Allenylic Carbonates in Enantioselective Iridium ... - ACS Publications

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Cite This: J. Am. Chem. Soc. 2018, 140, 4697−4704

Allenylic Carbonates in Enantioselective Iridium-Catalyzed Alkylations David A. Petrone,† Mayuko Isomura,† Ivan Franzoni,‡ Simon L. Rössler,† and Erick M. Carreira*,† †

ETH Zürich, Vladimir-Prelog-Weg 3, HCI, 8093 Zürich, Switzerland Department of Chemistry, University of Toronto, Toronto, Canada, M5S 3H6

‡

S Supporting Information *

ABSTRACT: An enantioconvergent C(sp3)−C(sp3) coupling between racemic allenylic electrophiles and alkylzinc reagents has been developed. An Ir/(phosphoramidite,olefin) catalyst provides access to highly enantioenriched allenylic substitution products (93−99% ee) with complete regiocontrol (>50:1 rr in all cases) over the corresponding 1,3-diene isomers which are obtained predominantly when other metal catalysts are emplyed. The synthetic utility of the products obtained was highlighted in a variety of stereoselective transition metal-catalyzed difunctionalization reactions. Furthermore, a combination of experimental and theoretical studies provide support for a putative reaction mechanism wherein enantiodetermining C−C coupling occurs via nucleophilic attack on a highly planarized aryl butadienylium π-system that is coordinated to the Ir center in an η2-fashion.

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INTRODUCTION Significant advances in the chemistry of chiral phosphoramidites have made Ir a workhorse in the field of asymmetric allylic substitution.1,2 Yet, despite extensive research efforts focused on allylic systems,3,4 only a single report of Ir-catalyzed allenylic substitution exists. In 2004, Takeuchi reported the substitution of tertiary allenylic acetates 1 using malonates and proposed the intermediacy of Ir(III) η3-butadienyl species A (Scheme 1A).5

issues in a general fashion while imparting high levels of enantiocontrol. Ir-phosphoramidite complexes display a high aptitude for catalyzing the asymmetric allylation of a diverse array of nucleophiles via a unified mechanism.3,4,6 Therefore, we envisioned that this class of catalyst could serve as a general solution for the absence of asymmetric Ir-catalyzed allenylations.3,4,6 In considering this approach, we note that these catalysts display reduced efficiencies in asymmetric allylations for substrates with increasing allyl substitution. Except for in limited cases, only branched allylic alcohols possessing monosubstituted alkenes and linear disubstituted allylic alcohols have been employed, to the best of our knowledge.7 Consequently, at the outset we were uncertain whether allenyl substrates would be compatible with Ir catalysts modified by phosphoramidite ligands. Herein, we report our efforts to develop the first enantioconvergent alkylative coupling of racemic allenylic carbonates 3 using Ir catalysis.8 The use of mild and easily prepared alkyl organozinc nucleophiles9,10 allows for the formation of chiral, substituted allene products 4 with high levels of asymmetric induction (Scheme 1B). Importantly, the conserved allene moiety is still available for use as a functional group handle in a wide array of stereoselective transformations.

Scheme 1. Iridium-Catalyzed Allenylic Substitutiona

a

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E = electron-withdrawing group; PG = protective group.

BACKGROUND The unique structural features of allenes have intrigued chemists for more than a century, during which time they have served as ideal platforms for the discovery of new reactivities.11 A central theme in both organic and organo-

This finding leads to several questions: (1) could nucleophiles beyond malonate salts be employed in this class of transformations; (2) does the nature of the nucleophile impact the overall regioselectivity (allenes versus 1,3-dienes); and (3) could a class of catalyst be identified which addresses the above © 2018 American Chemical Society

Received: February 5, 2018 Published: February 28, 2018 4697

DOI: 10.1021/jacs.8b01416 J. Am. Chem. Soc. 2018, 140, 4697−4704

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Journal of the American Chemical Society

ranging screening activities. Indeed as shown in Table 1, we examined an array of parameters.23 A catalyst system comprised

metallic chemistry has been the application of allenes as substrates in catalytic transformations, wherein these motifs act as valuable linchpins for the synthesis of functionalized olefins.12 Many permutations exist wherein two functional groups are added across one of the two orthogonal allene carbon−carbon double bonds and commonly involve13 Cu,14 Rh,15 Pd,16 Au,17 or organic catalysts (Scheme 2A).18

Table 1. Screening of Reaction Parameters and the Effect on Enantioselective Ir-Catalyzed Allenylic Alkylationa

