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Alkyne-Directed Chemo- and Enantioselectivity. Highly enantioselective catalytic reductive coupling of allyl acetate with acetylenic ketones occurs in a chemoselective manner in the presence of aliphatic or aromatic ketones. This method was used to construct C14-C23 of pladienolide D in half the steps previously required.

An inside job: Enantioselective phthalide synthesis was achieved through internal redox allylation of o-phthalaldehydes. Oxidative esterification is balanced by reductive carbonyl addition to achieve an overall redox-neutral process. This method enabled formal syntheses of ent-spirolaxine methyl ether and CJ-12,954.

Ruthenium-catalyzed reductive couplings of paraformaldehyde with dienes, alkynes, and allenes provide access to products of hydrohydroxymethylation that cannot be formed selectively under the conditions of hydroformylation. In certain cases, the regioselectivity of the CC coupling can be inverted by using nickel catalysts. With iridium catalysts, methanol engages in redox-neutral regioselective hydrohydroxymethylations.

As organic molecules are defined as compounds composed of carbon and hydrogen, stereoselective, atom-efficient methods for skeletal assembly involving the addition, removal or redistribution of hydrogen in the absence of discrete oxidation level adjustment, or non-constructive functional group interconversions represent a natural endpoint in the evolution of synthetic methods.[1] This view of synthetic efficiency tacitly recognizes the value of merged redox-construction events (“redox-economy”),[2] chemo- (site-), regio- and stereoselectivity,[3] protecting group-free chemical synthesis,[4] as well as the minimization of preactivation, defined as the degree of separation between reagent and feedstock.[5] Transformations and strategies that adhere to these ideals are inherently process-relevant.[6] Application of these concepts to the chemistry of carbonyl addition reveals a significant opportunity for invention. In classical carbonyl additions, premetallated C-nucleophiles and carbonyl compounds are typically generated through separate streams of redox reactions: discrete alcohol-to-aldehyde oxidation is frequently used to generate carbonyl electrophiles, and discrete reduction is often used to generate organometallic nucleophiles. By exploiting the native reducing capability of alcohols, redox-triggered carbonyl additions may be developed wherein hydrogen exchange between alcohols and unsaturated reactants generates transient electrophile- nucleophile pairs en route to products of formal alcohol C-H functionalization via carbonyl addition.[7] To date, three major mechanistic pathways for C-C coupling have been identified wherein alcohol oxidation is balanced by (A) C=C π-bond hydrometallation, (B) C-X reductive cleavage, and (C) transfer hydrogenolysis of oxametallacycles (Figure 1). Figure 1 Redox-economy in carbonyl addition: three general mechanistic pathways identified for redox-triggered alcohol C-H functionalization. In this review, catalytic enantioselective methods for C-C bond formation via alcohol CH functionalization are surveyed on the basis of the aforementioned mechanistic pathways. Related redox-triggered alcohol C-H functionalizations that have not been rendered asymmetric are covered elsewhere,[7] For most alcohol C-C couplings described in this account, corresponding aldehyde reductive couplings are reported,[7] but for the sake of brevity are not covered. Dehydrogenative C-C bond formations that result in formal alcohol substitution, that is, so-called “borrowing hydrogen” or “hydrogen auto-transfer” processes, radical mediated alcohol C-C couplings and processes involving stoichiometric oxidants are not discussed.[8,9]

Direct ruthenium-catalyzed CC coupling of alkynes and vicinal diols to form β,γ-unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p-toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne-diol hydrohydroxyalkylations contribute to a growing body of merged redox-construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by-products.

