Monoalkoxide Pyrrolide MAP Complexes

The anti isomers have been observed in the solution for bisalkoxide complexes when the photolysis of syn isomers was conducted at low temperatures [57]. syn Isomers are the norm for MAP species, although anti isomers have now been observed through the aforementioned photolysis of the syn species. 

The rates of interconversion of the syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes. Little is known about the rates of interconversion of syn-and anti-protons for MAP species [2c], although the rates of syn/anti interconversions in bisalkoxide Mo imido alkylidene complexes have been found to vary over approximately six orders of magnitude [57]. It is not surprising that the sterically demanding NArj jg52 ligand would destabilize the syn isomer for steric reasons and lead to mixtures that contain both syn and anti species, as observed. 

6-Me2CgH3 ligands were found to have approximately equal amounts of syn- and anti-alkylidene isomers in solution, which allowed for a study of the interconversion of the two by employing EXSY methods. The values 

The reactions between an alkylidene and a terminal olefin are routinely employed to prepare new alkylldenes, usually from a neopentylidene or neophylidene. In all such reactions, the intermediate that leads to the new alkylidene, an a,a -disubstituted metallacyclobutane (Eq. (1.6)), must be formed, but it is rarely stable enough to be observed. However, a 14-electron Mo vinylalkylidene MAP complex, sy -Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT), was successfully prepared and isolated by treating Mo(NAr)(CH-f-Bu)(Me2Pyr)(OHMT) with excess4-methyl-l,3-pentadiene [14]. 

Mo(NAr)(CHCH=CMe2)(Me2Pyr)(OHMT) is relevant to the Z-selective homocoupling of 1,3-dienes by molybdenum and tungsten MAP complexes. The formation of the desired M=CHR complex can be complicated by the reformation of M=CHR or the metathesis of R CH=CH2 (when added in large excess) to give R CH=CHR and ethylene, which lead to the formation of methylidenes and unsubstituted metaUacyclobutane complexes. 

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This review will focus on isolated and characterized high-oxidation state molybdenum and tungsten alkylidene and metallacyclobutane complexes. Attention will be directed largely toward monoalkoxide pyrrolide (MAP) complexes because they have yielded the majority of new results in the last several years. MAP species have been found to be especially efficient in several Z-selective olefin metathesis reactions, such as homocoupling, cross-coupling, ethenolysis, and ROMP (see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7). Most of what is presented here has appeared since a review in 2009 [4]. 

Based on the inversion of the stereogenic metal center s absolute configuration after each metathesis cycle, monoalkoxide pyrrolide (MAP) complexes undergo Z-selective ROMP to syndiotactic polynorbornene (Scheme 5.14). 

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