Scheme 2. Prior Work Involving Allene Functionalizationa

a

Reactions were conducted on 1.0 mmol scale. bDetermined by 1H NMR analysis of the unpurified reaction mixture. cDetermined by SFC analysis using chiral stationary phase. d0.75 mol % of [Ir(cod)Cl]2. eE/ Z = >50:1. fCatalyst was preformed using nPrNH2; see ref 25. g [Rh(cod)Cl]2 (0.75 mol %) was used. h0.75 mol % of {PdCl(η3C3H5)}2 and 3.0 mol % of PPh3 were used. iE/Z = 2.6:1. − = not determined.

a

FG = functional group; LG = leaving group; E = electronwithdrawing group.

A much less explored characteristic of allenes is their ability to serve as assisting groups in metal-catalzyed substitution reactions (Scheme 2B).11g Palladium has been almost exclusively used to catalyze such transformations wherein oxidative addition of activated allenylic alcohol derivatives results in the formation of Pd(II) η3-butadienyl species (e.g., B). In the presence of soft nucleophiles (e.g., malonates and amines), species B leads to allenylic substitution products via outer-sphere nucleophilic attack (path a),19 while nucleophiles which undergo transmetalation (e.g., organozincs) lead to 1,3dienes as E/Z mixtures via inner-sphere pathways (path b).20 Only soft nucleophiles have been reported in asymmetric variants to provide allenes wherein the stereogenic unit is either the allene moiety itself19b−d or the carbon atom directly adjactent to the allene.19e,f,21 Unfortunately, the propensity of palladium to generate achiral 1,3-dienes in the presence of hard or organometallic nucleophiles has precluded their use in asymmetric allenylic substitution. The dichotomous behavior of palladium encouraged us to investigate Ir/phosphoramidite catalysts in asymmetric allenylic substitution reactions given their uniform behavior across a diverse array of nucleophile classes.3,4,6,9c

of [Ir(cod)Cl]2 (0.75 mol %) and chiral phosphorus−olefin ligand (R)-L1 (3 mol %) in combination with commercial 4ethoxy-4-oxobutylzinc bromide (1.2 equiv, 0.5 M in THF) in Et2O at room temperature for 18 h was found to be optimal. Under these conditions, allene (S)-4aa was obtained in 74% yield with 97% ee and >50:1 selectivity over achiral diene 4aa′ (Table 1, entry 1). In toluene the reaction performed as well as in Et2O, and (S)-4a could be obtained in identical yield and nearly identical selectivity (entry 2). Since the use of basic organozinc reagents precluded the use of free allenylic alcohols, carbonyl-based protecting groups were employed. In this regard, Boc was key in obtaining an efficient alkylation, whereas other groups such as Ac or Troc led to significantly lower yields or complete decomposition of starting material, respectively (entries, 3 and 4). Altering the concentration of either (±)-3a or the organozinc reagent was found to be especially detrimental to the yield, while it had little effect on the enatio- or regioselectivity (entries 5 and 6). In the absence of either the [Ir(cod)Cl]2/(R)-L1 catalyst or the individual [Ir(cod)Cl]2 or (R)-L1 components, neither (S)-4aa nor 4aa′ was observed (entry 7).

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RESULTS AND DISCUSSION The allenylic alcohol derivatives that serve as substrates can be easily prepared in three steps from readily available materials. Addition of ethynylmagnesium bromide to various aldehydes furnishes racemic secondary propargyl alcohols, which can be converted into the corresponding allenylic alcohols in a straightforward manner via Ma’s robust Cu-catalyzed Crabbé homologation reaction.22 These products are then converted to the corresponding carbonates or acetates via standard synthetic procedures. The robustness of the catalyst system we have developed involving (P,olefin) ligand L1 with Ir(I) lends itself to wide 4698


