Catalytic α-Allylation of Enones with Alcohols via [Gold(I)]-Mediated [3,3]-Sigmatropic Rearrangement of Propargylic Carboxylates

Article abstract of DOI:10.1002/adsc.201600113

The site-selective α-allylic alkylation of enones with alcohols via gold-triggered formation of nucleophilic allenoates by means of [3,3]-sigmatropic rearrangements of propargylic carboxylates is reported. A range of α-alkylated enones is obtained in high yields in the presence of the gold complex [JohnPhosAu(ACN)]SbF₆ (2 mol%) as the promoting agent. Examples of allylation/Friedel-Crafts alkylation sequences are also provided, delivering densely functionalized dihydroindenes and dihydrobenzo[7]annulenes in a one-pot procedure in moderate yields. The role of the Br?nsted acidity (i.e., pivalic acid) delivered during the reaction course in assisting the formation of carbocationic intermediates is documented.

Full text of DOI:10.1002/adsc.201600113

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DOI: 10.1002/adsc.201600113
Very Important Publication
Catalytic a-Allylation of Enones with Alcohols via [Gold(I)]-
Mediated [3,3]-Sigmatropic Rearrangement of Propargylic
Carboxylates
Elisabetta Manoni,a Mario Daka,a Marco M. Mastandrea,a Assunta De Nisi,a
Magda Monari,a and Marco Bandinia,*
a
Department of Chemistry “G. Ciamician”, Alma Mater Studiorum – University of Bologna, via Selmi 2, 40126, Bologna,
Italy
Fax: (+39)-(0)51-209-9456: phone: (+39)-(0)51-209-9751; e-mail: marco.bandini@unibo.it
Received: January 25, 2016; Revised: February 19, 2016; Published online: April 18, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600113.
Abstract: The site-selective a-allylic alkylation of
enones with alcohols via gold-triggered formation
of nucleophilic allenoates by means of [3,3]-sigma-
tropic rearrangements of propargylic carboxylates is
reported. A range of a-alkylated enones is obtained
in high yields in the presence of the gold complex
[JohnPhosAu(ACN)]SbF6 (2 mol%) as the promot-
ing agent. Examples of allylation/Friedel–Crafts al-
kylation sequences are also provided, delivering
densely functionalized dihydroindenes and dihydro-
benzo[7]annulenes in a one-pot procedure in mod-
erate yields. The role of the Brønsted acidity (i.e.,
pivalic acid) delivered during the reaction course in
assisting the formation of carbocationic intermedi-
ates is documented.
Figure 1. Gold(I)-promoted sigmatropic rearrangement of
propargylic esters. a) classic “nucleophilic” trapping; b) less
explored “electrophilic” trapping.
Keywords: alcohols; allylation; enones; gold cataly-
sis; rearrangement
In this line, the still largely underemployed latter
approach has been very recently and elegantly pio-
neered by Hashmi and co-workers who documented
The gold-catalyzed [3,3]-sigmatropic rearrangement on the “electrophilic” trapping of the titled allenoates
of propargylic carboxylates[1] is one of the most ex- with acetal derivatives to perform the a-alkylation of
ploited organic transformations in homogenous enones.[5a]
gold(I) and gold(III) catalysis,[2] to access chemical di-
In line with our ongoing research interests on the
versity in a direct and straightforward manner.[3] The gold-assisted rearrangements of propargylic carboxy-
regioselective [3,3]-migration generally results in the lates,[4g] along with allylation reactions,[6] we consid-
formation of substituted allenoates that commonly ered the possibility to merge these chemical events
undergo nucleophilic trapping by means of gold-trig- enabling the direct a-allylic alkylation of enones with
gered electrophilic activation of the allenyl frame- alcohols as environmentally benign alkylating
work (Figure 1, pathway a).[4] On the contrary, the ex- agents.[7] In particular, we envisioned that the in situ
ploitation of the intrinsic nucleophilicity of the alle- delivered allenoate could undergo a direct condensa-
noate species has been by far considerably less ex- tion with allylic alcohols upon activation by Lewis or
plored in organic synthesis up to now (Figure 1, path- Brønsted acid catalysis.
way b).[5]
An outline of the proposed synthetic approach is
depicted in the Scheme 1.
