Axially Chiral Triazoloisoquinolin-3-ylidene Ligands in Gold(I)-catalyzed Asymmetric Intermolecular (4 + 2) Cycloadditions of Allenamides and Dienes

Rúa Jenaro de la Fuente, s/n –Campus Vida – Universidade de Santiago de Compostela -15782 Santiago de Compostela – ciqus.usc.es Axially Chiral Triazoloisoquinolin-3-ylidene Ligands in Gold(I)- Catalyzed Asymmetric Intermolecular (4 + 2) Cycloadditions of Allenamides and Dienes Javier Francos, Francisca Grande-Carmona, Hélio Faustino, Javier Iglesias-Sigüenza, Elena Díez, Isaac Alonso, Rosario Fernández, José M. Lassaletta, Fernando López, José L. Mascareñas Accepted Manuscript This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Chemical Biology, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published wo rk see: https://pubs.acs.org/doi/10.1021/ja3065446 How to cite: Francos, J., Grande-Carmona, F., Faustino, H., Iglesias-Sigüenza, J., Díez, E., Alonso, I., Fernández, R., Lassaletta, J.M., López, F. and Mascareñas, J.L. (2012). Axially Chiral Triazoloisoquinolin-3- ylidene Ligands in Gold(I)- Catalyzed Asymmetric Intermolecular (4 + 2) Cycloadditions of Allenamides and Dienes. J. Am. Chem. Soc., 134, 35, 14322-14325. doi: 10.1021/ja3065446 Copyright information: © 2012 American Chemical Society Axially Chiral Triazoloisoquinolin-3-ylidene Ligands in Gold(I)-Catalyzed Asymmetric Intermolecular (4+2) Cycloadditions of Allenamides and Dienes Javier Francos,† Francisca Grande-Carmona,§ Hélio Faustino,† Javier Iglesias-Sigüenza,∫ Elena Díez,∫ Isaac Alonso,† Rosario Fernández*,∫ José M. Lassaletta,*,§ Fernando López,*,‡ and José L. Mascareñas,*,† †Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS) and Departamento de Química Orgánica. Unidad Asociada al CSIC, Universidad de Santiago de Compostela, 15782, Santiago de Compostela, Spain ‡ Instituto de Química Orgánica General CSIC, Juan de la Cierva 3, 28006, Madrid, Spain §Instituto Investigaciones Químicas (CSIC-USe), Avda. Américo Vespucio, 49, 41092 Sevilla, Spain ∫Departamento de Química Orgánica, C/ Prof. García González, 1, 41012 Sevilla, Spain Supporting Information Placeholder ABSTRACT: The first highly enantioselective intermolecular (4+2) cycloaddition between allenes and dienes is reported. The reaction provides good yields of optically active cyclohexenes featuring diverse substitution patterns and up to three stereocenters. Key for the success of the process is the use of newly designed axially chiral N-heterocyclic carbene-gold catalysts. Catalytic asymmetric Diels-Alder (DA) cycloadditions are among the most effective strategies to construct optically active six-membered carbocycles.1 In the last decades there have been many reports on enantioselective intermolecular versions of these annulations, which are typically promoted by chiral Lewis acids or by organocatalysts. In most of the cases, however, these transformations are circumscribed to alkenyl dienophiles equipped with carbonyl-activating groups (e.g. -unsaturated aldehydes, ketones, esters, amides).2,3 Enantioselective intermolecular DA reactions involving other types of dienophiles are much less frequent and, particularly, those between allenes and dienes are virtually unexplored.4 We have recently reported a gold–catalyzed intermolecular (4+2) cycloaddition between allenamides and dienes.5-7 The transformation provides a simple, versatile and stereoselective entry to a variety of