nations Decade of Education for Sustainable Development (2005–2014) and the United Nations 2030 Agenda for Sustainable Development, this review is presented, bearing in mind that green chemistry is essential to contribute to sustainability. This work has compiled all the information relating to green chemistry metrics, so that stakeholders can select an appropriate model, under the Green Chemistry Protocol, to evaluate how much green is a process. The review was organized considering the following convenient sections: the mass valuation, the recognition of the human health and environmental impact, metrics using computational programs (software and spreadsheets), and finally global metrics. This review was developed by consulting the principal databases, since the appearance of the first green chemistry textbook in 1998. A massive number of references were attained involving the keywords proposed below, with six languages observed, highlighted by the English language. It is important to emphasize that the 12 Principles of Green Chemistry are conceptual and offer little quantitative information. In addition, almost all the reported metric green propositions do not consider the 12 principles and few papers offer how to obtain an appropriate evaluation about the greenness of a research. In this sense, it is convenient to note that only in the Spanish literature are there two metrics that consider all the principles. Finally, to our knowledge, and after a deep search in the literature, it is the first review that covers the different features of green chemistry: mass, environment/human health. and in some cases, the use of computational programs.

Green and facile sol-gel synthesis of the mesoporous SiO2-TiO2 catalyst by four different activation modes

The most environmentally friendly protocol for obtaining mesoporous SiO2-TiO2 catalysts has been sought. Water has been employed as a green solvent, the energy input has been minimized, and three further principles (1, 3, and 12) of Green Chemistry have been considered. Four different modes for promoting the reaction have been comparatively evaluated, namely near-infrared and microwave electromagnetic irradiations, ultrasound, and traditional mantle heating. Brunauer-Emmett-Teller (BET) analyses of the catalysts produced revealed that the non-conventional activation modes afforded both large surface areas (335-441 m2 g-1) and smaller crystal sizes (7.2-15.3 nm) than the mantle heating process. These modes also generated the catalysts in shorter reaction times than traditional mantle heating, 10-30 min versus 3 h, with anatase as the sole crystalline phase. The photocatalytic degradation of 4-chlorophenol has been carried out to assess the catalytic efficiencies of the hybrid materials. The catalyst synthesized with microwave assistance showed the best mineralization activity (97%), followed by those prepared with ultrasound, near-infrared, and mantle heating. The materials have been extensively characterized by FTIR, XRD, DRS-UV/Vis, SEM, 29Si MAS NMR, and BET analyses. To the best of our knowledge, this is the first such comparative assessment of green energetic alternatives in developing a sol-gel process.

Green Approach Extraction of Perezone from the Roots of Acourtia platyphilla (A. Grey): A Comparison of Four Activating Modes and Supercritical Carbon Dioxide

Perezone, a sesquiterpene quinone, is a very important molecule due to its pharmacological activities in addition to the fact that it is considered to be the first secondary metabolite isolated in the new world (America–Mexico, 1852). This study aims to offer a green comparative study about the extraction of the target molecule from the roots of the vegetable specimen Acourtia platyphilla (A. Grey). The study was performed comparing five different modes of extraction: supercritical CO₂, electromagnetic infrared and microwave irradiations, mechanical-wave ultrasound versus typical mantle heating procedure. An exhaustive comparative-discussion of the obtained results is provided. It is worth noting that the corresponding quantifications were established using ¹H NMR, correlating appropriately the integrals of the vinylic proton H-6 of perezone with the aromatic singlet of p-dinitrobenzene employed as an internal reference. It is also important to highlight that the four presented procedures are novel modes to extract perezone. Finally, a complementary study about the solubility of the target sesquiterpene quinone related to the use of supercritical CO₂ is also reported.

Alternative mass spectrometry techniques for the validation of the fragmentation pattern of capsaicin and dihydrocapsaicin

RATIONALE

Capsaicinoids are prevalent secondary metabolites in many natural and synthetic pharmacological compounds. To date, several soft ionization studies related to capsaicinoids have been reported; they all proposed a common fragmentation pattern based on a rearrangement of the aromatic double bonds and the fragmentation of the various positional acyl chains. However, the mechanism has never been validated by high-resolution analyses. Consequently, in this work, a validated fragmentation mechanism of the main capsaicinoids, capsaicin (1) and dihydrocapsaicin (2), is offered.

METHODS

In order to propose and validate a common electron ionization (EI) fragmentation mechanism for the target analytes, the following mass spectrometric methods were employed: collision-induced dissociation (CID) by means of linked scans (LS), reinforcing the methodology by high-resolution mass spectrometry (HRMS), in addition to appropriate deuterium-labeled experiments performed using gas chromatography/mass spectrometry (GC/MS) and direct analysis in real time (DART).

RESULTS

In a first stage, a common EI fragmentation pattern comprising two pathways was proposed for compounds 1 and 2; then, the suggested mechanism was validated by CID-LS together with HRMS complemented by DART-deuterium-labeling studies. The obtained results are indicative that the corresponding molecular ions were conveniently observed, m/z 305 and m/z 307; it is worth noting that the common base peak is in correspondence with a tropylium ion derivative (m/z 137), as a consequence of a McLafferty rearrangement. In addition to these highlighted fragments, other common ions, m/z 122 and m/z 94, and their corresponding trajectory, were confirmed using the same approach. Finally, the proposed mechanism was complementarily validated by deuterium-labeling studies, taking into account the two exchangeable hydrogens present in the phenolic and the amidic moieties.

