Olefin isomerization by a ruthenium carbenoid complex. Cleavage of allyl and homoallyl groups

Catalysis at the Interface of Ruthenium Carbene and Ruthenium Hydride Chemistry: Organometallic Aspects and Applications to Organic Synthesis

“…The sequence is illustrated for the particularly remarkable conversion 46 Ǟ 48, where the allylamine is selectively isomerized to intermediate 47 in the presence of an allyl ether moiety (Scheme 12, b). This finding is surprising, as the deprotection of allyl ethers using a ruthenium metathesis catalyst has been reported shortly afterwards by Cossy et al [62] The conditions applied in this study are, however, different from those used by Alcaide et al For example, second-generation catalyst 4 catalyzes the isomerization of allyl ether 49 to a vinyl ether 50 in chlorinated solvents at ambient temperature. Cleavage of 50 with aqueous HCl yields deprotected alcohol 51 (Scheme 12, c).…”

Isomerization Reactions of Allylic Alcohols into Ketones with the Grubbs Reagent

“…The formation of the one-carbon degraded methyl ketone had been recorded by Hoye and Zhau in 1999 [2] and the mechanism shown to be as depicted in Scheme 2. The Grubbs reagent attacks compound A to produce another carbene complex B, which rearranges into enol C. Then, tautomerization to D and reductive elimination affords the degraded ketone E. However, our observation of the formation of the isomerized ketone 2 was the first example, although some isomerization reactions using Ru complexes had been reported [3][4][5]. We became interested in the scope of these reactions and herein we report the results of the reaction of some different allylic alcohols with the Grubbs reagent.…”

“…The organic layer was washed with brine, dried (Na 2 SO 4 ), and was evaporated to afford a residue, which was purified by silica gel column chromatography (5% hexane-EtOAc) to give alcohol 6 (245. 5 …”

Ruthenium‐Catalyzed Ring‐Closing Metathesis: Recent Advances, Limitations and Opportunities

“…Particularly relevant from the perspective of product contamination are catalytic side-reactions accessible under the conditions of the desired metathesis chemistry. Key among these undesirable side reactions are olefin isomerization [69,70,[80][81][82][83] (vide infra), and Kharasch functionalization via addition of chloroform across olefinic bonds [84,85] Such "unexpected" chemistry has in some cases been exploited to good effect: Snapper's group, for example, has developed an efficient one-pot route to vinyl ketones via hydrolysis of Kharasch products [84] (Scheme 7). An instructive review of the problems and potential inherent in the isomerization chemistry has recently appeared [42].…”

“…Olefin isomerization is a particularly problematic side-reaction in RCM, as it transforms a terminal olefin into a less reactive internal olefin, successful RCM of which affords a ringcontracted product. Isomerization activity is exacerbated for catalysts of type C2, containing an N-heterocyclic carbene (NHC) ligand [80][81][82][83]98,99], and appears to be worse in aromatic solvents [80,89], While it remains unclear whether the Ru-alkylidene itself can induce isomerization, a ruthenium hydride contaminant [98] or decomposition product [100,101] is widely regarded as the culprit. (Most disconcerting, however, is the recent finding that substrate itself can mediate deactivation and formation of hydride species: Section 3.2) [100].…”