Allenylic Carbonates in Enantioselective Iridium-Catalyzed Alkylations

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Allenylic Carbonates in Enantioselective Iridium-Catalyzed Alkylations David A. Petrone, Mayuko Isomura, Ivan Franzoni, Simon L. Rössler, and Erick M Carreira J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01416 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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Journal of the American Chemical Society

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.

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 undesired 1,3-dienes isomers which are obtained predominantly in the case of other metal catalysts. The synthetic utility of the products obtained was highlighted in a variety of stereoselective transition metal-catalyzed difunctionalization reactions. Furthermore, a combination of computational and experimental studies supports a putative reaction mechanism wherein enantiodetermining C−C coupling occurs via nucleophilic attack on a highly planarized aryl butadienyl -system that is coordinated to the Ir center in an η2-fashion. INTRODUCTION Significant advances in the chemistry of chiral phosphoramidites has 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 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 (chiral allenes vs. achiral 1,3-dienes); and 3) could a class of catalyst be identified which addresses the above issues in a general fashion while imparting high levels of enantiocontrol. Scheme 1. Iridium catalyzed allenylic substitution.

Herein, we report our efforts in developing 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 allow 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 transformations. 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 reactivity.11 A central theme in both organic and organometallic 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 Au17 or organic catalysts (Scheme 2A).18 Scheme 2. Prior work involving allene functionalization.

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

Given the distinct ability of robust phosphoramiditebased catalysts to promote asymmetric allylic substitution by a diverse array of nucleophiles via a uniform mechanism, we examined them as a means to address the absence of asymmetric allenylic substitution reactions using iridium catalysts.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.

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

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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 transmetallation (e.g. organozincs) lead to 1,3-dienes 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 itself,19e,f or the carbon atom directly adjactent to the allene.19g,h,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 substituion. The dichotomous behaviour 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

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 Cucatalyzed 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 it self to wide ranging screening activites. Indeed as shown in Table 1, we examined an array of parameters.23 A catalyst system comprised of [Ir(cod)Cl]2 (0.75 mol%) and chiral phosphorous‒olefin ligand (R)-L1 (3 mol%) in combination with commercial 4-ethoxy-4-oxobutylzinc bromide (1.2 equiv, 0.5 M in THF) in Et2O at room temperature for 18 hours 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 identically as in Et2O, and (S)4aa could be obtained with identical yield and selectivity (entry 2). Since, the use of basic organozinc reagents precluded the use of free allenylic alcohols, carbonyl-based 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 enatioselectivity 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’ were observed (entry 7). 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 infra). 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-BINOLderived (R)-L3 led to a significant drop in yield to 36% with 97% ee (entry 10), while SPINOL-derived (R)-L4 led to a

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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 Ircatalyzed branch-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 as obtained in 30% yield along with significant amounts of products resulting from substrate decomposition. 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 13). Table 1. Screening of reaction rarameters and the effect on enantioselective Ir-catalyzed allenylic alkylation.a

Entry 1 2 3 4 5 6 7 8d 9 10 11 12f 13g 14h

variation from the “standard” conditions None toluene instead of Et2O OAc instead of OBoc OTroc instead of OBoc [(±)-3a] = 0.5 M [RZnBr] = 0.75 M no [Ir]/L1, no [Ir] or L1 1:1 [Ir]:L1 L2 instead of L1 L3 instead of L1 L4 or L5 instead of L1 L6 instead of L1 [Rh] instead of [Ir] [Pd]/PPh3 instead of [Ir]/L1

yield (%)b 74 74 22 50:1 >50:1 >50:1 10:1e >50:1 >50:1 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 nitriles, and products 4g and 4h were 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 the allenylic alkylation to generate product (S)-4m in 64% yield and 96% ee. Thiophene containining 4o was obtained in only 35% yield, albiet with 97% ee and >50:1 rr, respectively, while alkynyl (S)-4p was obtained in 60% yield with only 4% ee. Highly electron-deficient (S)-4q could only be obtained in 35% yield despite using higher catalyst loading and longer reaction times. Other limitations to this method include 1,1-

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) were determined by 1H NMR analysis of the unpurified reaction mixtures. bReaction was run using [Ir] = 2.0 mol% and (R)-L1 = 4 mol%. c Reaction was run using [Ir] = 5.0 mol% and (R)L1 = 10 mol%.

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(0)-catalyzed diboration, which utilizes vinyl iodide 7 as a key co-catalyst, 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 above transformations differentially functioanlize all three allenyl carbon atoms.


