Schrock carbenes synthesis

The living nature of the discussed cyclopolymerisation of the 1,6-heptadiyne derivative in the presence of Schrock carbene as the catalyst has been demonstrated by the synthesis of a block copolymer with 2,3,-dicarbomethoxynor-bornadiene [25]. 

In 1964, Fischer prepared and characterized unambiguously the first metal carbene complex 3 obtained by nucleophilic attack of phenyl lithium at tungsten hexacarbonyl followed by 0-alkylation. This was followed by Schrock s synthesis of a high oxidation state metal alkylidene complex 4 obtained by a-hydrogen abstraction from tris(neopentyl) tantalumfv) dichloride (Scheme 1.1)/ ... 

The metal-carbene complexes are nowadays extensively used as reagents in organic synthesis. Fischer carbene complexes have become valuable building blocks in various stoichiometric reactions such as the aldol reaction, benzannu-lation, cycloaddition, cyclopropanation, and Michael reaction. In particular, Fischer alkenyl carbene complexes have been widely used in cycloadditions [2c,2f,5]. On the other hand, a typical reaction of Schrock carbene complexes is olefin metathesis (Figure 5.2) [6]. Schrock s olefin metathesis catalyst is a high reactivity, but air- and moisture-sensitive reagent. 

Scheme 7 Synthesis of the first Schrock-type carbene complex...

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

Two possible catalytic systems for olefin mclathesis are the commercially available carbene complexes 61 and 62. Molybdenum complex 62. developed by Schrock,25 was the first to be introduced. Nevertheless, it has the disadvantage of being both unstable and difficult to access. The newer Gruhbs catalyst 61 is now preferred because it is stable and also easy to synthesi/e. Both catalysts are also appropriate for starting materials containing heteroatoms (see alst) Chapter 16). 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

Olefin metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis, and the reaction has been generally accepted to proceed through a series of metallacyclobutanes and carbene complexe intermediates [40-43]. For this type of reaction, the most widely used catalysts include an alkoxyl imido molybdenum complex (Schrock catalyst) [44] and a benzylidene ruthenium complex (Grubbs catalyst) [43]. The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

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