Two Distinct Cyclizations of 2-Propenyl-1-ethynyl Benzenes with

Sep 14, 2016 - This work reports the development of two catalytic cyclizations of 2-propenyl-1-ethynylbenzenes with aryldiazo esters. Cationic gold ca...
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Two Distinct Cyclizations of 2-Propenyl-1-ethynyl benzenes with Aryldiazo Esters Using Au and Rh/Au catalysts respectively Sachin Bhausaheb Wagh, Yu-Chen Hsu, and Rai-Shung Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02330 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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Two Distinct Cyclizations of 2-Propenyl-1-ethynyl benzenes with Aryldiazo Esters Using Au and Rh/Au catalysts respectively Sachin Bhausaheb Wagh, Yu-Chen Hsu and Rai-Shung Liu*

Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan, ROC email: [email protected]

KEYWORDS: gold and rhodium catalysis, 2-propenyl-1-ethynylbenzenes, cyclopropanation, 1H-indenes, tetrahydro-1H-cyclopropa[b]naphthalenes.

ABSTRACT: This work reports the development of two catalytic cyclizations of 2-propenyl-1ethynylbenzenes with aryldiazo esters. Cationic gold catalyst produces 2-substituted 3-alkenyl1H-indenes with substrates over a reasonable scope. Our mechanistic study suggests that arydiazo esters attack at the cyclopropyl moieties of gold carbene intermediates, followed by skeletal rearrangement of resulting intermediates. In the presence of Rh2(esp)2 additive, the same gold catalyst alters the chemoselectivity of these reactants to afford tetrahydro-1Hcyclopropa[b]naphthalenes with excellent stereoselectivity. Herein, Rh(II) catalyst catalyzed the reactions of the same 1,6-enynes with diazo species to form cyclopropenes initially, and a

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cationic Au(I) catalyst allows their subsequent reactions with the tethered alkenes. Preferable Eselectivity and synergistic effects of Au/Rh catalysts are rationalized in a postulated mechanism.



INTRODUCTION 1,n-Enynes (n = 5-7) are readily available and widely used to construct various carbocyclic

frameworks through metal-catalyzed cycloadditions,1 cycloisomerizations2 and metathesis reactions.3 In the presence of Au(I) and Pt(II) catalysts, 1,6-enynes I commonly generate reactive cyclopropyl gold carbenes A that can be trapped with various C-, N- and O-centered nucleophiles at the cyclopropyl carbons to yield functionalized products II (path a, eq 1).2,4 Diphenylsulfur oxide or pyridine-based oxides attack preferably at the gold carbene moieties of these intermediates to yield oxidation products, i.e. formylbicyclo[3.1.0]hexanes III (Y = O, path b).5,6 In the context of diazo nucleophiles, Dixneuf reported the first cyclizations of 1,6-enynes I using

(C5Me5)RuCl(COD)

catalyst

(COD

=

1,5-cyclooctadiene)

to

yield

alkenylbicyclo[3.1.0]hexanes III (Y = CHFG); these products arose from a carbene/alkyne metathesis to generate alkenylruthenium carbenes B initially (eq 2).7 Notably, compounds III might be alternatively produced from a diazo attack at cyclopropyl gold carbenes A using Pt(II) or Au(I) catalysts.4 Beyond the preceding scope, this work reports the development of two cyclizations of 1,6-enynes 1 with diazo esters using Au(I) and combined Rh(II)/Au(I) catalysts8

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respectively, affording 2-substituted 3-alkenylindenes 3-4 and 2-alkylidene-1a,2,7,7a-tetrahydro1H-cyclopropa[b]naphthalenes 6 efficiently (eqs 3-4).

