Allylic Alcohols as Substrate for Ruthenium‐Catalyzed CC Coupling Allylation Reactions. Preliminary Communication

Abstract

Allylic alcohols, rather than halides, acetates, or carbonates can be used directly in the FriedelCrafts-type coupling with various phenols. The use of a Ru IV , rather than a Ru II , precursor promotes the formation of one H + per cycle so that a large excess of acid is never present in the reaction mixture. Consequently, the leaving group in the oxidative addition reaction is H 2 O, thereby avoiding the production of an unnecessary by-product.An increasing variety of metal-mediated catalytic transformations involving allyl compounds have been developed, and these tools are rapidly becoming indispensable in organic synthesis [1 -5]. Generally, p-or s-allyl species are regarded as important intermediates in these various transformations [6] [7]. Typically, allyl sources such as halides, acetates, or carbonates have been employed since the allyl moiety often requires a leaving group to be effective; however, the direct activation of an allyl alcohol is both economically and environmentally more desirable, in that the leaving group is not wasted.One finds only a modest number of reports in which an allyl alcohol is employed as substrate [8 -13], and Akita and co-workers [8] have summarized several mechanistic possibilities for the use of an alcohol, e.g., either an oxidative addition reaction or a dehydration pathway. The former is not favored because the OH group is thought to be a poor leaving group. The latter mechanism works under acidic conditions; however, this often requires excess acid and/or severe reaction conditions [8].We have recently shown [14] that [Ru(Cp*)(MeCN) 2 )(h 3 -PhCH=CHCH 2 )](PF 6 ) 2 ((Cp* = 1,2,3,4,5-pentamethylcyclopenta-1,3-dien-1-yl); 1) facilitates Friedel-Crafts type aromatic allylation reactions of phenols and related arene compounds under relatively mild conditions, using allyl carbonate substrates. There is now a modest literature involving metal-catalyzed Friedel-Crafts reactions [15], but little involving ruthenium [16]. We report here an extension of this chemistry and show that the dicationic catalyst precursor 1, which contains Ru IV rather than Ru II , affords the CÀC coupling products starting from a selection of alcohols, as indicated in Scheme 1. The Table gives a list of the substrates and products tested. The reactions were, in many cases, fairly rapid (often complete conversion in less than 20 min) and regioselective in that the attack occurred at the least-substituted allyl C-atom, and proceeded to completion under relatively mild conditions in MeCN solution. Several of the tested alcohols

Cationic Ruthenium‐Cyclopentadienyl‐Diphosphine Complexes as Catalysts for the Allylation of Phenols with Allyl Alcohol; Relation between Structure and Catalytic Performance in O‐ vs. C‐Allylation

“…[11] Based upon the observed dependence of the allylation selectivity on the structure of the applied phosphine bidentate ligand and its chelating strength, we favour a hypothesis that C-allylation also requires activation of the phenol via its O atom at ruthenium (Scheme 3). If the isomeric p-allyl species C' and D' are more abundantly present, due to weak chelation or space around ruthenium, reductive elimination of O-allylated product will be relatively slow, as this proceeds via the s-allyl intermediate B'.…”

“…[11] Often intermedate protonation of allyl alcohol is implicated to increase the reactivity of allyl alcohol for allylation reactions. [10,11,22,23] The strong increase in allylation reaction rate observed with addition of a catalytic quantity of a strong acid such as HOTs (Table 5), can thus be explained by protonation of the allyl alcohol, making H 2 O the better leaving group.…”

“…[11,12] Pregosin and co-workers have described a system that is able to C-allylate phenols. [11] Finally, a CpRu(diphosphine) system was reported earlier with which both O-and C-allylated products were formed.…”

“…[11,12] Pregosin and co-workers have described a system that is able to C-allylate phenols. [11] Finally, a CpRu(diphosphine) system was reported earlier with which both O-and C-allylated products were formed. It was proposed that under the applied reaction conditions a thermal uncatalyzed Claisen rearrangement of O-allylated product occurred resulting in a mixture of O-and C-allylated products.…”

Scope of the allylation reaction with [RuCp(PP)]+ catalysts: changing the nucleophile or allylic alcohol

“…[8,9] This system is rather unique; in all of the reported allylation reactions with Rucomplexes as catalyst and allyl alcohol as allylating agent strong acids are present to promote the reactivity of allyl alcohol. [4,12,13] One could imagine the phenol to act as an acid (pK a = 10) to activate allyl alcohol for allylation. Given the low acidity of aliphatic alcohols, it would be interesting to see if the allylation of aliphatic alcohols proceeds with a similar catalytic system in the absence of acid.…”