Current Organic Chemistry - Volume 17, Issue 5, 2013

Microwave irradiation as a modern approach to various organic transformations, specially in combination with the use of homogeneous or heterogeneous catalysts, is a well-established strategy for the preparation of a range of important compounds. However, due to the recent enormous expansion of the field of modern, catalytic transformations of different types, we present the title subject in the form of two reviews (Part A: General and Part B: Catalysis). In Part A, we provided a general overview of the underlying principles of the application of microwaves in organic synthesis. Furthermore, we presented a selection of examples of catalyst-free applications of principles of the green chemistry, such as use of water as the solvent, application of neat reaction conditions and reactions taking place on solid supports. In Part B [1], we described the reactions carried out using microwaves in the combination with various types of catalysts (transition-metal-catalyzed transformation, organocatalysis, phase-transfer catalysis, the use of inorganic catalysts and ionic liquids, etc.). In addition, we looked at the syntheses and transformations of different organic targets with the emphasis on heterocycles, especially on those containing a 2H-pyran-2-one moiety in their structure. A particular interest is also dedicated to the environmentally benign reaction conditions.

In this review we provide selected recent examples of applications of microwave irradiation to carry out various transformations accelerated by a variety of catalysts. In particular, here we present transition-metal-catalyzed transformations (with the application of palladium, copper, rhodium and other metal-containing compounds), organocatalysis, heterogeneous catalysis, phase-transfer catalysis, the application of solid-supported catalysts, the use of inorganic catalysts (bases, acids and salts) and ionic liquids, etc. Combination of microwaves and catalysts represents an efficient and general pathway applicable in the syntheses and transformations of different organic targets, especially heterocyclic and some other compounds. The emphasis is dedicated to those compounds containing a 2H-pyran-2-one moiety in their structure and their subsequent transformations into (hetero)aromatic derivatives.

Impact of microwave radiation on organic chemistry has been assessed by workers time to time. This review covers selected recent examples form cross-coupling reactions, multicomponent reactions along with some cycloaddition reactions to reveal the impact of microwave on organic synthesis.

Current achievements in the microwave-assisted synthesis of quinoline and isoquinoline are discussed in this review. Monoazanaphthalenes are important scaffolds in organic synthesis and especially in drug design. Due to their frequent appearance in biologically active compounds, drugs and natural products, they are claimed to be privileged structures. Synthetic routes to these compounds have been known for more than one hundred years but their availability is still the main limiting factor. Conventional methods are laborious, time consuming and do not offer satisfactory yields or purity of the products. On the other hand, microwave-assisted protocols do not provide enough versatility in designing a molecular scaffold, i.e. substitution pattern. Most of the known microwave-assisted syntheses consist of the modification of monoazanaphthalene rings and as such are limited to several easily available scaffolds.

According to the concept of green chemistry, energy requirements of chemical processes should be minimized. Microwaveassisted synthesis has provided significant energy saving for the chemical transformations in comparison with conventional oil-bath heating. Reusability, stability and ease of handling are the significant points of solid-supported reagents, which make them an interesting field for an organic chemist. The combination of microwave (MW) irradiation with solid-supported reagent becomes an interesting and popular theme in synthetic organic chemistry. In this paper, we wish to review the recent applications of this important technique in the synthesis of organic compounds with the hope to increase awareness of using this combination among organic chemists. We have also highlighted the advantages and compared them with previously reported methods in order to show the importance and merits of this protocol.

Over the last 20 years, many chemical reactions or processes have been studied using microwave irradiation as heating source. Nowadays, microwave-assisted functionalization of materials and nanomaterials is a rapidly growing field of research. Indeed, for their elaboration with specific and relevant properties, post synthetic modification of the surface is one of the critical steps. Most of the time, they could not be synthesized directly with their properties and so their surfaces need to be tailored for their final applications. In the last few years various surfaces (polymer resins, porous materials, metal organic frameworks, inorganic nanoparticles, carbon nanotubes…) have been evaluated with reported reaction rate accelerations and higher yields. The present contribution will give an overview of this field and will focus on the advantages of microwave functionalization and future trends of this field of research.

The spread of microwave (MW) equipment has brought about a tremendous development in synthetic organic chemistry. This environmentally friendly methodology, associated often with solventless conditions, has also had a positive impact on organophosphorus chemistry, allowing new reactions to be carried out, or increasing the rate, selectivity and yield. In special cases, MW irradiation may replace phase transfer or other kinds of catalysts. Reactions, such as the derivatization of phosphinic acids, the inverse Wittig protocol, Diels– Alder cycloadditions, fragmentation-related phosphorylations, phospha-Michael additions, Kabachnik–Fields condensations, the addition of >P(O)H species to carbonyl compounds, substitution of α-hydroxyphosphonates, alkylation of CH-acidic compounds, C–P couplings, Arbuzov reactions and transesterifications served as model reactions under MW conditions.

Depending on the conditions, the alcoholysis of simple dialkyl phosphites [(RO)2P(O)H] under microwave conditions may result in the formation of mixed derivative [(RO)(R'O)P(O)H] and the fully transesterified species [(R'O)2P(O)H] in different ratios. On one hand, this may be a method of choice for the preparation of less common dialkyl phosphites with alkyl chains of higher carbon atom number, on the other hand, the mixed derivatives are valuable building-blocks due to the chiral phosphorus atom. The thermal accomplishments led, in most cases, to uncomplete conversions.