Notably, the two cyclizations suppress effectively competitive reactions. In the presence of gold catalysts, 1,6-enynes 1 undergo catalytic cycloisomerizations to yield 2-alkenylindenes 1’9 that might be convertible to cyclopropylindane products 1’-c via a subsequent cyclopropanation (eq 5). Particularly astonishing is the efficient transformations of cyclopropene intermediates 5 into tetrahydro-1H-cyclopropa[b]naphthalenes 6 (eq 4), which contradicts an early report of Lee10 that gold complex catalyzes the rearrangement of closely related cyclopropene 5’ into furan-2-(5H)-one III and methyl 1H-indene-1-carboxylate IV efficiently (eq 6). Such a rearrangement is completely circumvented with our 1,6-enyne substrates 1.

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RESULTS AND DISCUSSION Scheme 1 shows our initial tests of the cyclizations of diazo ester 2a with acyclic 1,6-

enynes I and benzene-bridged 1,6-enynes 1a using IPrAuCl/AgNTf2 in dichloromethane (DCM, 25 °C). 1,6-Enyne I underwent a rapid cycloisomerization in a brief period (1 h), affording diene I’ in a large proportion (76%) with diazoester 2a (70%) unreacted. Clearly, cyclopropyl gold carbenes A generated from 1,6-enyne I failed to react with diazo species 2a due to its facile rearrangement to diene I’. To our pleasure, 1,6-enyne 1a afforded 2-substituted 3-dimethylvinyl1H-indene 3a, over a 3.5 h period; the yield was 73%. Herein, cycloisomerization product 1a’ Scheme 1. Catalytic reactions of two 1,6-enynes with phenyldiazo ester.

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was isolated in 5% yield. Structural characterization of compound 3a relied on X-ray diffraction of its 3,5-dinitrobenzoate derivative 3a’11 through a two-step functionalization. According to our control experiment (eq 8), the formation of compound 3a is not derived from diene 1a’. Commonly gold catalysts including PPh3AuNTf2, P(t-Bu)2(o-biphenyl)AuNTf2, P(OPh)3AuNTf2 and IPrAuSbF6 were effective, yielding desired 3a in 63-71% yields. Table 1 shows the scope of reactions using various 1,6-enynes 1b-1j and phenyldiazo ester 2a (1.1 equiv); the reactions were operated with IPrAuCl/AgNTf2 ( 5 mol %) in DCM (25 °C, 1.5-4 h) . For 1,6-enynes 1b-1d bearing alterable para-substituents (X = Cl, Me, OMe), their corresponding products 3b-3d were obtained in 76-83% yields (entries 1-3). These gold cyclizations were applicable to other 5-substituted 1,6-enynes 1e and 1f (Y = Cl, OMe), rendering desired compounds 3e and 3f in satisfactory yields (68-83%, entries 4-5). We tested the reactions of additional alkenyl-substituted 1,6-enynes 1g-1h (R1, R2 = -(CH2)5-, R1 = R2 = Et), Table 1. Cyclizations of 1,6-enynes with diazo ester 2a

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which afforded desired 3g and 3h in 63-67% yields (entries 6-7). 1,6-Enynes 1i and 1j bearing trans-configured alkenyl substituents (E/Z > 10:1) were applicable substrates to afford expected dienes 3i (E/Z > 20:1) and 3j (E/Z > 12:1) satisfactorily (entries 8-9). We have examined the reactions on those 1,6-enynes bearing vinyl and trans-styryl groups, but the reactions were unsuccessful. To expand the scope of reactions, we tested various aryldiazo esters 2b-2i (1.1 equiv) with 1,6enyne 1a and IPrAuCl/AgNTf2 in DCM (25 °C, 2-6 h); the results are summarized in Table 2. For para-substituted phenyldiazo esters 2b-2c (R = Me, Cl), 2-substituted 3-alkenyl-1H-indene compounds 4b-4c were produced in 69-72% yields. 3-Methoxy and 3,5-dimethoxy reagents 2d and 2e were suitable for these cyclizations to generate desired 4d and 4e in 91% and 57% yields respectively. For 2-bromophenyldiazo ester 2f, the yield of its resulting product 4f was 64%. Diazo species 2g and 2h bearing distinct esters (R’ = n- and t-Bu) were also compatible with these reactions, yielding products 4g and 4h in 89% and 66% with a n-butyl being more efficient than tert-butyl. For 2-napththyldiazo ester 2i, its resulting product 4i was produced efficiently. Unsubstituted diazo ester HC(=N2)CO2Et (2 equiv) was inapplicable because of its rapid diazo decomposition to form diethyl fumarate. Table 2. Cyclizations with various aryldiazo esters