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Journal of the American Chemical Society Interestingly, when the [Ir]:(R)-L1 ratio was changed from 1:2 to 1:1, the yield, regioselectivity, and enantioselectivity all significantly dropped to 19%, 10:1 rr, and 92% ee, respectively (entry 8). This leads us to believe that a 1:2 Ir:(P,olefin) complex is likely operative (vide inf ra). Structural/electronic permutations of L1 led to less satisfactory reaction outcomes (entries 9−12). The use of (S)-L2, which contains a symmetrically difluorinated iminostillbene motif, led to the allene product being obtained in 65% yield with −95% ee (entry 9). Octahydro-BINOL-derived (R)-L3 led to a significant drop in yield to 36% with 97% ee (entry 10), while SPINOLderived (R)-L4 led to a complete decomposition of starting material. In addition to the requirement for a chiral phosphoramidite ligand containing a BINOL backbone, the olefin motif is similarly crucial. When dihydro analog (S)-L5 was employed, neither alkylation product was observed (entry 11). Interestingly, phosphoramidite (S,S,S)-L6, which is a privileged ligand in Ir-catalyzed branched-selective allylic substitution,13 led to a switch in alkylation regioselectivity. In this regard, only diene 4aa′ was obtained in 48% yield with >50:1 E:Z (entry 12). Furthermore, when [Rh(cod)Cl]2 was used instead of [Ir(cod)Cl]2,24 the E-diene was the only observed alkylation product which was obtained in 30% yield along with significant amounts of products resulting from substrate decomposition (entry 13). To highlight the contrasting behavior of Pd, an experiment was conducted involving a {PdCl(η3-C3H5)}2/PPh3-based catalyst under otherwise identical reaction conditions. This led to diene 4aa′ being obtained as the sole alkylation product in 71% yield as a 2.6:1 mixture of E:Z isomers (entry 14). The scope of the enantioselective Ir-catalyzed allenylic alkylation was then examined. Since Et2O and toluene provided nearly identical results during optimization, all organozinc reagents and substrates were examined using both solvents. Furthermore, both Riecke organozinc bromides and their corresponding Knochel-type LiCl adducts10b were found to be suitable nucleophiles in the allenylic alkylation reaction (Table 2). This transformation exhibits broad functional group tolerance, wherein organozinc reagents containing esters, nitriles, alkyl chlorides, and alkenes can be employed as nucleophiles, and the desired allenes were obtained as the exclusive alkylation products in all cases (>50:1 rr). Carbonyl derivatives (S)-4aa−4ac were obtained in 66−69% yield and 95−99% ee, whereas protected alcohols (S)-4ad and (S)-4ae were obtained in 69% and 58% yield with 95% and 96% ee, respectively. Alkyl chloride containing allene (S)-4af was obtained in 62% yield with 97% ee, while 1,7-enallene (S)4ag was afforded in 57% yield with 93% ee. Ester (S)-4aa was used to establish the absolute configuration of the products by X-ray crystallographic analysis. This was accomplished by the reduction of (S)-4aa to the corresponding primary alcohol and its subsequent conversion to the p-bromophenyl carbamate.23 Various allenylic carbonate derivatives 3a−n were found to undergo the allenylic alkylation, and they proceeded to give products (S)-4a−n as single regioisomers (>50:1 rr) with 96− 99% ee (Table 3). Electron-withdrawing, fluorine-based substitutions were well tolerated under the reaction conditions, and products containing a p-OCF3 (4b), -CF3 (4c), or -F (4d) group could be obtained in 66−74% yield with 97−99% ee. Other halogens such as p-Cl (4e) and -Br (4f) were also suitable, and the corresponding products were furnished in 70% yield with 99% ee and 69% yield with 98% ee, respectively. The reaction conditions were compatible with both ester and

Table 2. Scope of Organozinc Bromides for the Ir-Catalyzed Enantioselective Allenylic Alkylation Reactiona

a Reaction conditions: [Ir] = 1.5 mol %, (R)-L1 = 3.0 mol %, (±)-3a (1.0 mmol), RZnBr or RZnBr·LiCl (1.2 equiv, 0.5 M in THF), Et2O (c = 0.7 M), rt, 24 h unless otherwise noted. Isolated yields provided. Enantiomeric excess values (ee) were determined by SFC analysis using a chrial stationary phase. Regiochemical ratios (rr’s) were determined by 1H NMR analysis of the unpurified reaction mixtures. b Reaction was run using the corresponding RZnBr·LiCl adduct. c Reaction solvent = toluene.

