The role of the functional group in double bond migration in allylic systems catalysed by ruthenium hydride complexes

Add time:09/08/2019    Source:sciencedirect.com

The influence of functional group Q on course and rate of the reaction of 3-B, 3-C, 3-Si, 3-Sn, 3-N, 3-P, 3-O, 3-S, 3-Se, 3-Cl, 3-Br, and 3-I substituted allylic systems of type Q–CH2CHCH2 with hydride ruthenium complexes (primarily [RuClH(CO)(PPh3)3]) has been researched. It has been proved that the basic factors for the reaction course are: coordinating properties of Q group, C–Q bond strength, tendency of Q-allyl to oxidative addition to [Ru], and steric effect of Q. When Q has weak (Q = alkyl, aryl, RO, ArO, …) or medium (Q = Ph3CS, RC(O)NR, …) complexing power, migration of the double bond in Q-allyl occurs. Most often, mixtures of (E)- and (Z)-1-(propenyl) derivatives are the isomerisation products. Sometimes, mainly or solely products with E configuration (e.g. Q = ArNCOMe, PhSO2) have been formed, and in other cases—mainly or solely products with Z configuration (e.g. Q = Ph3CS, Ph3CO, MeCONH). Basing on mechanistic investigations and quantum calculations, it has been shown that E-selectivity of double bond migration results mainly from specific coordination effects, but also from steric effects (of Q and ligands). On the other hand, Z-selectivity of isomerisation results from steric effects. It has been also proved that when Q has strong complexing power (Q = EtS, PhS, PhS(O), Me2N, PhCHN, allyl2P, …), the double bond migration is not observed at all. Catalytically inert ruthenium complexes are then formed in the reaction mixture. Some of them have been isolated and their structures have been defined. The bond migration has not been observed also when allylic system has been undergoing an oxidative addition to [Ru] (e.g. Q = Cl, Br, I). A similar influence of Q on reactions between Q-allyl and [Ru]–H groups has also been observed in reactions of Q-allyl with [RuH2(CO)(PPh3)3] and hydride complexes generated in situ (e.g. [RuCl2(PPh3)3] + Li[AlH4], {[RuCl2(1,5-COD)]x} + CaH2). Solvent effects in isomerisation reactions of allyl phenyl ether and allylbenzene or safrole have been researched as well. A very good, quantitative consistence of solvent effect (its complexing power) on the isomerisation with the observed effect of Q group has been shown. The requirements for Q–CH2CHCH2, enabling its isomerisation to Q–CHCHCH3, have been defined.

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