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Scheme 3. Synthetic applications of the allenylic alkylation products.

Our group has previously reported examples of Ircatalyzed substitution of racemic allylic carbinols wherein the rates of consumption for the two substrate enantiomers can be 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 these reactions and the subject of this study, a series of control experiments were performed to provide insight into the reaction mechansim (Scheme 4). First, 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% isolated yield of the alkylation product (S)-4n which was obtained with 96% ee (Scheme 4A). The corresponding 2-naphthyl analog (±)3a led to the observation of a similar effect 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 hours was also compared (Scheme 4C). When (S)-3n (>99% ee) and (R)-3n (97% ee) were employed 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 oberservations, 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 the 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 pre-catalyst for effective allenylic alkylation reaction of (±)-3n to afford (S)-4n in 71% yield with 96% ee and >50: 1 rr. Scheme 4. Mechanistic studies for the asymmetric Ir-catalyzed allenylic alkylation.

The experimental approach discussed above was combined with theoretical investigations in order to gain



further insight into the Ir-catalyzed allenylic alkylation. 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 (exo-Ir-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 due to the severe steric clashing between the η3-butadienyl fragment and the iminostilbene moiety of the (P,olefin) ligand bound solely through phosphorus (illustrated in grey) making this structure highly disfavored.

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

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 a η2-fashion through the terminal alkene. Interestingly, all isomers display significant delocalization of the positive charge across the highly planarized -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-

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documented phenomenon which has been involked to rationalize the unique reactivity observed in serveral classes of reactions including Co-mediated (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-Ir-3’ and trans-Ir-3, respecitively. Furthermore, complex trans-Ir-3’ was found to be 15.7 kcal mol-1 higher in energy than cis-Ir-3. 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 electron rich Ir center and the delocalized positive charge may be attributed to a π-backbonding electron donation from an opportunely positioned orbital of the electron rich Ir atom to this positively charged π-accepting substrate-derived ligand.37 Furthermore, analogous DFT studies to those discussed above were performed on the previously reported Ir(III) η3-allyl species, and the LUMO distribution and NPA results that were obtained were in good aggreement with the present system.6b,23 We hypothesize that the steric hindrance created by the ligands around the Ir center would preclude the reaction of an external nucleophile at C2, thus, favoring the reaction which is obserd 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).

Scheme 5. DFT studies for the asymmetric Ir-catalyzed allenylic alkylation.


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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, in contrast to the commonly proposed Ir(I)→(III)→(I) cycle. SUMMARY

ASSOCIATED CONTENT The information is available free of charge via the internet at http://pubs.acs.org. Experimental details of synthetic procedures, X-ray data, and computational details (PDF). AUTHOR INFORMATION

C(sp3)−C(sp3)

The first enantioconvergent coupling between racemic allenylic electrophiles and alkylzinc reagents has been developed. A simple Ir(phosphorous,olefin)-based catalyst system is used to enable the highly asymmetric preparation (93-99% ee) of allene products over the corresponding dienes 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 posttransformation functionalizations as means to increase their complexity. Mechanistic studies revealed that this stereoconvergent process does not operate via a substratedependent 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 unusal Ir(I) intermediate containing a planar η2-bound butadienylium fragment that possesses a discrete positive charge delocalized within its π-system. Further studies regarding this class of enantioconvergent reactions are in progress with the aim of developing other efficient and selective transformations.