nitriles, and products 4g and 4h were both obtained in 69% yield with 98% ee. Biphenyl 4i was obtained in 64% yield and 98% ee under the reaction conditions. Similar efficiencies were observed when substrates containing various electron-donating groups were tested (3j−m). For example, p-Me, m-OMe, and m-OBn groups were tolerated, leading to products 4j−l in 61− 65% yield with 97−98% ee. Substrates containing ortho substituents (3m) also underwent allenylic alkylation to generate product (S)-4m in 64% yield and 96% ee. Thiophene-containing 4o was obtained in only 35% yield, albeit with 97% ee and >50:1 rr, respectively, while alkynyl (S)4p was obtained in 60% yield with only 4% ee. Highly electrondeficient (S)-4q could only be obtained in 35% yield despite using a higher catalyst loading and longer reaction times. Other limitations to this method include 1,1-disubstituted allenes, as well as alkyl-, alkenyl-, and 2-pyridyl-containing substrates. Since the alkylation products retain the valuable allene functionality, they can be easily diversified. We sought to highlight the utility of these products by their use as substrates in a series of stereoselective transition-metal-catalyzed allene transformations (Scheme 3). Cheng’s Pd-catalyzed diboration, which utilizes vinyl iodide 7 as a key cocatalyst, was applied to (S)-4aa (96% ee) and afforded (Z)-8 in 88% yield with >20:1 rr and 5.4:1 Z:E.16c Furthermore, Tsuji’s Pd/dppf-catalyzed arylamination using PhI and pyrrolidine afforded trisubstituted allylic amine (Z)-9 in 83% yield with >20:1 rr and >20:1 Z:E.26 Both of these reactions proceed without erosion of enantiopurity and involve the initial addition of PinB−Pd(II) or Ph−Pd(II) to the allene to generate a Pd(II)-η3-allyl species. Breit’s redox-neutral Rh-catalyzed allene hydrocarboxylation provided allylic alcohol-derivative 10 bearing vicinal stereocenters with an anti configuration in 61% yield with >20:1 rr and >20:1 dr.15d Finally, Hoveyda’s Cu-catalyzed protoboration was employed to provide vinyl pinacolboronate 11 in 86% yield with 19:1 rr. Boronate 11 was oxidized to afford ketone 12 in 93% yield, which resembles products obtained via Rh-catalyzed 1,4-addition to enones.14c,27 It is interesting to note that the 4699


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Our group has previously reported examples of Ir-catalyzed substitution of racemic allylic carbinols wherein the rates of consumption for the two substrate enantiomers are markedly different.6b In some cases, this effect is manifested as a highly efficient and selective kinetic resolution transformation.4b,e Due to the obvious parallels between previously reported allylic substitutions and the allenylic substitution focused on in this study, a series of control experiments were performed to provide insight into the reaction mechansim (Scheme 4). First,

Table 3. Scope of Racemic Electrophiles for the Ir-Catalyzed Enantioselective Allenylic Alkylation Reactiona

Scheme 4. Mechanistic Studies for the Asymmetric IrCatalyzed Allenylic Alkylation

a

Reaction conditions: [Ir] = 3.0 mol %, (R)-L1 = 6 mol %, (±)-3 (1.0 mmol), RZnBr (1.2 equiv, 0.5 M in THF), Et2O (c = 0.7 M), rt unless otherwise noted. Isolated yields provided. Enantiomeric excess values (ee) were determined by SFC analysis using a chrial stationary phase. Regiochemical ratios (rr’s) were determined by 1H NMR analysis of the unpurified reaction mixtures. bReaction was run using [Ir] = 2.0 mol % and (R)-L1 = 4 mol %. cReaction was run using [Ir] = 5.0 mol % and (R)-L1 = 10 mol %.

we studied the outcome of the reaction involving racemic allenylic carbonate 3n at partial conversion. Therein, 3n was recovered with only 6% ee at 60% conversion, along with 41%

above transformations differentially functionalize all three allenyl carbon atoms.

