Jonathan D. Moseley

A critical assessment of the greenness and energy efficiency of microwave-assisted organic synthesis

The question “why should microwave chemistry be green?” is evaluated in the context of the twelve principles of green chemistry, with a focus on the 6th principle: design for energy efficiency. A significant number of publications on microwave-assisted organic transformations during the past 25 years describe this non-classical heating technology as being “green”, assuming that microwave dielectric heating is more energy efficient than classical conductive heat transfer methods. In this Perspective article, we critically assess the energy efficiency of microwave-assisted transformations in the context of scaling-up this technology to production quantities.

The Newman−Kwart Rearrangement: A Microwave Kinetic Study

The kinetic profile of the Newman-Kwart rearrangement has been evaluated using microwave heating. After first demonstrating equivalence between conventional convective heating and microwave heating, data was gathered and analyzed to determine the effects of substituent, solvent, and concentration on the reaction order. Reaction rate constants, Arrhenius constants, and activation energies have been determined. The reaction rate shows strong sensitivity to the substituent and modest sensitivity to the solvent. At high concentrations, the reaction order increases from the previously reported first-order to a mixed first/second-order reaction. Overall, this re-evaluation of the Newman-Kwart rearrangement has shown the reaction rate order to be more complex than previously thought. In addition, microwave heating has proven ideal for the rapid collection of data to facilitate this type of kinetic study.

The molecularity of the Newman-Kwart rearrangement

It was recently reported that the venerable Newman-Kwart rearrangement (1→2) proceeds via mixed first- and second-order kinetics. Prior to this, the rearrangement had been considered to proceed exclusively via an intramolecular O(Ar)→S(Ar) migration. A new bimolecular pathway, possibly involving an 8-membered cyclic transition state, was proposed to account for reaction rates that increased disproportionately with substrate concentration under microwave heating conditions. We report a reanalysis of the kinetics and molecularity of the rearrangement of N,N-dimethyl O-(p-nitrophenyl)thiocarbamate 1a in N,N-dimethylacetamide solvent. Using HPLC, isotopic labeling ((2)H, (18)O, (34)S), and ESI-ICRMS methods, we show that there is no evidence for a bimolecular pathway en route to 2a, with near-perfect exponential decay in 1a at concentrations ranging from 0.11 to 4.70 M. Instead, it is demonstrated that under the microwave heating conditions, a delayed negative feedback signal to the microwave power balancing loop results in oscillatory reaction overheating. Due to higher tan δ in the solute, the amplitude of this oscillation increases with the concentration of 1a, and this phenomenon best accounts for the kinetic behavior previously misinterpreted as being mixed first- and second-order in nature.

Intramolecular azomethine ylide cycloaddition reactions to give octahydroindoles

The aldehyde 2-formyl-2-(pent-4-enyl)-1,3-dithiane 1, containing both alkene and aldehyde functional groups, is a good substrate for intramolecular dipolar cycloaddition reactions after condensation with various N-alkyl α-amino-esters. This paper reports the optimization, scope and stereoselectivity of this reaction to give octahydroindoles (2-azabicyclo[4.3.0]nonanes).

Medium ring synthesis by radical ipso-substitution

A new approach to eight and nine membered ring synthesis is described in which a radical ipso-substitution reaction features as a key step.

A new stereoselective approach to the manzamine alkaloids

The key step in a new, stereoselective approach to the manzamine alkaloids involves an intramolecular azomethine ylide cycloaddition reaction, which forms rings B and C simultaneously, together with three new chiral centres; this has allowed a rapid access to the core ABC ring system of manzamine A.