Schrock-type systems

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

Although classic ternary systems based on molybdenum, tungsten and rhodium have been widely used for these purposes [25-52], terminal alkynes are still best polymerized by well-defined Schrock-type metathesis initiators in... 

Figure 9 The it interaction between a carbene and metal for an octabedral system. Case (a) refers to a Fisher-type carbene and (b) to a Schrock-type carbene...

The M—Ccarbene bonds in both Fischer- and Schrock-type complexes are longer than typical M—Cco(term) bonds, but shorter than typical M—C single bonds, e.g. in (OC)5Cr=C(OMe)Ph, Cr-Ccarbene = 204 pm and Cr—Cco = 188 pm. This implies some degree of (fi -/j)7r-char-acter as indicated by resonance structures 23.51 and 23.52. The TT-system can be extended to the heteroatom in the Fischer-type system as shown in diagram 23.53. 

Recently, Weiss and Kraut reported an unusual NHC in which the two substituents at the nitrogen atoms are oxygen [101]. The carbene center can be viewed as the terminus of two, mutually independent two-step 71-redox systems. This electronic configuration is expected to interact with high flexibility with a metal center coordinated to the carbenic center that is predicted to act as either 71-donor (Schrock-type carbene) or 71-acceptor (Fischer-type carbene). The multiplicity at the carbonic center depends on the nature of the metallic fragment. The carbene could not be isolated but the authors were able to trap it in the presence of palladium salts (Scheme 35). 

Schrock-type initiators of the type Mo(NAr )(OR )2(CHCMe2R) possess a tetrahedral geometry. The addition of phosphanes such as PMe3 or amines such as quinucHdine to Mo(NAr)(OCMe(CF3)2)2(CH-6Bu) allows, in analogy to W-based systems [37], the observation and isolation of two isomeric adducts [143]. The attack of the Hgand was found to occur preferably at the CNO-face [144] that is, the face defined by the N, carbon, and one O-substituent - a fact which is in accordance... 

Both, Schrock-type carbenes and NHCs have been used for this purpose selected systems are reviewed in the following subsection. 

Recently, metathesis of cyclooctane was reported using the tandem system having the pincer-ligated iridium complexes for hydrogenation/dehydrogenation and Schrock-type Mo-alkylidene complexes for olefin metathesis [78]. However,... 

Building off of seminal work on the development of iridium complexes for dehydrogenation - hydrogenation catalysts, an improved, homogeneous, dual-catalytic system for alkane metathesis has recently been discovered [101, 102]. Goldman and Brookhart found that a combination of an Ir-pincer-based catalyst with Schrock-type, Mo- or W-alkylidene complexes (as the olefin metathesis catalyst) transformed alkanes into lower and higher new alkanes (Figure 2.12). 

Acyclic Diene Metathesis Polymerization. Acyclic diene metathesis (ADMET) and the other metathesis polymerization methods are closely related to the catalyst system employed. Besides classical catalysts, Schrock-type alkyli-denes and Grubbs-type carbenes can be used in ADMET polymerization. 

A report by van der Schaaf et al. is further worth to be mentioned [Me(CF3)2CO]2 (NPh)W(CH2SiMe3)2 and Cl(NPh)W(CH2SiMe3)3 are transformed into Schrock-type initiators by irridation and were used for the photoinduced ROMP (PROMP) of norbomene and dicyclopentadiene [168]. The main advantage of these thermally very stable PROMP systems is their latency in pure monomers in the absence of light and the easier synthesis [26,27]. 

Schrock-type catalysts. Following the synthetic route described for the synthesis of a polymer-supported chiral biphenyl-based Schrock catalyst, analogous systems based on the 2,6-dichlorophenylimido and adamantylimido ligand were prepared (Figure 8). 

Fig. 3.2 (a) Chatt- and (b) Schrock-type molybdenum systems for N2 reduction. 

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