CONCLUSIONS

A common validated EI fragmentation pattern for both capsaicin and dihydrocapsaicin was established using appropriated mass spectrometric methods together with convenient hydrogen/deuterium labeling. This study provides a new alternative to validate mechanisms of fragmentation of important natural products.

Green approach to the extraction of major capsaicinoids from habanero pepper using near-infrared, microwave, ultrasound and Soxhlet methods, a comparative study

A simple method for the extraction of two major capsaicinoids from habanero peppers, using near-infrared irradiation, microwave irradiation or ultrasound as the energy source and ethanol as the solvent, was compared with Soxhlet extraction. The extraction processes were monitored by gas chromatography/mass spectrometry. The new processes offer better overall yields and a higher ratio of capsaicin to dihydrocapsaicin than Soxhlet extraction. The physical effect of the different energy sources on the peppers was determined using scanning electron microscopy. Extraction of capsaicin and dihydrocapsaicin using near-infrared irradiation, which has not been previously reported, was shown to be a simple and efficient alternative extraction procedure.

A DFT Study of the Geometrical, Spectroscopical and Reactivity Properties of Diindolylmethane-Phenylboronic Acid Hybrids

The structure of the ortho-, meta- and para- hybrid diindolylmethane-phenylboronic acids and their interactions were optimized with by a quantum chemical method, using density functional theory at the (DFT) level. Thus, infrared bands were assigned based on the scaled theoretical wavenumbers by correlating the respective experimental data of the molecules. In addition, the corresponding ¹H-/¹³C-/¹¹B-NMR experimental and theoretical chemical shifts were correlated. The target molecules showed a poor treatment of the OH shifts in the GIAO method due to the absence of explicit solvent effects in these calculations; therefore, they were explicitly considered with acetone molecules. Moreover, the electron density at the hydrogen bond critical point increased, generating stabilization energy, from weak to moderate or weak to strong, serving as an indicator of the strength of the hydrogen bond between the different intermolecular interactions. Finally, some properties related to the reactive behavior of the target molecules associated with their cytotoxic effects and metabolic pathways were also calculated.

In silico Study of the Pharmacologic Properties and Cytotoxicity Pathways in Cancer Cells of Various Indolylquinone Analogues of Perezone

Several indolylquinone analogues of perezone, a natural sesquiterpene quinone, were characterized in this work by theoretical methods. In addition, some physicochemical, toxicological and metabolic properties were predicted using bioinformatics software. The predicted physicochemical properties are in agreement with the solubility and cLogP values, the penetration across the cell membrane, and absorption values, as well as with a possible apoptosis-activated mechanism of cytotoxic action. The toxicological predictions suggest no mutagenic, tumorigenic or reproductive effects of the four target molecules. Complementarily, the results of a performed docking study show high scoring values and hydrogen bonding values in agreement with the cytotoxicity IC50 value ranking, i.e: indolylmenadione > indolylperezone > indolylplumbagine > indolylisoperezone. Consequently, it is possible to suggest an appropriate apoptotic pathway for each compound. Finally, potential metabolic pathways of the molecules were proposed.

Infrared Irradiation: Toward Green Chemistry, a Review

This review provides a comprehensive overview of where infrared irradiation has been employed, mainly as regards activating green mode for natural products extractions, as well as to favor a reaction, highlighting its actual importance. It is also underlined that infrared irradiation heating has been around for a long time; however, only in the last eighteen years have many of its advantages been applied to satisfy a wide range of chemical processes, natural products extractions, and for the promotion of many kinds of reactions. In addition, it is brought to light that near infrared irradiation is more efficient than middle and far infrared irradiations, being easily controllable and with the quality of a fast responding heat source. Thus, the main objective of this review is to offer infrared irradiation as an alternative clean energy source to activate reactions, in addition to favor the selective extraction of natural products, all of which is within the Green Chemistry protocol. Some recent results from our laboratory are also included.

Green Production of Indolylquinones, Derivatives of Perezone, and Related Molecules, Promising Antineoplastic Compounds

A green approach to produce the indolyl derivatives from four natural quinones (perezone, isoperezone, menadione, and plumbagin) was performed; in this regard, a comparative study was accomplished among the typical mantle heating and three nonconventional activating modes of reaction (microwave, near-infrared, and high speed ball milling or tribochemical), under solventless conditions and using bentonitic clay as a catalyst. In addition, the tribochemical production of isoperezone from perezone is also commented on. It is also worth noting that the cytotoxicity of the synthesized indolylquinones in human breast cancer cell was tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, with the 3-indolylisoperezone being the most active. The structural attribution of the target molecules was performed by typical spectroscopic procedures; moreover, the experimental and computed1H and13C NMR chemical shifts data, with previous acquisition of the corresponding minimum energetic structures, were in good agreement.