Schrock molybdenum metathesis catalysts

Acyclic Diene Metathesis (ADMET) Polymerization represents a versatile route towards the synthesis of linear well-defined unsaturated polymers [1,2]. The reaction developed by Wagener et al. occurs in the presence of highly active tungsten and molybdenum metathesis catalysts such as Schrock alkylidenes, e.g. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

Fig. 3 Olefin metathesis catalysts Schrock tungsten (Cl) and molybdenum (C2) alkylidene complexes, Grubbs first- (C3) and second-generation (C4) catalysts, Hoveyda-Grubbs second-generation catalyst (C5), and Grubbs third-generation catalyst (C6)...

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

This approach would neither be possible, nor conceivable, without the advent of modern olefin metathesis catalysts. Figure 3 shows a few of the most commonly used catalysts. In this work, we initially relied upon Schrock s Molybdenum catalyst 6 (7) to effect the ring closures, but now exclusively rely upon the second generation Grubbs ruthenium catalyst 7 (8). 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

Schrock and Hoveyda have reported the synthesis and activity of a number of chiral molybdenum-based catalysts for enantioselective olefin metathesis (for example, 97a, b) (Scheme 14) to date, the majority of successful... 

Hoveyda and Schrock attached (97a) to polymer via attached styrene groups yielding the first reported supported chiral molybdenum olefin metathesis catalyst, (290) (Scheme 27). Supported complex (290) is less active than (97a), but it gives similar ranges of ees for enantioselective transformations like desymmetrization. The catalyst is recyclable and, even though the conversions have eroded, the enantioselectivity is still relatively high. Table 14. 

Connon SJ, Blechert S. Recent developments in olefin crossmetathesis. Angew. Chem., Int. Ed. 2003 42 1900-1923. Schrock RR, Hoveyda AH. Molybdenum and mngsten imido alkylidene complexes as efficient olefin-metathesis catalysts. Angew. Chem. Int. Ed. 2003 42 4592-4633. 

Grubbs metathesis catalysts in general have less catalytic activity than Schrock catalysts, but are less sensitive to oxygen and water. They are also substantially less expensive than the molybdenum and tungsten catalysts. The catalyst having R = cyclohexyl, X = Cl, and R = phenyl has received particular attention and is marketed as Grubbs s catalyst. One requirement of these catalysts is the presence of... 

Recently, Floveyda and Schrock have developed molybdenum-based catalysts (i.e. 744a,b) for an asymmetric ring-opening metathesis (AROM) reaction (Scheme 121) (98JA4041, 98JA9720, 99JA11603). Using a tandem AROM/RCM sequence, they examined the asymmetric synthesis of several heterocyclic compounds. For example,... 

Fig. 3. Three well-defined metathesis catalysts Schrock s molybdenum alkylidene (1) and Grubbs first generation (2) and second generation (3) benzylidene catalysts.

A very active heterogeneous olefin metathesis catalyst was prepared by reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes or with Schrock-type carbyne complexes [L3WsC(Bu )j (L = Cl, O(Bu ), neopentyl).Surface species of the type shown in Scheme 2... 

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

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