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To elucidate the mechanism, we prepared deuterated sample d1-1a bearing 94% deuterium content at its alkynyl proton, and its product d1-3a comprised 91% deuterium at the alkenyl position (eq 7). In the presence of D2O (5 equiv), the reaction gave d1-3a’ with the CHPh proton bearing 65% deuterium (eq 8). We prepared cycloisomerization product 1a’ from 1,6-enyne 1a using IPrAuNTf2, but the yield was only 21%. Notably, a subsequent cyclopropanation of species 1a’ with diazo ester 2a and gold catalyst yielded a cyclopropane derivative 1a’-c efficiently; but this cyclopropane derivative was not convertible to observed 3a with gold catalyst (eq 9). Accordingly, compound 1a’ is unlikely to be an intermediate in the present system. The high stereoselectivity of compound 1a’-c is due to electronic effect of the ester, as noted in related Rh(I)-and Au(I)-catalyzed cyclopropanations.12,13

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Based on the above data, we postulate a mechanism of formation of compound 3 as depicted in Scheme 2. The key step involves an attack of the diazo nucleophile at cyclopropyl gold carbenes A at its cyclopropyl ring, yielding C(3)-addition product B. To rationalize the C(2)-regioselectivity, we postulate a Roskamp rearrangement14 to liberate nitrogen, inducing a 1,2-alkyl migration to form a stable tertiary carbocation C. We envisage that the acidity of the C(2)-H proton of species C is enhanced by neighboring phenyl and ester groups, rendering a proton abstraction by an ester to form oxonium intermediate D. The carbocation and gold centers of species D exert a push-pull effect to assist the cleavage of the weak C-C bond between the two quaternary carbons, yielding observed product 3a. This mechanism rationalizes the result with the D2O experiment as the hydroxyl proton of species D is labile and exchangeable with water (eq 7). Although an alternative transformation C→D’ can not be excluded, gold-catalyzed ring cleavage of species D’ is unlikely to give desired 3a.15 Scheme 2. A postulate mechanism

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We develop another new cyclization of the same reactants using different catalysts (Scheme 3). We innovated Dixneuf’s reactions using Rh2(OAc)4 (2 mol %) to functionalizes the alkyne initially, producing cyclopropenyl product 5a in DCM with 81% yield (25 °C, 1.5 h). After isolation, species 5a was treated with IPrAuNTf2 (4 mol %) in DCM (25 °C, 14 h), producing 2alkylidene-1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalenes

6a

with

excellent

stereoselectivitiy (E/Z > 20:1); the yield was 91%. We performed a one-pot operation of this sequence through initial treatment of Rh2(OAc)4 (2 mol%) in DCM (25 °C, 1.5 h), followed by gold-catalyzed reaction in the same solution, but the overall yield (58%) is smaller than the combined yield (73%) of the two-step operations. We successfully improved the efficiency of this reaction sequence with Rh2(esp)2,16 the overall yield was increased up to 81%. To achieve a single operation, Rh2(esp)2/IPrAuNTf2 (2/4 mol %) was loaded with the reactants in DCM (25 °

C); the overall yield was further increased to 88% over a brief period (3 h). This one-step

operation not only optimizes the product yields, but also shortens reaction time. The structure of compound 6a was elucidated by 1H NOE effect to establish the E-configuration. Scheme 3. A distinct cyclization of 1,6-enyne 1a with diazo 2a