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Catalytic a-Allylation of Enones with Alcohols
Table 1. Optimization of the catalytic system.[a]
Scheme 1. Working hypothesis for the present [Au(I)]-cata-
lyzed a-allylic alkylation of enones with alcohols.
Run [Au] (2 mol%)
1a/ Yield of 4aa
2a
Z:E[c]
[%][b]
1
[Au(IPr)NTf2]
1a
1a
40[d]
23
95:5
95:5
97:3
–:–
95:5
–
95:5
95:5
95:5
97:3
97:3
95:5
98:2
98:2
98:2
It is worth mentioning that the present approach
could represent a complementary protocol to Baylis–
Hillman-type condensations that, besides the great
diffusion, found limited applications in allylating pro-
cedures.[8]
At the outset of the investigation the propargylic
acetate 1a and activated trans-1,3-diphenylpropen-ol
2a were selected for the parameter optimization and
a survey of reaction conditions was carried out
(Table 1). Interestingly, while carbene-gold catalysis
{[Au(IPr)Cl]/AgNTf2 2 mol%}[9] [IPr=1,3-bis(2,6-dii-
sopropylphenyl)imidazol-2-ylidene] proved only mod-
erate efficiency as catalyst (Table 1, entry 1), the de-
sired allylated compound 4aa was isolated in higher
yield (65%) and good cis-diastereoselectivity in the
presence of JohnPhosAuCl/AgNTf2 (2 mol%, entry 3,
Table 1).[10]
2[e] Ph3PAuNTf2
3
4
JohnPhosAuCl/AgNTf2 1a
JackiePhosAuNTf2
65
1a
1a
1a
1a
1a
1a
12[d]
38[d]
nr
5[e] Me-Dalphos-AuNTf2
6
AgNTf2
7[e] JohnPhosAuOTs
8[e] JohnPhosAuTFA
9[e] JohnPhosAuBArF
48
45
61
74
86
71
80
87
10
11
JohnPhosAu(ACN)SbF6 1a
JohnPhosAu(ACN)SbF6 2a
12[f] JohnPhosAu(ACN)SbF6 2a
13[g] JohnPhosAu(ACN)SbF6 2a
14[h] JohnPhosAu(ACN)SbF6 2a
15[i] JohnPhosAu(ACN)SbF6 2a
76
[a]
All the reactions were carried out under a nitrogen at-
mosphere. Ratio 1a/2a:3a:[Au]=1:1.5:0.02 unless other-
wise specified. All reagents employed and the products
obtained should be considered as racemic mixtures.
Isolated yield after flash chromatography.
Determined via GC-MS on the reaction crude.
Large amount of unreacted acetate 1a was recovered.
Reaction time 48 h.
The catalyst was prepared in situ by treating equimolar
amounts of LAuCl and AgX.
Trifluoromethyltoluene was used as the solvent.
Benzene was used as the solvent.
This value was the highest among the phosphine-
based complexes tested (entries 4 and 5).
[b]
[c]
[d]
Subsequently the effect of the counterion was in-
vestigated (Table 1, entries 7–9) leading to the conclu-
sion Àthat weakly coordinating anions (i.e., BArFÀ and
SbF6 ) performed better than the more nucleophilic
ones.[11] In particular, commercially available silver-
free [JohnPhosAu(CH3CN)SbF6][12] resulted in the
isolation of 4aa in 74% yield (entry 10). Therefore,
due to the well-known impact of the structure of
propargylic esters on their gold-assisted migratory at-
titude, we surmised that replacement of an acetyl
group (1a) with a bulkier pivalate unit (2a) could
[e]
[f]
[g]
[h]
[i]
Dichloroethane was used as the solvent.