cyclohexenyl products incorporating an exo enamide group and up to two new stereocenters. The reaction is better carried out using AuCl as catalyst, but can also be promoted by other gold (I) catalysts such as IPrAuCl/AgSbF6 (IPr=1,3-bis(diisopropyl-phenyl)imidazole-2- ylidene), although in this case it is somewhat less selective with respect to a competitive (2+2) annulation that provides cyclobutane side adducts.5a,8 On the above bases, we were challenged to explore the viability of achieving this type of allene-diene cycloadditions in an enantioselective manner, using chiral NHC-gold catalysts. Curiously, despite the wide use of racemic NHC-gold catalysts,9 applications of their chiral counterparts are very scarce;10 and only recently ee values above 90% have been reported for a couple of reactions, both of them promoted by chiral acyclic diaminocarbene gold complexes.10g,h Herein, we demonstrate that a newly designed chiral NHC-gold(I) complex, Au8, in which the carbene gold ligand is embedded in the cyclic backbone of an axially chiral unit, is able to catalyze the (4+2) cycloaddition between allenamides and a large number of dienes with total regio- and stereo-selectivity, and excellent enantioselectivity. We initially focused on C2–symmetric dihydroimidazole NHC–gold complexes Au1-Au5, incorporating the 1,2- diphenylethylene backbone.11,12 These complexes promoted the (4+2) cycloaddition between 1a and 2a to afford the desired cycloadduct 3a in moderate to good yields and complete stereoselectivity;13 however, the enantioselectivity was consistently poor (Table 1). We were then curious to know the performance of chiral triazolylidene-gold complexes. Triazolebased NHCs have been successfully used in asymmetric oganocatalysis,14 however, their organometallic complexes and, in particular, the gold counterparts, are essentially unexplored.15 Interestingly, Au(I) complex Au6, prepared from Bode`s triazolylidene ligand,16 promoted the cycloaddition in just 15 min at –50 °C, providing 3a in an excellent 91% yield, albeit with low ee. Table 1. Preliminary screening of chiral NHC-gold catalyst entry [Au] t (min) 3a, yield (%)a ee (%)b 1 Au1 60 79 10 2 Au2 60 88 4 3 Au3 60 74 4 4 Au4 60 76 23 5 Au5 60 51 24 6 Au6 15 91 16 a Isolate yield. b Determined by HPLC on chiral stationary phases. In view of the good catalytic activity of Au6, we decided to explore other triazole-based NHC ligands that could generate a more effective chiral environment at the proximity of the gold center. Relying on our recent work on imidazo[1,5-a]pyridin-3-ylidene (A)17 and [1,2,4]triazolo[4,3-a]pyridin-3-ylidene (B)18 NHC architectures, we envisioned that gold complexes of type C (Figure 1) could be particularly well poised for this task. The rigid bicyclic structure of these NHC units should fix the relative orientation of the C(carbene)-Au bond while forcing the C(5)-aryl substituent in close proximity to this reacting center, and therefore might favor an efficient transfer of axial chirality. Additionally, the asymmetric induction might be further tuned by modulating the steric demands of substituents R, and particularly R’.19 Figure 1. Design of new chiral NHC ligands. To test the efficacy of these ligands we prepared the complexes of Au7 (R’ = Me) and Au8 (R’ = Cy), according to the reaction sequence indicated in the Scheme 1. The process involves a selective Suzuki coupling between 