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These one-step cyclizations with dual catalysts are compatible with 1,6-enynes over a wide scope, including various substituted alkenes. Notably, the resulting products 6b-6o were produced excellent E-selectivity except compound 6c as a E/Z mixtures (E/Z = 5:1). The structures of compounds 6c, 6f and 6m were established by 1H NOE effects. The reactions were applicable to 1,6-enynes bearing mono- and 1,1-disubstituted alkenes (entries 1-2), affording the corresponding tetrahydro-1H-cyclopropa[b]naphthalene products 6b-6c in satisfactory yields (> 74%). For trans-1,2-disubstituted alkene derivatives (R1 = H, R2 = n-butyl, cyclopropyl and phenyl), the corresponding products 6d-6f were obtained in 67%, 85% and 60% yields respectively (entries 3-5). These reactions worked well for 1,6-enynes bearing tri-substituted alkenes including a E-configured alkene (entries 6-7), the reaction products 6g and 6h were produced satisfactorily (78% and 72% yields) and stereoselectiviely. 4- and 5-Phenyl Table 3. Cyclizations of 1,6-enynes/diazo species using Rh/Au catalysts.

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substituents of 1,6-enynes greatly affected the efficiencies of the second cyclizations, with electron-rich substituents (X = OMe; Y = OMe) yielding desired 6i and 6k in 77-79% yields (entries 10 and 12), whereas 4- or 5-chloro derivatives gave desired 6j and 6l in relatively low yields (41 and 51%, entries 9 and 11). We tested the reactions with various aryldiazo esters 2c and 2j, which operated well with this reaction sequence; resulting products 6m-6n were produced with 77% and 64% yields respectively. We tested this reaction on tert-butyl diazoacetate 2h to yield desired 6o in 73% yield (Scheme 4). For a cyclohexene-bridged 1,6-enyne 1p, its corresponding product, octahydro-1Hcyclopropa[b]-naphthalene 6p was produced with 41% yield in DCM (25 °C, 3 h). We performed

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each individual reaction to seek mechanistic insight. The initial Rh2(esp)2-catalyzed cyclopropenation of 1,6-enyne 1p gave cyclopropenyl species 5p in 63% yield. After isolation, species 5p was treated with gold catalyst to afford 6p in 31% yield over a protracted period (12 h), but in the presence of Rh2(esp)2 (2 mol %), the yield of 6p was increased to 51% over a brief period (3.5 h). Synergistic effect between Rh/Au is significant not only for the 5p→6p transformation, but also for the initial step 2a→5p because the overall yield 41% is larger than the combined yield 32% (63 x 51). To rationalize this effect, we speculate that LAu+ might complex with basic Rh2(esp)2 to form an acid-base pair V; this Rh2(esp)2/LAuNTf2 complex increases the electrophilicity of Rh-carbenes toward the cyclopropenation of alkynes. Meanwhile, resulting intermediate 5p can quickly diffuse into the proximate LAu+ center to stabilize key carbocation VII (see Scheme 5) before a cyclopropane cleavage. In contrast, Rh2(OAc)4 will not form a complex with IPrAu+ because its carboxylate groups are not basic enough as good donors. Scheme 4. Reactions with additional substrates

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Scheme 5 rationalizes the formation of tetrahydro-1H-cyclopropa[b]naphthalene 6a from cyclopropenyl species 5a. We envisage that π-complexation of LAu+ induces a transformation of cyclopropenyl species F into alkenylgold carbene G;17 herein, a positive charge of species VII locates mainly at the benzylic carbon to cleave the CH-C(Ph) bond selectively. This mechanism explains well our observations that chloro at the 4-phenyl carbons (entries 11, Table 3) is unfavorable for the formation of benzylic cation VII, thus giving products in relatively low yields. Scheme 5. Rationales for E-configured products

The high E-selectivity of products 6a is somewhat surprising because phenyl and ester have similar sizes. We envisage that the ester of carbene species H exerts a static interaction with LAu+, as depicted by state H. This process facilitates a cyclopropanation reaction, in which a cleavage of the Au-Cα bond is accompanied with a coordination of ester to LAu+, as shown by state I. The effect of ester on the stereoselectivity of cyclopropanation is well known in Rh carbene chemistry.12,13 Accordingly, these cyclopropene/alkene reactions can avoid formation of side products such as furan-2-(5H)-one III and methyl 1H-indene-1-carboxylate IV (eq 6). We note that the formation of side products III and IV were produced at high temperature over a protracted period (80 °C, 18 h, eq 6), indicative of slow reactions. 

