Z-selective homocouplings

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. 

Alternatively, monoaryloxide-pyrrolide (MAP) complexes of Mo and W have been developed. It was found that the methyUdene species of these complexes were quite stable toward bimolecular decomposition, yet very reactive [20]. As such, MAP catalysts are very efficient in reactions, including the ethenolysis of methyl oleate [21], enantioselective RCM [22], and Z-selective homocouplings and cross metatheses [23, 24]. MAP catalysts are further discussed in detail in Chapters 1, 6, and see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7. 

Marinescu SC, Schrock RR, MiiUer P, Takase MK, Hoveyda AH. Room-temperature Z-selective homocoupling of a-olefms by tungsten catalysts. Organometal-lics. 2011 30(7) 1780-1782. 

Fig. 28 Z-Selective homocoupling of terminal alkenes catalyzed by W-MAP complexes...

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]. 

The Schrock lab focused on the development of new catalysts to address the Z-selectivity for a simplified cross metathesis homocoupling of terminal alkenes (Fig. 28) [61,62]. While various Mo-MAP catalysts including the optimal ones for the previous systems failed to provide homocoupled internal olefins in high Z-selectivity, it was discovered that the less reactive W-based MAP complexes A and B supported by the sterically bulky HIPTO or the 3,3 -bismesityl-aryloxide... 

Z-selectivity of the product through equilibration of the isomers that would be facile if Mo-methylidene is allowed to accumulate. The more stable alkoxy-substituted alkylidene does not undergo homocoupling due to an electronic mismatch, but can efficiently undergo productive cross metathesis with terminal alkenes in this way, a selective cross metathesis can be achieved. 

Earlier this year, the Grubbs group reported the preparation of the Ru-based catalyst with a chelating iV-heterocyclic carbene (NHC) ligand that catalyzes highly Z-selective olefin metathesis (Fig. 31) [69, 70]. This catalytic system provided similar levels of efficiency and selectivity to the W-alkylidene complexes for homocoupling reactions. The reason for the Z-selectivity is not clear at this point. Extension of the substrate scope of this catalytic system is expected. 

On the other hand, some cyclometallated complexes, such as 59 depicted in eqn (4.4), provided homocoupling products in up to 95% Z selectivity. ... 

A wide series of oxidants, spanning from TiCLj to iodine, has been used in the oxidative homocoupling of chiral 3-arylpropionic acid derivatives aimed at the preparation of lignans. The /f,/f-selectivity in the reactivity of 34 has been explained by a radical coupling mechanism (equation 20). The initially formed lithium (Z)-enolate may transform into the titanium enolate 35, which undergoes oxidation to the radical intermediate 36 via a single electron transfer process. The iyw-Z-type radicals 36 couple each other at the less hindered S-side si face) to give the R,/f-isomers 37 stereoselectively. 

Zhang, H., Wei, J., Zhao, E, Liang, Y., Wang, Z., Xi, Z. (2010). Selective synthesis of dibenzo[a,c]cyclooctatetraenes via palladium-catalyzed [4+4] eycUc homocoupling of boiylvinyl iodobenzene derivatives. Chemical Communications, 46, 7439—7441. 

Silver(I) carbonate functioned as an co-oxidant with TEMPO. Tricyclohexylphosphine was employed to suppress homocoupling between heteroarenes. Substituted thiophenes, furans, and indoles could be selectively olefinated (C5-alkenylation for thiophenes and furans, C3-alkenylation for indoles, E/Z <99 1). Unsubstituted thiophenes produced poor yields (24%) however, formyl, acetyl, and ketyl substituents were well tolerated. For electron-deficient substrates, tricyclohexylphosphine was reduced to 10 mol % to achieve good conversions. A variety of ketones could be employed using 2-methyl thiophene as a coupling partner. 

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