1,3-dichloroisoquinoline 4 and boronic acids 5a,b, (6a,b),20 followed by Buchwald-Hartwig amination with BocNHNHBoc (7a,b).21 After deprotection, formylation and cyclization (8a,b), the racemic mixtures were resolved by chiral HPLC.22 Ensuing alkylation with 1-adamantyl bromide [9a,b(Br)] and anion exchange23 gave the triazolium salts 9a(Cl) and 9b(Cl) and, finally, metallation with Ag2O (10a,b) followed by transmetallation with AuCl·Me2S gave the desired gold complexes Au7 and Au8. Scheme 1. Synthesis of Au7 and Au8a a Reagents and conditions: (a) Pd(PPh3)4, CsF, DME, Reflux, 15 h; (b) BocNHNHBoc, Pd2(dba)3, dppf, CsCO3, toluene; (c) HCl 4M dioxane; (d) HCOOH, reflux; (e) i) POCl3, toluene, reflux, ii) HPLC chiral resolution (f) 1-BrAd, AcOH, reflux; (g) Dowex 22 (Cl); (h) Ag2O, CHCl3, MS 4Å; (i) AuCl·Me2S, toluene. The X-ray structure of complex (Ra)-Au8 (Figure 2) was used for the assignment of the absolute configuration of the chiral axis and for the quantification of the steric demand of the ligand, measured as percentage of buried volume (%Vbur) around the gold center. Using the SambVca software developed by Cavallo and co-workers,24 an extremely high %Vbur value of 46.2, among the highest described for monodentate NHCs,19b was calculated for the carbene ligand in (Ra)-Au8. Additionally, the analysis of this structure confirmed that there might be substantial differences in the accessibility of either prochiral face of the allyl-cation gold intermediate that is presumably formed by activation of the allenamide.5a,b Figure 2. X-ray structure of (Ra)-Au8. H atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Au(1)- C(1) 1.968(6), Au(1)-Cl(1) 2.2685(13), Au(1)-C(11) 3.026, C2- N(3)-C(1)-Au(1) 10.9(8), N(3)-C(2)-C(11)-C(20) 84.9(6). Gratifyingly, complex Au7/AgSbF6 catalyzed the cycloaddition of 1a and 2a, providing the expected cycloadduct 3a in good yield and a promising 63% ee (Table 2, entry 1). Importantly, the cyclohexyl-substituted derivative Au8, provided a similar yield but an excellent 90% ee (entry 2). This ee value could be improved by using AgNTf2 as silver salt (entry 3),25 and further increased up to >99% by lowering the temperature (entry 4). As can be seen in entries 5 and 6, the presence of different types of substituents at the aryl group of the diene did not significantly affect the enantioselectivity of the process, so 3b and 3c could be isolated with 94% and 96% ee, respectively.26 The presence of substituents at the internal position of the diene is well tolerated (entry 7) and, 1,4- disubstituted dienes such as 2e and 2f also participate in the cycloaddition providing a direct and diastereoselective access to 1,4-cis disubstituted cyclohexenyl products (3e and 3f) with moderate to good yields, and excellent ee’s (entries 8 and 9).27,28 Dienes lacking aryl substituents such as (E)-penta-1,3- diene (2g) or (E)-3-methylpenta-1,3-diene (2h) are also suitable substrates, providing the corresponding adducts with ee’s varying from 91 to 94% (entries 10 and 11). Even challenging 1,4-dialkyl-substituted dienes (e.g. 2i, 2j) provided satisfactory results under the standard conditions (entries 12 and 13), producing the expected adducts with complete chemo-,29 regio- and diastereo-selectivity, and ee’s close to 90%.30 Excellent enantioselection was obtained in the cycloaddition of oxazolidinone-diene 2k, which provides a N-substituted chiral cyclohexene (entry 14). Other allenamides, like 1b (entry 15) or, more importantly, terminally substituted derivatives such as 1c, do also provide excellent results. For instance, cycloaddition of 2d with allenamide 1c provided a 6:1 mixture of diastereisomeric cycloadducts 3dc and 3dc’ with 75% combined yield and 96% ee (entry 16). Gratifyingly, the diastere- oselectivity of this reaction could be increased by performing the reaction with catalyst Au7/AgSbF6 at –50 ºC, which provided exclusively 3dc, still with a good yield an excellent 93% ee (entry 17). Finally, the excellent performance and wide scope of this catalyst was again demonstrated in the cycloaddition of 2f to 1c, which provided the cyclohexenyl adduct 3fc, featuring three new stereogenic centers with complete regio- and diastereoselectivity as well as an excellent 91% ee (entry 18).31 Table 2. Catalyst identification and scope of the enantioselective (4+2) DA cycloaddition of allenamides and dienes.a entry Diene, 2 Cat Product, 3b T (°C) t (h) 3, yield (%)c ee (%)d 1 2a (Sa)-Au7/AgSbF6 (R)-3a –50 0.75 82 63 2 2a (Ra)-Au8/AgSbF6 (S)-3a –50 1 81 90 3 2a (Ra)-Au8/AgNTf2 (S)-3a –50 1 82 94 4 2a (Ra)-Au8/AgNTf2 (S)-3a –78 3 88 >99 5 2b, Ar: 3,4,5- (OMe)3C6H2 (Ra)-Au8/AgNTf2 (S)-3b –50 1 58e 94 6 2c, Ar: 4-Br-C6H4 (Ra)-Au8/AgNTf2 (S)-3c –50 1 55e 96 7 2d, R4 = Me; R2 = H, (Ra)-Au8/AgNTf2 (S)-3d –50 1 88 95 8f 2e, R4 = H; R2 = Ph (Sa)-Au8/AgNTf2 (2R,5S)-3eg 0 0.25 48 96 9 2f, R4 = H; R2 = Me (Ra)-Au8/AgNTf2 (2S,5R)-3f –50 1 85 94 10 2g, R4 = H (Ra)-Au8/AgNTf2 (R)-3g –50 3 71 91 11 2h, R4 = Me (Ra)-Au8/AgNTf2 (R)-3h –78 2 56 94 12 2i, R2 = Me (Ra)-Au8/AgNTf2 (2R,5R)-3i –50 12 56 87 13 2j, R2 = CH2OTBS (Ra)-Au8/AgNTf2 (2R,5R)-3j –50 12 50 89 14 2k (Ra)-Au8/AgNTf2 (R)-3k –50 1 69 >99 15h 2d (Ra)-Au8/AgNTf2 (S)-3db –50  rt 3 50 90 16i 2d (Ra)-Au8/AgNTf2 (2S,6R)-3dc: (2S,6S)-3dc’ –50–15 2 75, (dr = 6:1)j 96k 17i 2d (Sa)-Au7/AgSbF6 (2R,6S)-3dc –50 16 64, (dr > 20:1)l 93 18i 2f (Sa)-Au8/AgNTf2 (2R,5R,6S)-3fcg +10 3 51 91 a Conditions: Diene (3 equiv) and allenamide (1 equiv) were added to a cooled solution of (Ra)-Au8 and AgX in CH2Cl2 (0.1 M) unless otherwise noted. Conv. > 99%. b N* = (2-oxo)oxazolidin-3-yl, N** = (2-oxo)pyrrolidin-1-yl. The absolute configuration of (S)-3c was determined by X-ray diffraction analysis, see the Supporting Information. The absolute configuration of all other products 3 was assigned by analogy. c Isolated yields. d Determined by HPLC on chiral stationary phases. e Unoptimized yield. f Carried out with 4 equiv of diene. g Carried out with (Sa)-Au8, instead of (Ra)-Au8.h Carried out with allenamide 1b. i Carried out with allenamide 1c. j Ratio of (2S,6R)-3dc: (2S,6S)-3dc’ (crude1H-NMR). k Same ee values (97%) were observed for both diastereoisomers, (2S,6R)-3dc and (2S,6S)-3dc’. l Ratio of (2R,6S)-3dc: (2R,6R)-3dc’ (crude1H-NMR). In summary, we described the first examples of a highly enantioselective intermolecular (4+2) cycloaddition between allenes and dienes, which also represents the first asymmetric intermolecular (4+2) cycloaddition promoted by a chiral carbophilic metal complex. The reaction provides a versatile and practical approach to a variety of optically active cyclohexene products which are not obviously accessible using alternative methodologies. The success in the asymmetric induction relies on the development of a novel class of designed ligands featuring a triazole unit embedded in a rigid axially chiral cyclic frame. These ligands might find utility in other metalcatalyzed asymmetric processes; in particular, the excellent results obtained with catalyst Au-8 augurs well for further applications in other gold-catalyzed transformations. ASSOCIATED CONTENT Supporting Information. Full experimental procedures, characterization data, crystallographic data and HPLC traces. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * joseluis.ma[email protected], fernando.lop[email protected], [email protected], jm[email protected] ACKNOWLEDGMENT This work was supported by the Spanish MINECO, (SAF2010- 20822-C02, CTQ2010-15297, CTQ2010-14974, and Consolider Ingenio 2010 CSD2007-00006), the ERDF funds, the Xunta de Galicia (INCITE09209084PR, GRC2010/12) and the Junta de Andalucía (2008/FQM-3833 and 2009/FQM-4537, predoctoral fellowship to F. G.-C). HF acknowledges the Fundação para a Ciência e Tecnologia (FCT, Portugal) and POPH/FSE for a Ph. D. grant (Grant SFRH/BD/60214/2009). Johnson-Matthey PLC is acknowledged for a gift of metal complexes. REFERENCES (1) For general reviews on asymmetric Diels-Alder reactions, see e.g.: (a) Corey, E. J. Angew. Chem. Int. Ed. 2002, 41, 1650. (b) Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis, G. Angew. Chem. Int. Ed. 2002, 41, 1668. (2) For selected examples of DA reactions catalyzed by chiral Lewis acids, see: (a) Schotes, C.; Mezzetti, A. J. Am. Chem. Soc. 2010, 132, 3652. (b) Shibatomi, K.; Futatsugi, K.; Kobayashi, F.; Iwasa, S.; Yamamoto, H. J. Am. Chem. Soc. 2010, 132, 5625. (c) Liu, D.; Canales, E.; Corey, E. J. J. Am. Chem. Soc. 2007, 129, 1498. (d) Payette, J. N.; Yamamoto, H. J. Am. Chem. Soc. 2007, 129, 9536, and references therein. (3) For selected examples with chiral organocatalysts, see: (a) Momiyama, N.; Konno, T.; Furiya, Y.; Iwamoto, T.; Terada, M. J. Am. Chem. Soc. 2011, 133, 19294. (b) Jia, Z.-J.; Jiang, H.; Li, J.-L.; Gschwend, B.; Li, Q.-Z.; Yin, X.; Grouleff, J.; Chen, Y.-C.; Jorgensen, K. A. J. Am. Chem. Soc. 2011, 133, 5053. (c) Northrup, A. B.; MacMillan, D. W. C. J. Am. Chem. Soc. 2002, 124, 2458, and references therein. (4) Asymmetric DA reactions with allenyl dienophiles are limited to chiral auxiliary strategies. See: (a) Henderson, J. R.; Chesterman, J. P.; Parvez, M.; Keay, B. A. J. Org. Chem. 2010, 75, 988 and references therein. (b) Ikeda, I.; Honda, K.; Osawa, E.; Shiro, M.; Aso, M.; Kanematsu, K. J. Org. Chem. 1996, 61, 2031. (c) Node, M.; Nishide, K.; Fujiwara, T.; Ichihashi, S. Chem. Commun. 1998, 2363. For isolated examples of enantioselective hetero-DA reactions involving allenyl dienophiles and Lewis base catalysts, see: (d) Wang, X.; Fang, T.; Tong, X. Angew. Chem. Int. Ed. 2011, 50, 5361. (e) Ashtekar, K. D.; Staples, R. J.; Borhan, B. Org. Lett. 2011, 13, 5732. (5) (a) Faustino, H.; López, F.; Castedo, L.; Mascareñas, J. L. Chem. Sci. 2011, 2, 633. See also: (b) Wang, Q. R.; Wang, G. A.; Zou, Y.; Li, Z. M.; Goeke, A. Adv. Synth. Catal. 