CONCLUSION

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In summary, two catalytic cyclizations of 1,6-enynes with aryldiazo esters18-19 were developed using Au(I) and Rh(II)/Au(I) complexes, respectively. The use of a cationic gold catalyst produced 2-substituted 3-alkenyl-1H-indenes with substrates over a reasonable scope. Our control experiments indicate that the resulting 3-alkenyl-1H-indenes are irrelevant for their cycloisomerization reactions. Instead, we postulate an attack of diazo ester at the cyclopropyl gold carbene intermediates as the key step. In the second approach, combined Rh(II)/Au(I) catalysts implement one-step cyclizations of the same 1,6-enynes with aryl diazo esters to afford tetrahydro-1H-cyclopropa[b]naphthalenes with excellent stereoselectivity. Rationales for the preferable E-selectivity are provided in a postulated mechanism. 

ASSOCIATED CONTENT

Supporting Information This supporting information is available free of charge via the Internet at http://pubs.acs.org. Detailed experimental procedures and spectral data (PDF) X-ray crystal structure data for compound 3a’ (CCDC 1497193) 

AUTHOR INFORMATION

Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT

The authors thank the financial support of this work from Ministry of Science and Technology, Taiwan.

 References

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(1) Reviews for catalytic cycloadditions of enynes; see (a) Wender, P. A.; Verma, V. A.; Paxton, T. H. Acc. Chem. Res. 2008, 41, 40-49. (b) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49-92. (c) Inglesby, P. A.; Evans, P. A. Chem. Soc. Rev. 2010, 39, 2791-2805. (d) Garayalde, D.; Nevado, C. ACS. Catal. 2012, 2, 1462-1479. (e) Abu Sohel, S. M.; Liu, R.-S. Chem. Soc. Rev. 2009, 38, 2269-2281. (2) Selected reviews for cycloisomerizations: (a) Jimenez-Nunez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326-3350. (b) Obradors, C.; Echavarren, A. M. Acc. Chem. Res. 2014, 47, 902-912. (c) Trost, B. M. Acc. Chem. Res. 1990, 23, 34-42. (d) Le Paith, J.; Cuervo Rodriguez, D.; de Rien, S.; Dixneuf, P. H. Synlett. 2000, 1, 95-97. (e) Zhang, L.; Sun, J.; Kozmin, S. A. Adv. Synth. Catal. 2006, 348, 2271-2296. (3) For metathesis reaction, see selected reviews: (a) Fu, G. C.; Miller, S. J.; Grubbs, R. H. Acc. Chem. Res. 1995, 28, 446-452. (b) Tranka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 1829. (c) Lloyd-jones, G. C. Org. Biomol. Chem., 2003, 1, 215-236. (d) Mori, M. J. Mol. Catal. A: 2004, 213, 73-79. (e) Villar, H.; Frings, M.; Bolm, C. Chem. Soc. Rev. 2007, 36, 55-66. (4) (a) Leseure, L.; Toullec, P. Y.; Jenet, J. P.; Michelet, V. Org. Lett. 2007, 9, 4049-4052. (b) Buzas, A. K.; Istrate, F. M. Gagosz, F. Angew. Chem. Int. Ed. 2007, 46, 1141-1144. (c) Amijs, C. H. M.; Ferrer C.; Echavarren, A. M. Chem. Commun. 2007, 698-700. (d) Lopez, s.; Gomez, E. H.; Galan, P. P.; Oberhuber, C. N.; Echavarren, A. M. Angew. Chem. Int. Ed. 2006, 45, 6029-6032. (e) Schelwies, M.; Dempwolff, A. L.; Rominger, F. Helmchen, G. Angew. Chem. Int. Ed. 2007, 46, 5598-5601. (f) Nevado, C.; Cardenas, D. J.; Echavarren, A. M. Chem. Eur. J. 2003, 9, 2627-2635.

