Schrock complexes

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 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. 

Alkylidene carbonyl iridium complexes, reactions, 7, 275 Alkylidene compounds, NLO properties, 12, 121 Alkylidene-containing complexes, in molybdenum complexes, Schrock-type complexes, 5, 524 a-Alkylidene cyclic carbonyl compounds, isomerization,... 

Another clear example of an equilibrium metathesis reaction is the ROMP of cyclopentene initiated by the tungsten analogue of 2. The reaction may be totally reversed by placing the system under vacuum thereby stripping off the monomer units from the living metal carbene complexes (Schrock 1989). 

Characteristic Fischer-Type Carbene Complex Schrock Type Carbene Complex... 

Reaction of [(CH3)3CCH2]3TaCl2 with (CH3)3CCH2Li gives a tantalum-carbene complex (Schrock, 1974). Labeling studies indicate that the reaction proceeds via a-elimination from an intermediate RsTa compound. Carbene complexes have been considered as possible intermediates in olefin metathesis... 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

Scheme 4 Schrock-type and Fischer-type carbene complexes...

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

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

Fig. 1 A,B Dominant orbital interactions in Fischer-type carbene complexes (A) and Schrock-type carbene complexes (B)...

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. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The synthesis of these complexes can easily be accomplished by substitution of one or both PCy3 groups of 3 by NHC ligands. The X-ray structure of 6 shows significantly different bond lengths the Schrock double bond to the CHPh group is 1.821(3) A, while the NHC bond to the l,3-diisopropylimidazolin-2-ylidene is 2.107(3) A. Complexes with imidazolidin-2-ylidenes were also synthesized and screened in an extensive study by Fiirstner [153], who found that the performance of those catalysts depends strongly on the application and that... 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

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