2011, 353, 550. For thermal intramolecular (4+2) cycloadditions of allenamides and dienes, see: (c) Lohse, A. G.; Hsung, R. P. Org. Lett. 2009, 11, 3430. (d) Horino, Y.; Kimura, M.; Tanaka, S.; Okajima, T.; Tamaru, Y. Chem. Eur. J. 2003, 9, 2419. (6) For reviews on gold-catalyzed cycloadditions, see: (a) López, F.; Mascareñas, J. L. Beilstein J. Org. Chem. 2011, 7, 1075. (c) Fernández, I.; Mascareñas, J. L. Org. Biol. Chem. 2012, 10, 699. (7) For intramolecular enantioselective (4 + 2) cycloadditions of allenes and dienes, see: (a) Alonso, I.; Trillo, B.; López, F.; Montserrat, S.; Ujaque, G.; Castedo, L.; Lledós, A.; Mascareñas, J. L. J. Am. Chem. Soc. 2009, 131, 13020. (b) González, A. Z.; Toste, F. D. Org. Lett. 2010, 12, 200. (c) Teller, H.; Flugge, S.; Goddard, R.; Fürstner, A. Angew. Chem. Int. Ed. 2010, 49, 1949. For a related (4+3) intramolecular enantioselective cycloaddition of allenedienes, see: (d) Alonso, I.; Faustino, H.; López, F.; Mascareñas, J. L. Angew. Chem. Int. Ed. 2011, 50, 11496. (8) For examples of (2+2) cycloadditions of allenamides, see: (a) Faustino, H.; Bernal, P.; Castedo, L.; López, F.; Mascareñas, J. L. Adv. Synth. Catal. 2012, 354, 1658. (b) Li, X.-X.; Zhu, L.-L.; Zhou, W.; Chen, Z. Org. Lett. 2012, 14, 436. (c) Suárez-Pantiga, S.; Hernández-Díaz, C.; Piedrafita, M.; Rubio, E.; González, J. M. Adv. Synth. Catal. 2012, 354, 1651. (9) (a) Nolan, S. P. Acc. Chem. Res. 2011, 44, 91. (b) Gaillard, S.; Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45, 778. (c) Diez- González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (10) (a) Espinet, P.; Bartolome, C.; Garcia-Cuadrado, D.; Ramiro, Z. Inorg. Chem. 2010, 49, 9758. (b) Arnanz, A.; González-Arellano, C.; Juan, A.; Villaverde, G.; Corma, A.; Iglesias, M.; Sánchez, F. Chem. Commun. 2010, 46, 3001. (c) Wang, W.; Yang, J.; Wang, F.; Shi, M. Organometallics 2011, 30, 3859. (d) Wilckens, K.; Lentz, D.; Czekelius, C. Organometallics 2011, 30, 1287. (e) Yang, J.; Zhang, R.; Wang, W.; Zhang, Z.; Shi, M. Tetrahedron: Asymmetry 2011, 22, 2029. (f) Yamada, K.-I.; Matsumoto, Y.; Selim, K. B.; Yamamoto, Y.; Tomioka, K. Tetrahedron 2012, 68, 4159. (g) Wang, Y.-M.; Kuzniewski, C. N.; Rauniyar, V.; Hoong, C.; Toste, F. D. J. Am. Chem. Soc. 2011, 133, 12972. (h) Handa, S.; Slaughter, L. M. Angew. Chem. Int. Ed. 2012, 51, 2912. For a review covering asymmetric NHC-gold catalysis, see: (i) Wanga, F.; Liua, L.-J.; Wanga, W.; Li, S.; Shi, M. Coord. Chem. Rev. 2012, 256, 804. See also, (j) Widenhoefer, R. A. Chem. Eur. J. 2008, 14, 5382. (k) Sengupta, S.; Shi, X. Chem. Cat. Chem. 2010, 2, 609. (l) Pradal, A.; Toullec, P. Y.; Michelet, V. Synthesis 2011, 1501. (11) For other types of transition-metal catalysts incorporating these C2-symmetric chiral NHC ligands, and their applications, see: (a) Funk, T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 1840. (b) Chaulagain, M. R.; Sormunen, G. J.; Montgomery, J. J. Am. Chem. Soc. 2007, 129, 9568. (d) Selim, K.; Matsumoto, Y.; Yamada, K.; Tomioka, K. Angew. Chem. Int. Ed. 2009, 48, 8733. (e) Matsumoto, Y.; Yamada, K.; Tomioka, K. J. Org. Chem. 2008, 73, 4578. (f) Lee, K.; Hoveyda, A. H. J. Org. Chem. 2009, 74, 4455 and references therein. (g) See also ref. 10i. (12) See the Supporting Information for details concerning the synthesis of these chiral NHC-gold complexes. See also refs 11f and 10f. (13) (2+2) Cycloadducts were not detected. (14) (a) Bugaut, X.; Glorius, F. Chem. Soc. Rev. 2012, 41, 3511. (b) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606. (c) Moore, J. L.; Rovis, T. Top. Curr. Chem. 2009, 290, 77. (15) For the preparation of a chiral triazolylidene-gold complex, see: (a) Paul, S.; Schweizer, W. B.; Ebert, M.