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(5) (a) Witham, C. A.; Mauleon, P.; Shapiro, N. D.; Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 5838-5839. (b) Shi, S.; Wang, T.; Yang, W.; Rudoph, M. Hashmi, A. S. K. Chem. Eur. J. 2013, 19, 6576-6580. (6) Hung, H.-H.; Liao, Y.-C.; Liu, R.-S. J. Org. Chem. 2013, 78, 7970-7976. (7) Monnier, F.; Bray, C. V-L.; Castillo, D.; Aubert, V.; Toupet, L.; Mealli, C.; Derien, S.; Dixneuf, P. H. J. Am. Chem. Soc., 2007, 129, 6037-6049. (8) For bimetallic Au/M catalysis, see selected examples: (a) García-Dominquez, P.; Nevado, C. J. Am. Chem. Soc. 2016, 138, 3266-3269. (b) Shi, Y.; Roth, K. E.; Ramgren, S. D.; Blum, S. A. J. Am. Chem. Soc. 2009, 131, 18022-18023. (c) Peng, H.; Akhmedov, Liang, Y.-F.; Jiao, N.; Shi, X. J. Am. Chem. Soc. 2015, 137, 8912-8915. (d) Mezailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2013, 15, 3626-3629. (e) Hansmann, M. M.; Hashmi, A. S. K.; Lautens, M. Org. Lett. 2013, 15, 3226-3229. (f) Zhu, Z.; Chen, K.; Xu, Q.; Shi, M. Adv. Synth. Catal. 2015, 357, 30813080. (g) Miura, T.; Tanaka, T.; Matsumoto, K. Murakami, M. Chem. Eur. J. 2014, 20, 1607816082. (9) (a) Mezailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133-4136. (b) Gawade, S. A.; Bhunia, S. Liu, R.-S. Angew. Chem. Int. Ed. 2012, 51, 7835-7838. (c) Madhushaw, R. J.; Lo, C.-Y.; Hwang, C.-W.; Su, M.-D.; Shen, H.-C.; Pal, S.; Shaikh, I. R.; Liu, R.-S. J. Am. Chem. Soc. 2004, 126, 15560-15565. (10) Bauer, J. T.; Hadfield, M. S.; Lee, A.-J. Chem. Commun. 2008, 6405-6407. (11) X-ray crystallographic data of compound 3a’ were deposited at the Cambridge Crystallographic Data Center. (CCDC 1497193)

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(12) (a) Bruzinski, P. R.; Lake, D. H.; Kong, N.; Fall, J. M.; Davies, H. M. L. J. Am. Chem. Soc. 1996, 118, 6897-6907. (b) Thompson, J. L.; Davies, H. M. L. J. Am. Chem. Soc. 2007, 129, 6090-6091. (13) (a) Briones J. F.; Davies, H. M. L. J. Am. Chem. Soc. 2012, 134, 11916-11919. (b) Prieto, A.; Fructos, M. R.; Requejo, M. M. D.; Perez, P. J.; Galan, P. P.; Delpont, N.; Echavarren, A. M. Tetrahedron. 2009, 65, 1790-1793. (14) (a) Hashimoto, T.; Miyamoto, H.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc. 2009, 131, 11280-11281. (b) Li, W.; Wang, J.; Hu, X.; Shen, K.; Wang, W.; Chu, Y.; Lin, L.; Liu, X.; Feng, X. J. Am. Chem. Soc. 2010, 132, 8532-8533. (c) Doyle, M. P.; Trudell, M. L.; Terpstra, J. W. J. Org. Chem. 1983, 48, 5146-5148. (15) As noted in eq 9, IPrAuNTf2 failed to transform cyclopropane derivative 1a’-c into 2substituted 3-alkenylindene 3a. This observation suggests that ally cation intermediate J if formed, is unlikely to yield species 3a. Instead, species J will undergo irreversible transformation into species 1a’-c if the transformation D’ → J is feasible.