Scheme 3. Synthetic Applications of the Allenylic Alkylation Products

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Journal of the American Chemical Society isolated yield of the alkylation product (S)-4n which was obtained with 96% ee (Scheme 4A). It is important to note that a similar effect was observed for the 2-naphthyl analog (±)-3a at partial conversion.23 In a second experiment, when enantiopure substrate (S)-3n (>99% ee) was subjected to the standard reaction conditions using (±)-L1 instead, the corresponding alkylation product 4n was obtained as a racemate (Scheme 4B). Therefore, in stark contrast to some previously described Ir/(phosphoramidite,olefin)-catalyzed asymmetric allylic substitutions,6b these results suggest that the analogous allenylic displacement does not proceed with significant enantiospecificity. The efficiency with which the individual substrate enantiomers are converted to product under the standard conditions for 16 h was also compared (Scheme 4C). When (S)-3n (>99% ee) and (R)-3n (97% ee) were employed as substrates in separate experiments, alkylation product (S)-4n was obtained in both cases in 66% yield with 96% ee and in 62% yield with 95% ee, respectively. These results parallel those obtained by using the corresponding racemic substrate (±)-3n (Table 3 and Scheme 4B). Formally, retention of absolute stereoconfiguration was observed with one enantiomer, with inversion being obtained in the other. Together, these results support two conclusions: (1) in contrast to our previous observations, a substrate-dependent kinetic resolution is not operative and the catalyst has virtually no preference for one specific substrate enantiomer; and (2) the stereochemical outcome of the final C−C bond-forming event is under complete catalyst control and depends on the enantiomer of the Ir catalyst. Complex (R,R,R)-Ir-1 was prepared in 54% yield by combining [Ir(cod)Cl]2 (0.5 equiv), (R)-3n (3.5 equiv), and (R)-L1 (2 equiv) in chloroform followed by trituration with pentane. The off-white powder obtained via filtration of the reaction mixture was readily characterized by NMR spectroscopy (1D and 2D experiments).23 Unfortunately, we had little success growing crystals of (R,R,R)-Ir-1 which were suitable for X-ray diffraction studies. However, upon preparing the enantiomeric complex (S,S,S)-Ir-1, X-ray quality crystals could be obtained via vapor diffusion into an equimolar mixture of the two enantiomeric complexes (Scheme 4D).28,29 These studies confirmed unambiguously that binding of the allene occurs via the terminal double bond, which results in considerable bending of the allene (∠C1−C2−C3 = 145.7°). The terminal carbon of the allene is also further from the metal center than the central carbon (C1−Ir = 2.182 Å vs C2−Ir = 2.084 Å) which may reflect the different hybridiziations of C1 and C2.30 Finally, we were able to show that (R,R,R)-Ir-1 could act as a precatalyst for effective allenylic alkylation reaction of (±)-3n to afford (S)-4n in 71% yield with 96% ee and >50:1 rr. The experimental approach discussed above was combined with theoretical investigations in order to gain further insight into the Ir-catalyzed allenylic alkylation process. Takeuchi’s proposed Ir(III) η3-butadienyl species (A, Scheme 1A),5 in addition to the previous isolation and characterization of several η3-butadienyl complexes of Ir(III)31 and other transition metals,32 led us to initially propose the involvement of an analogous Ir(III) species possessing two (P,olefin) ligands (exoIr-2, Figure 1).6b However, efforts to optimize this structure or related isomers by density functional theory calculations at the M06-2X/def2-SVP level of theory were unsuccessful. This was partly due to severe steric clashing between the η3-butadienyl fragment and the iminostilbene moiety of the (P,olefin) ligand

Figure 1. Initially postulated Ir(III) η3-butadienyl species bearing two (phosphoramidte, olefin) ligands.

bound solely through phosphorus (illustrated in gray) which makes this structure highly disfavored. Conversely, four cis and trans isomers of complex Ir-3 were located instead (Scheme 5A).23,33 In each of these complexes, the substrate-derived butadienylium fragment binds to the metal center in an η2-fashion through the terminal alkene. Interestingly, all isomers display significant delocalization of the positive charge across the highly planarized π-systems which include the substrate arene. These computational models may explain the lack of reactivity observed for substrates bearing alkyl groups and attenuated reactivity for substrates possessing electron-deficient arenes (Table 3). In the former case, the absence of an aromatic moiety precludes stabilization via delocalization of the positive charge, while destabilization would result in the latter. The stabilization of carbocations by proximal transition metal centers is a well-documented phenomenon which has been invoked to rationalize the unique reactivity observed in serveral classes of reactions including Comediated (Nicholas reactions) or Cu-catalyzed propargylic substitutions.34−36 Structure cis-Ir-3 was found to be the most stable, being 4.4 and 5.0 kcal mol−1 lower in energy than cis-Ir3′ and trans-Ir-3, respecitively. Furthermore, complex trans-Ir3′ was found to be 15.7 kcal mol−1 higher in energy than cis-Ir3. Comparing these computed structures, the relative arrangement of the butadienylium fragment in cis-Ir-3 (illustrated in green) reduces steric clash with the pocket created by the chloride and (P,olefin) ligands (Scheme 5A and 5B). In order to evaluate potential electrophilic sites of cis-Ir-3, we analyzed the orbital composition of the lowest unoccupied molecular orbital (LUMO) and calculated the natural population analysis (NPA) charges. The LUMO of cis-Ir-3 is completely localized on the butadienylium fragment (Scheme 5C). The LUMO distribution shows two potential electrophilic sites at C2 (28.9%) and C4 (31.3%), and NPA reveals computed charges for C2 and C4 of +0.168 and +0.255, respectively.23 Of note, a small LUMO distribution on the Ir atom (5.0%) was calculated. This result may help to understand the role of iridium in the stabilization of the positively charged butadienylium fragment. The metal center coordinates to the terminal double bond moiety via orbitals that are orthogonal to the plane of the delocalized π-system. A direct interaction between the electronrich Ir center and the delocalized positive charge may be attributed to a π-backbonding electron donation from an opportunely positioned orbital of the Ir atom to this positively charged π-accepting substrate-derived ligand.37 Furthermore, analogous DFT studies to those discussed above were performed on our previously reported Ir(III) η3-allyl species bound by two (phosphoramidite,olefin) ligands, and the LUMO distribution and NPA results that were obtained are in good aggreement with the present system.6b,23 We hypothesize that the steric hindrance created by the ligands 4701