-O.; Gilmour, R. Organometallics 2010, 29, 4424. For isolated examples of achiral triazolylidene-gold complexes, see: (b) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24, 2411. (c) Alcarazo, M.; Stork, T.; Anoop, A.; Thiel, W.; Fürstner, A. Angew. Chem. Int. Ed. 2010, 49, 2542. (16) He, M.; Struble, J. R.; Bode, J. W. J. Am. Chem. Soc. 2006, 128, 8418. (17) (a) Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Fernández, R.; Brown, J. M.; Lassaletta, J. M. J. Am. Chem. Soc. 2005, 127, 3290. See also (b) Burstein, C.; Lehmann, C. W.; Glorius, F. Tetrahedron 2005, 61, 6207. (18) (a) Iglesias-Sigüenza, J.; Ros, A.; Díez, E.; Alcarazo, M.; Álvarez, E.; Fernández, R.; Lassaletta, J. M. Dalton Trans. 2009, 7113. See also (b) Ma, Y.; Wei, S.; Lan, J.; Wang, J.; Xie, R.; You, J. J. Org. Chem. 2008, 73, 8256. (19) For recent reviews analyzing the steric properties of NHC ligands, see: (a) Dröge, T.; Glorius, F. Angew. Chem. Int. Ed. 2010, 49, 6940. (b) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841. (20) Ford, A.; Sinn, E.; Woodward, S. J. Chem. Soc., Perkin Trans. 1, 1997, 927. (21) Arterburn, J. B.; Rao, K. V.; Ramdas, R.; Dible, B. R. Org. Lett. 2001, 3, 1351. (22) The racemic mixtures 8a,b were resolved by preparative HPLC (Chiracel IA). See the Supp. Info for further details. (23) The anion exchange to Cl prevents the presence of Cl/Br mixtures after transmetallation of the NHC-AgBr intermediate with AuCl·SMe2. (24) (a) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo L. Eur. J. Inorg. Chem., 2009, 1759; (b) http://www.molnac.unisa.it/OMtools.php. The calculated value (H atoms omitted) was obtained using the standard parameters: radius of sphere = 3.5 Å; distance from sphere = 2.1 Å; mesh step = 0.05. (25) Equivalent results to those of entries 2 and 3 were obtained when the cationic catalyst was previously filtered through celite. For a pertinent discussion on the “silver effect”, see: Wang, D.; Cai, R.; Sharma, S.; Jirak, J.; Thummanapelli, S. K.; Akhmedov, N. G.; Zhang, H.; Liu, X.; Petersen, J. L.; Shi, X. J. Am. Chem. Soc. 2012, 134, 9012 and references therein. (26) A highly electron-withdrawing p-NO2 substituent at the aryl group of the diene still provided the reaction product with 92% ee, although in just 11% yield. On the other hand, furan did not participate in this type of cycloaddition. See the Supp. Info for details. (27) The synthetic potential of the exo-enamide group was explored with cycloadduct (S)-3d by transforming it into a hydroxylmethyl group, upon treatment under acidic conditions and subsequent reduction with NaBH4.5a (28) The reaction of (1Z,3E)-2f with 1a under standard conditions or even at higher temperatures did not provide any (4+2) cycloadduct, suggesting that a concerted cycloaddition between the diene (in a s-cis conformation) and the gold-allyl cation derived from 1a could be taking place. Both, concerted (4+2) or (4+3) cycloadditions, this later followed by a 1,2-ring contraction, could be equally feasible. See the Supp. Info., and references 5a,b and 7a for related mechanistic information. (29) In contrast to the reaction of 2i and 1a promoted by IPrAuCl/ AgSbF6,5a we haven’t observed (2+2) products when using Au8/AgNTf2. (30) Cycloaddition experiments of allenamide 1a with dienes 2i, 2a and 2h employing gold catalysts featuring chiral phosphoramidites or bisphosphine ligands led to low ee’s and/or yields. See the Supp. Info. for details. (31) This reaction fails with AuCl or IPrAuCl/AgSbF6,5a which further highlights the potential and efficiency of Au8.