Rhodium‐Catalyzed Enantioselective Intermolecular Hydroalkoxylation of Allenes and Alkynes with Alcohols: Synthesis of Branched Allylic Ethers

Abstract

Regio-and enantioselective additions of alcohols to either terminal allenes or internal alkynes provides access to allylic ethers by using a Rh I /diphenyl phosphate catalytic system. This method provides an atom-economic way to obtain chiral aliphatic and aryl allylic ethers in moderate to good yield with good to excellent enantioselectivities.Allylic ethers are key components of bioactive molecules and natural products and serve as versatile synthetic intermediates for further construction of numerous structures [1] through a wide variety of reactions such as [2,3] and [3,3] sigmatropic rearrangements and olefin metathesis reactions, among many others.[1b, 2, 3] Significant progress towards their synthesis has been achieved recently by employing transition metal catalyzed allylic substitution. In most cases, allyl acetates [4] or carbonates [5] have been used as electrophiles. However, recent studies show that even allylic alcohols can serve as allyl precursors.[6] To date, premetalated alkoxides have been used as nucleophiles along with metals to soften the hard nucleophilic character of an alkoxide (such as tin, [4a] boron, [7] zinc, [4b,c, 5f] and copper [5a,c-e] ). Following on the initial work with palladium catalysts of the groups of Trost [8] and Yamamoto, [9] we recently reported on a series of rhodium-catalyzed pronucleophile addition reactions to allenes and alkynes, reactions that can be regarded as atom-economic alternatives to allylic substitution chemistry displaying complementary branch regioselectivity (Scheme 1). [10] Thus, hydroamination, [11] hydroesterification, [12] hydroacylation, [13] and hydrothiolation [14] have been achieved to afford versatile enantioenriched branched allylic products. In this context, it would be highly interesting if simple and non-deprotonated alcohols could be added atom economically to allenes or even to alkynes, thus giving valuable branched allylic ether products.We herein report on the implementation of such a transformation allowing the highly regio-and enantioselective addition of simple and functionalized alcohols, including methanol and ethanol, to terminal allenes and internal alkynes to give a wide range of valuable building blocks for synthetic organic chemistry.Initial reactions were conducted with 3-phenylpropyl allene (1 a) and ethanol (2 b) as model substrates. First reactivity assays indicated that the presence of an acidic cocatalyst was necessary. The reaction proved to work using a rhodium(I)/DPEphos/diphenyl phosphate catalyst in dichloromethane (DCM) at 40 8C for 18 hours and furnished the desired allylic ether in 34 % yield (Table 1, entry 1). [15] Inspired by this result, various chiral bidentate diphosphine ligands with different backbones were subjected to this model reaction. While many standard privileged chiral ligands failed to catalyze this reaction, [16] the chiral ferrocene-type ligand L4 proved to be the best (entries 5 and 6). To our delight, switching the solvent from DCM to 1,2-dichloroethane (DCE) gave th...

Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

“…Very recently, we were also able to couple various alcohols enantioselectively with either terminal allenes or methylsubstituted alkynes (Table 5), 17 which are substrates previously used by Yamamoto. 4 This was made possible by the addition of a catalytic amount of a phosphoric acid ester.…”

Enantioselective hydroalkoxylation of 1, 3-dienes via Ni-catalysis

“…[4] In contrast to carbon and nitrogen-based nucleophiles, chalcogen nucleophiles are underdeveloped as coupling partners. [5] In most cases, alkynes or allenes have been used as substrates for hydroalkoxylation, with high regioselectivity and enantioselectivity, [5a, 6] albeit using precious metal catalysts, such as Rh, [7] Ru, [8] Pd, [9] or Au [10] (Figure 1A). The addition of alcohols to conjugated dienes has attracted significant attention.…”

Rhodium-catalyzed asymmetric hydroamination of gem-difluoroallenes with anilines

“…Traditional methods for the preparation of the key allylmetal species often exploit allylic alcohols and the derivatives as precursors [39][40][41]. It has been reported by Breit et al that allenes can provide completely atom-efficient path to allylmetal intermediates for asymmetric hydrofunctionalization reactions [42][43][44][45][46][47][48][49][50][51][52]. Notably, some recent research has paid attention to the formation of fluorinated allylmetal species from fluorinated molecules [53][54][55][56][57].…”