Over the past half century, numerous protocols for carbonyl propargylation using allenylmetal reagents have been developed.[1] Allenic Grignard reagents were used by Prevost et al.[2a] in carbonyl additions to furnish mixtures of β-acetylenic and α-allenic carbinols, which led to them to coin the term “propargylic transposition.”[2a,b] Subsequent studies by Chodkiewicz and co-workers[2c] demonstrated relative stereocontrol in such additions. Shortly thereafter, Lequam and Guillerm[2d] reported that isolable allenic stannanes provide products of carbonyl propargylation upon exposure to chloral. Later, Mukaiyama and Harada[2e] demonstrated that stannanes generated in situ from propargyl iodides and stannous chloride reacted with aldehydes to provide mixtures of β-acetylenic and α-allenic carbinols. Related propargylations employing allenylboron reagents were first reported by Favre and Gaudemar,[2f] and propargylations employing allenylsilicon reagents were first reported by Danheiser and Carini.[2g] Asymmetric variants followed (Scheme 1). Allenylboron reagents chirally modified at the boron center engage in asymmetric propargylation, as was first reported by Yamamoto and co-workers[2h] and Corey et al.[2i] Allenylstannanes chirally modified at the tin center also induce asymmetric carbonyl propargylation, as was first reported by Minowa and Mukaiyama.[2j] Axially chiral allenylstannanes, allenylsilanes, and allenylboron reagents propargylate aldehydes enantiospecifically, as was first described by Marshall et al.,[2k,l] and Hayashi and coworkers,[2m] respectively. Finally, asymmetric aldehyde propargylation using allenylmetal reagents may be catalyzed by chiral Lewis acids or chiral Lewis bases, as was first reported by Keck et al.,[2n] and Denmark and Wynn,[2o] respectively. Scheme 1 Chirally modified allenylmetal reagents for carbonyl propargylation. Tf =trifluoromethanesulfonyl, Ts =para-toluenesulfonyl. Here, we report a new approach to carbonyl propargylation based on ruthenium-catalyzed C–C bond-forming transfer hydrogenation.[3–5] Specifically, upon exposure of 1,3-enynes 1a–1g to alcohols 2a–2o in the presence of [RuHCl(CO)(PPh3)3]/dppf (dppf =1,1′-bis(diphenylphosphino)ferrocene), hydrogen shuffling between reactants occurs to generate nucleophile–electrophile pairs that regioselectively combine to furnish products of carbonyl propargylation.[6] Under related transfer hydrogenation conditions and employing isopropanol as the terminal reductant, 1,3-enynes couple to aldehydes to furnish identical products of carbonyl propargylation. The observed regiochemistry is unique with respect to related enyne–carbonyl reductive coupling reactions that are catalyzed by rhodium[5,7] and nickel complexes, [8,9,10] which favor coupling at the acetylenic terminus of the enyne. Significantly, this protocol enables carbonyl propargylation from the alcohol or aldehyde oxidation level in the absence of preformed allenylmetal reagents (Scheme 2). Scheme 2 Divergent regioselectivity observed in metal-catalyzed enyne–carbonyl coupling. In connection with our efforts to exploit catalytic hydrogenation in C–C coupling reactions beyond hydroformylation,[5] we recently demonstrated that C–C bond formation may be achieved under the conditions of iridium- and ruthenium-catalyzed transfer hydrogenation.[11] These processes enable direct carbonyl allylation from the alcohol or aldehyde oxidation level by using commercially available allenes or dienes as allyl donors. Seeking to develop corresponding carbonyl propargylations, diverse iridium and ruthenium complexes were assayed for their ability to catalyze the coupling of enyne 1a and alcohol 2a. Gratifyingly, both [{Ir(cod)Cl}2]/biphep (biphep =diphenylphosphine, cod =cycloocta-l,5-diene) and [RuHCl(CO)(PPh3)3]/dppf catalyze the desired coupling. The ruthenium-based catalyst was most effective and, under optimized conditions, enyne 1a coupled to benzylic, allylic, and aliphatic alcohols 2a–2o to form homopropargyl alcohols 3a–3o in good to excellent yields (Table 1). To probe the scope of the enyne coupling partner, enynes 1b–1g were coupled to benzyl alcohol 2b under standard reaction conditions. Good to excellent yields of propargylation products 3p–3u were observed (Table 2). Substitution at the olefinic terminus of the enyne was found to diminish conversion to product. Finally, carbonyl allylation can also be achieved from the aldehyde oxidation level by employing isopropanol as the terminal reductant. Under standard reaction conditions, aldehydes 4a–4c couple to enyne 1a to provide the products of carbonyl propargylation 3a–3c, respectively, in good to excellent yield. Thus, carbonyl propargylation may be achieved from either the alcohol or aldehyde oxidation level (Table 3). The coupling products 3a–3u are remarkably resistant to over-oxidation to form the corresponding β,γ-acetylenic ketones. However, such over-oxidation is observed if cationic ruthenium complexes are employed as catalysts. This result suggests that, for the neutral ruthenium complexes employed in this study, the alkyne moiety of the coupling product blocks a coordination site required for β hydride elimination of the carbinol C–H bond. Other aspects of the catalytic mechanism, including determination of the structural and interactive features of the ruthenium complex that influence relative and absolute stereocontrol, are currently under investigation. Table 1 Carbonyl propargylation from the alcohol oxidation level by ruthenium-catalyzed transfer hydrogenation.a Table 2 Coupling of enynes 1b–1g to benzyl alcohol 2b by ruthenium-catalyzed transfer hydrogenation.a Table 3 Carbonyl propargylation from the aldehyde oxidation level by ruthenium-catalyzed transfer hydrogenation.a A general catalytic mechanism is likely to involve the following steps:[11] a) alcohol dehydrogenation to generate a ruthenium hydride is followed by b) enyne hydrometalation to generate an allenyl metal–aldehyde/nucleophile–electrophile pair, which undergoes c) carbonyl addition with propargylic transposition. Consistent with this interpretation, the coupling of enyne 1a to [D]-2b under standard reaction conditions provides [D]-3b, in which deuterium is incorporated at the benzylic position (>95%), the allylic methyl group (56%), and the allylic methine position (24%), thus suggesting reversible olefin-hydrometalation [Eq. (1)]. (1) Our collective studies on hydrogenative and transfer hydrogenative C–C coupling define a departure from the use of preformed organometallic reagents in carbonyl addition chemistry.[5,11] For such transfer hydrogenative coupling reactions, hydrogen embedded within isopropanol or an alcohol substrate is redistributed among reactants to generate nucleophile–electrophile pairs, thus enabling carbonyl addition from the aldehyde or alcohol oxidation level. In this way, carbonyl additions that transcend the boundaries of oxidation level are devised. In the present study, we have demonstrated that 1,3-enynes serve as allenylmetal equivalents under the conditions of transfer hydrogenative coupling, thus also enabling carbonyl propargylation from the alcohol or aldehyde oxidation level. These studies contribute to a growing body of catalytic methods for the direct functionalization of carbinol C–H bonds.[11,12] Future studies will focus on the development of related alcohol–unsaturate C–C coupling processes.