Chemistry Reference
In-Depth Information
their efforts in the design of microwave ovens specifically for use in laboratories. The first custom-built commercial
microwave synthesizer, to conduct chemical synthesis, was introduced in 2000. It was designed to produce a
uniform microwave field and was equipped with technology that could control the temperature of the chemical
reaction [8,9].
As a consequence, the amount of articles describing efficient rapid chemical synthesis promoted by microwave
irradiation has grown quickly from ~200 in 1995 to ~1000 in 2001 and ~3300 in 2009. The efficiency of microwave
flash-heating chemistry in reducing reaction times (from days or hours to minutes or seconds), reducing side
reactions, increasing yields, and improving reproducibility has been responsible for attracting many academic and
industrial research groups to microwave research projects. The current trend is that, in a few years, most chemists
will probably use microwave energy to heat chemical reactions [5,10].
Microwave-assisted heating under controlled conditions is an important technology for both organic and medicinal
chemistry. From a medicinal chemistry perspective, the competitive landscape of the pharmaceutical industry
demands constant innovation and accelerated timelines for the development of nascent programs. MAOS have
recently fascinated many pharmaceutical industries, not so much in terms of production of a large variety of
compounds in a reduced time, but more with the synthesis of novel, complex and decorated scaffolds. Nowadays,
the pharmaceutical companies have incorporated the microwave chemistry into their drug discovery process
introducing novel approaches to drug discovery and development [8,11,12].
MICROWAVE RADIATION
Microwaves are generated by a cavity magnetron, which is a high-powered vacuum tube, and are useful for
industrial, scientific and medical applications (e.g., microwave oven, radar, magnetic resonance etc).
Microwave is a form of electromagnetic energy (irradiation) in a frequency range of 0.3 to 300 GHz, corresponding
to wavelengths ranging from 1 cm to 1 m. The microwave region of the eletromagnetic spectrum (Fig.
1
) lies
between infrared and radio frequencies.
Figure 1:
The electromagnetic spectrum. Figure adapted from http://en.wikipedia.org/wiki/File:EM_spectrum.svg
Whithin this region only molecular rotation is affected, not molecular structure. Microwaves move at the speed of
light (300.000 Km/s) and the energy in microwave photons (0.037 Kcal/mol) is very low relative to the typical
energy required to cleave molecular bonds (80-120 Kcal/mol). Thus, it is clear that microwaves cannot “induce''
