Preferential formation of specific hexose and heptose in the formose reaction under microwave irradiation

To realize sustainable societies, the production of organic compounds not based on fossil resources should be developed. Thus, C1 chemistry, utilizing one-carbon compounds as starting materials, has been of increasing importance. In particular, the formose reaction is promising because the reaction produces sugars (monosaccharides) from formaldehyde under basic conditions. On the other hand, since microwave (MW) induces the rotational motion of molecules, MW irradiation often improves the selectivity and efficiency of reactions. In this study, the formose reaction under MW irradiation was thus investigated under various conditions. The formose reaction proceeded very fast using 1.0 mol per kg formaldehyde and 55 mmol per kg calcium hydroxide (Ca(OH)₂) as a catalyst at a high set temperature (150 °C) for a short time (1 min) to form preferentially specific hexose and heptose. The major products were isolated by thin layer chromatography and characterized by mass spectroscopy and NMR. These characterization data elucidated that the hexose and heptose were 2-hydroxymethyl-1,2,4,5-tetrahydroxy-3-pentanone (C6*) and 2,4-bis(hydroxymethyl)-1,2,4,5-tetrahydroxy-3-pentanone (C7*), respectively. On the basis of these observations, as well as density functional theory calculations, a plausible reaction pathway was also discussed; once 1,3-dihydroxyacetone is formed, consecutive aldol reactions favorably occur to form C6* and C7*.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Microwave Reactor Concepts: From Resonant Cavities to Traveling Fields

Microwave chemistry has been investigated for nearly thirty years with many notable results being published on apparent process enhancement due to microwave exposure. Conclusive proof of beneficial microwave-chemical interactions is lacking though, as are design rules for successful implementation of microwave-chemical processing systems. In this chapter, the main cause for this is asserted to be the current absence both of suitable instrumentation for research, and processing equipment that merges chemistry with electromagnetic aspects. Several concepts are presented to show how these challenges may be addressed.

Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals

Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N-O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N- and O-centred radicals. This article reviews the use of these precursors for studying the structures, reactions and kinetics of the released radicals. Two classes have been exploited for radical generation; one comprises carbonyl oximes, principally oxime esters and amides, and the second comprises oxime ethers. Both classes release an iminyl radical together with an equal amount of a second oxygen-centred radical. The O-centred radicals derived from carbonyl oximes decarboxylate giving access to a variety of carbon-centred and nitrogen-centred species. Methods developed for homolytically dissociating the oxime derivatives include UV irradiation, conventional thermal and microwave heating. Photoredox catalytic methods succeed well with specially functionalised oximes and this aspect is also reviewed. Attention is also drawn to the key contributions made by EPR spectroscopy, aided by DFT computations, in elucidating the structures and dynamics of the transient intermediates.

A new microwave-assisted thionation-heterocyclization process leading to benzo[c]thiophene-1(3H)-thione and 1H-isothiochromene-1-thione derivatives

The first example of a MW-assisted tandem thionation/S-cyclization process, leading to benzothiophenethione and isothiochromenethione derivatives, is reported.

Catalyst-free photoredox addition-cyclisations: exploitation of natural synergy between aryl acetic acids and maleimide

Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.

The oxime portmanteau motif: released heteroradicals undergo incisive EPR interrogation and deliver diverse heterocycles

Selective syntheses are now available for compounds of many classes, based on C-centered radicals, exploiting a diverse range of mechanisms. The prospect for chemistry based around N- and O-centered radicals is probably more favorable because of the importance of heterocycles as biologically active materials. Heteroradical chemistry is still comparatively underdeveloped due to the need for safe and easy ways of generating them. Oxime esters appeared promising candidates to meet this need because literature reports and our EPR spectroscopic examinations showed they readily dissociated on photolysis with production of a pair of N- and O-centered radicals. It soon became apparent that a whole suite of benign oxime-containing molecules could be pressed into service. The bimodality of the oxime motif meant that by suitable choice of functionality the reactions could be directed to yield selectively products from either the N-centered radicals or from the O-centered radicals. We found that on one hand photolyses of acetophenone oxime esters of carboxylic acids yielded alicyclics. On the other hand, aromatic and heteroaromatic acyl oximes (as well as dioxime oxalates) afforded good yields of phenanthridines and related heterocycles. Easily prepared oxime oxalate amides released carbamoyl radicals, and pleasingly, β-lactams were thereby obtained. Oxime carbonates and oxime carbamates, available via our novel 1,1'-carbonyldiimidazole (CDI)-based preparations, were accessible alternatives for iminyl radicals and hence for phenanthridine preparations. In their second modes, these compounds proved their value as precursors for exotic alkoxycarbonyloxyl and carbamoyloxyl radicals. Microwave-assistance was shown to be a particularly convenient procedure with O-phenyl oxime ethers. The iminyl radicals generated from such precursors with alkene, alkyne, and aromatic acceptor substituents furnished pyrrole, quinoline, phenanthridine, benzonaphthiridine, indolopyridine, and other systems. Microwave irradiations with 2-(aminoaryl)alkanone O-phenyl oximes enabled either dihydroquinazolines or quinazolines to be obtained in very good yields. The fine quality of the EPR spectra, acquired during photolyses of all the O-carbonyl oxime types, marked this as an important complement to existing ways of obtaining such spectra in solution. Quantifications enabled SARs to be obtained for key reaction types of N- and O-centered radicals, thus putting mechanistic chemistry in this area on a much firmer footing. Surprises included the inverse gem-dimethyl effect in 5-exo-cyclizations of iminyls and the interplay of spiro- with ortho-cyclization onto aromatics. Insights into unusual 4-exo-cyclizations of carbamoyl radicals showed the process to be more viable than pent-4-enyl 4-exo-ring closure. Another surprise was the magnitude of the difference in CO2 loss rate from alkoxycarbonyloxyl radicals as compared with acyloxyl radicals. Their rapid 5-exo-cyclization was charted, as was their preferred spiro-cyclization onto aromatics. The first evidence that N-monosubstituted carbamoyloxyls had finite lifetimes was also forthcoming. It is evident that oxime derivatives have excellent credentials as reagents for radical generation and that there is ample room to extend their applications to additional radical types and for further heterocycle syntheses. There is also clear scope for the development of preparative procedures based around the alkoxyl and aminyl radicals that emerge downstream from oxime carbonate and oxime carbamate dissociations.