A 2a:3a:[Au]=1:1:0.02 ratio was utilized. nr=no reac-
tion.
favour the shifting of the equilibrium toward the 95%, 4af). Electron-withdrawing as well as moderate
allene isomeric form.[13] Gratifyingly, under the latter electron-donating groups were tolerated. Differently,
conditions, 4aa was isolated in 86% yield with no ero- stronger EDGs such us OMe (para-position 3i) result-
sion of the Z:E ratio (entry 11).[5a] Additionally, other ed in disproportion of the alcohol to the correspond-
solvents such as DCE, benzene and CF3C6H5 have ing chalcone and 1,3-diphenylpropene derivatives.[14]
proved competence in the transformation but no sig- Analogously, the replacement of one or both the aro-
nificant variance in terms of chemical outcome was matic substituents with hydrogen atoms or alkylic
observed. Last but not least, AgNTf2 (Table 1, substituents, caused the failure of the process. This
À
entry 6) did not show any catalytic effect in the pres- evidence supports a SN1-type mechanism in the C C
ent transformation.
bond forming event. Additional insights into this
Scope and limitations of the protocol were initially aspect came by the reaction of the asymmetric alcohol
assessed by reacting a range of symmetric allylic alco- 3g with pivalate 2a that produced a 1:1 mixture of re-
hols 3b–i with propargylic pivalate 2a [JohnPhos- gioisomers.
Au(ACN)SbF6, 2 mol%, 1108C, 2 h, Scheme 2].
In several cases, trisubstituted dihydroindenyl com-
Generally, 1,3-diphenylallylic alcohols reacted pounds were also isolated in low amount (<10%). In-
smoothly under optimal conditions, delivering the cor- terestingly, indene 5ah was isolated as a major prod-
responding product in good to excellent yields (up to uct in (38%) in a 1.2:1 diastereomeric ratio. An allyla-
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Elisabetta Manoni et al.
Scheme 2. Scope of the reaction related on the use of different allylic alcohols. nr=no reaction.
tion/Friedel–Crafts-type alkylation is considered to be enone moiety exhibits a significant deviation from
responsible for the afore-described chemical outcome planarity forming a torsion angle of 52.3(3)8 between
(vide infra).
the C=C double bond and the carbonyl group. This
The structure of enones 4 was unambiguously de- particular arrangement allows weak intermolecular
termined by single crystal X-ray diffraction. In partic- p–p stackings among the phenyl groups linked to the
ular, the X-ray crystal structure of compound 4ae carbonyls as shown in Scheme 3.
showed that the enone unit adopts the Z configura-
Additionally, in order to further prove the flexibili-
tion.[15] The geometry of the molecule in the crystal is ty of the methodology, several propargylic pivalates
influenced by the high steric hindrance; in fact the (2b–i) featuring aliphatic as well as aromatic groups at
Scheme 3. Top: single crystal X-ray structure of one enantiomer of (Z,E)-4ae with 30% probability thermal ellipsoids.
Bottom: View along the a axis of the crystal packing of 4ae. (green dotted lines show the weak p–p stackings; the centroid-
centroid distance is 4.02 Š).
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Catalytic a-Allylation of Enones with Alcohols
Scheme 4. Scope of the reaction related to the use of differ-
ent propargylic pivalates.
Scheme 5. a) Cascade reaction for the synthesis of dihydro-
benzo[7]annulene 7. b) Favoured and unfavoured approach-
ing trajectories in the formation of the C C bond.
À
both terminals of the acetylenic moiety were conven-
iently synthesized and employed under best condi-
tions. The desired a-alkylated enones were obtained adding at the beginning a catalytic amount of pivOH
in moderate to very high yields (Scheme 4).
were unfruitful.
