Rapid metathesis reactions

EG Gillan, RB Kaner. Synthesis of refractory ceramics via rapid metathesis reactions between solid-state precursors. Chem Mater 8 333, 1996. 

Wallace C H, Rao L, Kim S-H, Heath J R, Nicol M and Kaner R B 1998 Solid-state metathesis reactions under pressure a rapid route to crystalline gallium nitride Appl. Phys. Lett. 72 596... 

The main reason for the rapid development of metathesis reactions on a laboratory scale (the reaction itself had been known for quite a long time) has been the development of active and robust second-generation ruthenium catalysts (6/3-14 to 6/3-16), which usually provide better yields than the first-generation Grubbs catalysts (6/3-9 or 6/3-13) (Scheme 6/3.2). This also reflects the huge number of domino processes based on ruthenium-catalyzed metathesis, which is usually followed by a second or even a third metathesis reaction. However, examples also exist where, after a metathesis, a second transition metal-catalyzed transformation or a pericyclic reaction takes place. 

Medicinal chemistry makes use of solid phase combinatorial chemistry as a (rapidly maturing) tool for lead discovery and optimization. Metathesis reactions are obviously useful in this context [45] as they can be used ... 

Solid-state metathesis reactions. For a number of compounds, solid-state metathesis (exchange) reactions have the advantages of a rapid high-yield method that starts from room-temperature solids and needs little equipment. The principle behind these reactions is to use the exothermicity of formation of a salt to rapidly produce a compound. We may say that for instance a metal halide is combined with an alkali (or alkaline earth) compound of a /7-block element to produce the wanted product together with a salt which is then washed away with water or alcohol. Metathesis reactions have been used successfully in the preparation of several crystalline refractory materials such as borides, chalcogenides, nitrides. 

Olefins can be divided into four categories on the basis of their propensity to homodimerize (Figure 2). Type I olefins are able to undergo rapid homodimerization and whose homodimers can equally participate in CM. A CM reaction between two olefins of this type will generally result in a statistical product mixture. Type II olefins homodimerize slowly, and, unlike type I olefins, their homodimers can only be consumed with difficulty in subsequent metathesis reactions. Type III olefins are unable to undergo homodimerization, but have the capacity to undergo CM with either type I or II olefins. As with type I olefins, the reaction between either two type II or type III olefins should result in non-selective CM. Type IV olefins are inert to olefin CM, but do not inhibit the reaction therefore, they can be regarded as spectators to CM. 

Electron-poor a-methylene-(3-lactams were reported to undergo cross-metathesis more rapidly and efficiently than more electron-rich analogs. Significantly, tetrasubstituted alkenes have for the first time been accessed by cross-metathesis reactions (III, Fig. 27), [308]. 

Cis- and fraws-cyclooctene, 100 and 102 respectively, and their derivatives 103-107, all undergo ROMP295 also 10862,362,109 and 11062, 111-113362, 114363,115364,116365, 118362, 119 and 120366,367. Only 101295 and 117362 fail to polymerize, perhaps due to unfavourable choice of catalyst and conditions. The trans monomer 102 gives a 43% cis polymer very rapidly in the presence of MoCl2(PPh3)2(NO)2/EtAlCl2368 and is polymerizable by 18110. With a catalyst of type 10 secondary metathesis reactions of the double bonds in the polymer of 100 cause the cis content to fall from 75% to 25% as the reaction proceeds271. 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

The first publication describing the synthesis of tetrafluoroborate and hexafluorophosphate ionic liquids by metathesis reaction from the corresponding alkali salts [13] opened up the way towards a commercial ionic liquid production. Nowadays, a number of commercial suppliers offer ionic liquids even in large quantities [14]. Moreover, the availability of many ionic liquids on a rapid delivery basis has been established through internationally operating distributors [15]. 

This work consists of an overview of the major developments in the alkene metathesis reaction since 1997. In view of the breadth of the subject area and the rapid pace of advancement in the field in recent years, this review is not intended to serve as a comprehensive survey, but rather as an account of how the development of novel catalyst systems has made a dramatic impact on the reaction in terms of scope and efficiency/selectivity. 

As is evident from the selected examples discussed here, domino metathesis reactions are fast becoming a method of choice for the rapid synthesis of complex ring structures from simple building blocks. The recent advances in catalyst technology and associated widening of the scope of alkene metathesis seems certain to further augment the importance of these relatively unexplored reactions in years to come. 

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