Schrock system

A chain termination with aldehydes as with the Schrock systems is not possible if using Grubbs catalysts. Vinylethers are used to cleave the polymer chain from the metal via formation of Fischer-type carbene complexes, which are metathesis inactive as reported by Grubbs, and an olefinic end group [189,190]. 

In 2011, Nishibayashi reported on a new Mo-based catalyst system supported by the tridentate PNP-pincer scaffold (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine) that can readily be modified in its steric and electronic properties (Scheme 34). This Hgand system was expected to provide sufficient steric protection of the Mo atom and being less susceptible to protonation and therefore mitigating the major drawbacks of the Schrock system. 

Proposed intermediates in the reduction of N2 at a LMo center (see L on the right) through the stepwise addition of protons and eiectrons in the Schrock system... 

In this regard the Schrock system is distinct from aU previously developed molybdenum N2 systems. 

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

Although RCM technology has been known for over 15 years, early examples utilized poorly defined and inefficient catalyst systems which exhibited limited functional group tolerance [2]. These factors made the process unsuitable for most synthetic applications. The recent explosive growth in the area can be attributed primarily to the work of Schrock [3] and Grubbs [4], who developed the stable, well-defined and efficient metathesis initiators 1,2 and 3 (Fig. 1). 

In parallel investigations, Danishefsky and coworkers accomplished the preparation of the 16-membered lactone of a model epothilone system via an alternative C9,C10 disconnection [14] (Scheme 4). In this case, coupling of epoxy-alcohol 17 with acids 18a and 18b afforded trienes 19a and 19b respectively. RCM of 19a under the influence of ruthenium initiator 3 produced dienes 20a as a 1 1 mixture of Z -isomers. Under identical conditions, cyclization of 19b produced a single product 20b (tentatively assigned as the Z-isomer). The variable stereoselectivity observed in these reactions was inconsequential since the olefinic functionality could be reduced to afford the corresponding saturated macrolactones. Schrock s molybdenum initiator 1 promoted the cyclization of 19a and 19b with similar efficacy [14]. 

For the cross-metathesis of functionalised alkenes the ill-defined classical catalyst systems currently offer very few advantages (cost and heterogeneous catalysis) over the more functional group tolerant Schrock and Grubbs alkylidene... 

Mononuclear ruthenium complexes were found to be superior to carbonyl clusters during a comprehensive comparison of a variety of catalysts in the reduction of acetone [49]. Without solvent, most catalysts were highly selective, although the activity was quite low. The addition of water to the system vastly increased yields, in agreement with Schrock and Osborrfs observations into rhodium-catalyzed hydrogenations (Table 15.9) [41],... 

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