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Journal of the American Chemical Society Scheme 5. DFT Studies for the Asymmetric Ir-Catalyzed Allenylic Alkylation

mediate containing a planar η2-bound butadienylium fragment that possesses a discrete positive charge delocalized within its πsystem. Further studies regarding this class of enantioconvergent reactions are in progress with the aim of developing other efficient and selective transformations.

around the Ir center would preclude the reaction of an external nucleophile at C2, thus, favoring the reaction which is observed to occur at C4. Furthermore, one of the two possible faces of the butadienylium fragment is blocked by the binaphthyl moiety of one of the two (P,olefin) ligands (Scheme 5D, path a). This model is supported by the fact that nucleophilic attack at C4 from the most accessible face leads to the experimentally observed major enantiomer (Scheme 5D, path b). Collectively, these results suggest that an unusual mechanism for this class of transformations may be operative, wherein the formal oxidation state of the metal center does not change, which stands in contrast to the commonly invoked Ir(I) → (III) → (I) cycle in asymmetric allylic substitution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b01416. Experimental details of synthetic procedures, X-ray data, and computational details (PDF) Crystallographic data for Ir-1 (CIF) Crystallographic data for (S)-14 (CIF)

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SUMMARY The first enantioconvergent C(sp3)−C(sp3) coupling between racemic allenylic electrophiles and alkylzinc reagents has been developed. A simple Ir-(phosphorus,olefin)-based catalyst system is used to enable the highly asymmetric preparation (93−99% ee) of allene products over the corresponding diene isomers with complete regiocontrol (>50:1 in all cases). The use of mild organozinc reagents leads to broad functional group tolerance and allows the asymmetric incorporation of an array of alkyl substituents onto the benzylic position. Since this reaction exploits, yet conserves, the allene, the products can be used in a wide variety of post-transformation functionalizations as a means to increase their complexity. Mechanistic studies revealed that this stereoconvergent process does not operate via a substrate-dependent kinetic resolution and that enantioselectivity is under catalyst control. Furthermore, DFT studies do not support the involvement of an η3-butadienyl Ir(III) intermediate. Instead, they support an unusual Ir(I) inter-

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

David A. Petrone: 0000-0001-9867-9178 Ivan Franzoni: 0000-0001-8110-6218 Simon L. Rössler: 0000-0002-5057-0576 Erick M. Carreira: 0000-0003-1472-490X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ETH Zürich and the Swiss National Science Foundation (200020_152898) are gratefully acknowledged for financial support. D.A.P. thanks NSERC for a postdoctoral fellowship. MI thanks the Funai Foundatation for a Funai Overseas 4702


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Scholarship. I.F. thanks the Collaborative Research and Training Experience (Create ChemNET) program for a postdoctoral fellowship. The authors thank Dr. Amalia Poblador-Bahamonde (Université de Genève) for access to her computational facility and Carmine Chiancone (Université de Genève) for technical support. Salome Püntener (ETH Zürich) is thanked for preliminary studies regarding allenylic carbonates in other substitutions. The authors thank Michael Solar and Dr. N. Trapp (ETH Zürich) for X-ray analysis and Dr. M.-O. Ebert, R. Arnold, R. Frankenstein, and S. Burkhardt for NMR measurements. Tobias Schnitzer and Patrick Hilpert (ETH Zürich) are acknowledged for their assistance with SFC analysis, and Prof. Helma Wennemers (ETH Zürich) is thanked for SFC access.

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