(16) See selected examples: (a) Hunter, A. C.; thapally, K.; Sharma, I. Eur. J. Org. Chem. 2016, 2260-2263. (b) Werlѐ, C.; Goddard. R.; Philipps, P.; Fares, C.; Fürstner, A., J. Am. Chem. Soc. 2016, 138, 3797-3805. (c) Bonge, H. T.; Pintea, B.; Hansen, T. Org. Biomol. Chem. 2008, 6, 3670-3672. (d) Goudreau, S. R.; Marcoux, D.; Charette, A. B. J. Org. Chem. 2009, 74, 470-473.

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(17) (a) Miege, F.; Meyer, C.; Cossy, J. Beilstein. J. Org. Chem. 2011, 7, 717-734. (b) “Goldcatalyzed reactions of cyclopropenes” Archambeau, A.; Miege, F.; Crossy, J.; Meyer, C. in Patai’s Chemistry of Functional Groups, John Wiley, 2015. (c) Miege, F.; Meyer, C.; Cossy, J. Org. Lett. 2010, 12, 4144-4147. (d) Miege, F.; Meyer, C.; Cossy, J. Chem. Eur. J. 2012, 18, 7810-7822. (e) Li, C.; Zeng, Y.; Wang, J. Tetrahedron Lett. 2009, 50, 2956-2959. (f) Zhu, Z.-B.; Shi, M. Chem. Eur. J. 2008, 14, 10219-10222. (18) For gold catalyzed reactions of diazo species: see reviews: (a) Liu, L.; Zhang, J. Chem. Soc. Rev. 2016, 45, 506-516. (b) Fructos, M. R.; Diaz-Requejo, M. M.; Pѐrez, P. J. Chem. Commun. 2016, 52, 7326-7335. (19) (a) Fructos, M. R.; Belderrain, T. R.; de Fremont, P.; Scott, N. M; Nolan, S. P.: DiazRequejo, M. M.; Perez, P. J. J. Am. Chem. Soc. 2004, 126, 10846-10847. (b) Fructos, M. R.; Belderrain, T. R.; Nicasio, M. C.; Nolan, S. P.; Kaur, H.; Dıaz-Requejo, M. M.; Perez, P. J. Angew. Chem. Int. Ed. 2005, 44, 5284-5288. (c) Li, Z.; Ding, X.; He, C. J. Org. Chem. 2006, 71, 5876-5880. (d) Yu, Z.; Ma B.; Chen, M.; Wu, H.-H.; Liu, L. Zhang, J. J. Am. Chem. Soc. 2014, 136, 6904-6907. (e) Barluenga, J.; Lonzi, G.; Tomas, M.; Lopez, L. A. Chem. Eur. J. 2013, 19, 1573-1576. (f) Zhang, D.; Xu, G.; Ding, D.; Zhu, C.; Li, J.; Sun, J. Angew. Chem. Int. Ed. 2014, 53, 11070-11074. (g) Lonzi, G.; Lopez, L. A. Adv. Synth. Catal. 2013, 355, 1948-1954. (h) Pagar, V. V.; Jadhav, A. M.; Liu, R.-S. J. Am. Chem. Soc. 2011, 133, 20728-20731. (i) Jadhav, A. M.; Pagar, V. V.; Liu, R.-S. Angew. Chem. Int. Ed. 2012, 51, 11809-11813. (j) Pagar, V. V.; Liu, R.S. Angew. Chem. Int. Ed. 2015, 54, 4923-4926. (k) Xi, Y.; Su, Y.; Yu, Z.; Dong, B.; McClain, E. J.; Lan, Y.; Shi, X. Angew. Chem. Int. Ed. 2014, 53, 9817-9821.

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