Corresponding Author* [email protected] NOTES The authors declare no competing financial interests. ACKNOWLEDGEMENTS ETH Zürich and the Swiss National Science Foundation (200020_152898) are gratefully acknowledged for financial support. DAP thanks NSERC for a postdoctoral fellowship. MI thanks the Funai Foundatation for a Funai Overseas Scholarship. IF thanks the Collaborative Research and Training Experience (Create ChemNET) program for a postdoctoral fellowship. The authors thank Dr. Amalia Poblador-Bahamonde (University of Geneva) for access to her computational facility, and Carmine Chiancone (University of Geneva) for technical support. Salome Püntener 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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50, 7961-7968; g) Mao, M.; Zhang, L.; Chen, Y.-Z.; Zhu, J.; Wu, L. ACS Catal. 2017, 7, 181-185; h) Zhu, J.; Mao, M.; Ji, H.-J.; Xu, J.-Y.; Wu, L. Org. Lett. 2017, 19, 19461949. [21] For select examples involving 1,3-dieny-2-yl halides, see: a) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 2089-2090; b) Ogasawara, M.; Ueyama, K.; Nagano, T.; Mizuhata, Y.; Hayashi, T. Org. Lett. 2003, 5, 217-219; c) Ogasawara, M.; Nagano, T.; Hayashi, T. J. Org. Chem. 2005, 70, 5764-5767. [22] Luo, H.; Ma, S. Eur. J. Org. Chem. 2013, 15, 3041. [23] For details, see the Supporting Information. [24] Turnbull, B.W.H.; Oliver, S.; Evans, P.A. J. Am. Chem. Soc. 2015¸137, 15374-15377. [25] Shu, C.; Leitner, A.; Hartwig, J.F. Angew. Chem. Int. Ed. 2004, 43, 4797-4800. [26] Shimizu, I.; Tsuji, J. Chem. Lett. 1984, 13, 233-236. [27] Takaya, Y.; Ogasawaram M,; Hayashi, T. J. Am. Chem. Soc. 1998¸ 120, 5579-5580. [28] Thermal ellipsoids are shown at the 20% probability level. Hydrogen atoms, co-crystallized solvent molecules and inconsequential disorder have been omitted for clarity in (R,R,R)-Ir-3. [29] Brock, C.P.; Schweizer, W.B.; Dunitz, J.D. J. Am. Chem. Soc. 1991, 113, 9811-9820. [30] For a review on transition metal-allene complexes, see: Shaw, B.L.; Stringer, A.J. Inoranica Chem. Acta Rev. 1973, 7, 1-10. [31] Boutadla, Y.; Davies, D.L.; Al-Duaij, O.; Fawcett, J.; Jones, R.C.; Singh, K. Dalton Trans. 2010, 43, 1044710457; b) MacDougall, T.J.; Trepanier, S.J.; Dutton, J.L.; Ferguson, M.J.; McDonald, R.; Cowie, M. Organometallics, 2011, 30, 5882-5893. [32] For select examples, see: a) Giulieri, F.; Benaim, J. J. Organomet. Chem. 1984, 276, 367-376; b) Hughes, R.P.; Lambert, J.M.J.; Rheingold, A.L. Organometallics 1985, 4, 2055-2057; c) Brisdon, B.J.; Hodson, A.G.W.; Mahon, M.F.; Molley, K.C. J. Organomet. Chem. 1988, 344, C8C10; d) Benyunes, S.A.; Green, M.; McPartlin, M.; Nation, C.B.M. J. Chem. Soc. Chem. Commun. 1989, 1887-1888; e) Rischer, R.A.; Fischer, R.W.; Herrmann, W.A.; Herdtweck, E. Chem. Ber. 1989, 122, 2035-2040; f) Bruce, M.I.; Hambley, T.W.; Liddell, M.J.; Snow, M.R.; Swincer, A.G. Tiekink, E.R.T. Organometallics, 1990, 9, 96-104; g) Brisdon, B.J.; Walton, R.A. Polyhedron, 1995, 14, 1259-1276; h) Wiedemann, R.; Wolf, J.; Werner, H. Angew. Chem. Int. Ed. 1995, 34, 1244-12246; i) Nishimura, A.; Ohashi, M.; Ogoshi, S. J. Am. Chem. Soc. 2012, 134 15692-15695. [33] The cis and trans descriptors refer to the geometry about the C2-C3 bond of the butadienyl fragment. [34] For select references, see: a) Nicholas, K.M. Acc. Chem. Res. 1987, 20, 207-214; b) Müller, T.J.J. Eur. J. Org. Chem. 2001¸ 2021-2023; c) Terada, M.; Ota, Y.; Li, F.; Toda, Y.; Kondoh, A. J. Am. Chem. Soc. 2016, 138, 11038-11043. [35] For an example of an Fe-stabilized Si cation, see: Müther, K.; Fröhlich, R.; Mück-Lichtenfeld, C.; Grimme, S.; Oestreich, M. J. Am. Chem. Soc. 2011, 133, 1244212444. [36] For reviews, see: a) Nishibayashi, Y. Synthesis 2012, 44, 489-503; b) Zhang, D.-Y.; Hu, X.-P. Tetrahedron Lett. 2015, 56, 283-295. [37] The proposed stabilizing interation of the butadienylium by the Ir(I) center resembles that of the one proposed for the coordination/activation allenes with cationic Au(I) species. See: Malacria, M.; Fensterbank, L.; Gandon, V. Top. Curr. Chem. 2011, 302, 157-182.



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Allenylic Carbonates in Enantioselective Iridium-Catalyzed Alkylations

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