Finally, not only secondary but also the tertiary al-
Finally, the unexpected Z-stereochemistry of the
lylic alcohol 6 has proven competence in the titled re- final enones can be rationalized in terms of different
action under optimal conditions. Interestingly, the bi- steric constraints of the stereoisomeric transition
cyclic trans-dihydrobenzo[7]annulene 7 was isolated states delivering the E- and Z-configurated C=C bond
in moderate yield (35%) but in high trans-diastereose- (see X-ray of 4ae in Scheme 3). In particular, the
lectivity (Scheme 5a and the Supporting Information). latter one led to a more accessible approaching trajec-
An allylic alkylation/Michael-type Friedel–Crafts al- tory due to the absence of detrimental interactions
kylation sequence is assumed for the final outcome.
between the allyl framework and the allene substitu-
Mechanistically, the reaction seems to involve the ents.
initial rearrangement of the propargylic carboxylate
In conclusion, an unprecedented gold(I)/H+ co-cat-
to the corresponding allenoate. This electron-rich p- alyzed a-allylation of enones with p-alcohols is docu-
system can spontaneously condense on the allylic car- mented. The reaction proved competence for a large
bocationic species derived from the p-alcohol via an variety of pivalic propargylate derivatives and specif-
SN1-type mechanism.
icity for stabilized carbocations. In most of the cases,
Interestingly, the key role of the in situ formed and unexpectedly high diastereoselectivity towards the
accumulating pivalic acid on the kinetics was unam- formation of the Z-stereoisomer (enone C=C double
biguously proved via dedicated control experiments. bond) was observed. Attempts to extend this protocol
As a matter of fact, the presence of different bases to different alkylating agents are currently under way
[i.e., 2,6-(t-Bu)2py, K2CO3, 1 equiv.] caused the com- in our laboratories.
plete suppression of the catalytic cycle.[16] All at once,
it revealed that the concomitant existence of p- and
s-acidity is required for the successful allylating pro-
cedure, with specific and distinct roles during the pro-
Experimental Section
cess: [Au(I)]=formation of the nucleophilic species,
[pivOH]=activation of the alcohol towards the nucle-
General Procedure
ophilic trapping. Unfortunately, attempts to lower the
reaction temperature (i.e., room temperature) by with anhydrous toluene (1 mL), the desired carboxylate 1/2
An oven-dried, two-necked, round-bottom flask was charged
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Elisabetta Manoni et al.
(0.1 mmol), the allylic alcohol 3 (0.15 mmol) and the [John-
PhosAu(ACN)]SbF6 (2 mol%). The flask was placed in
a pre-heated oil bath (1108C) and the mixture left stirring at
the same temperature for the indicated time. After cooling
to room temperature, the reaction crude was directly
charged into a column for purification by flash chromatogra-
phy (cyclohexane:AcOEt 98:2).
y) D.-H. Zhang, X.-Y. Tang, M. Shi, Acc. Chem. Res.
2014, 47, 913–924.
[3] a) K. Miki, K. Ohe, S. Uemura, J. Org. Chem. 2003, 68,
8505–8513; b) A. Correa, N. Marion, L. Fensterbank,
M. Malacria, S. P. Nolan, L. Cavallo, Angew. Chem.
2008, 120, 730–733; Angew. Chem. Int. Ed. 2008, 47,
718–721.
[4] For recent representative studies, see: a) C. H. M.
Amijs, V. López-Carrillo, A. M. Echavarren, Org. Lett.
2007, 9, 4021–4024; b) P. Mauleón, J. L. Krinsky, F. D.
Toste, J. Am. Chem. Soc. 2009, 131, 4513–4520; c) T.
de Haro, C. Nevado, Chem. Commun. 2011, 47, 248–
249; d) Y.-M. Wang, C. N. Kuzniewski, V. Rauniyar, C.
Hoong, F. D. Toste, J. Am. Chem. Soc. 2011, 133,
12972–12975; e) D. Wang, L. N. S. Gautam, C. Bollin-
ger, A. Harris, M. Li, X. Shi, Org. Lett. 2011, 13, 2618–
2621; f) S. J. Heffernan, J. M. Beddoes, M. F. Mahon,
A. J. Hennessy, D. R. Carbery, Chem. Commun. 2013,
49, 2314–2316; g) G. Cera, M. Chiarucci, F. Dosi, M.
Bandini, Adv. Synth. Catal. 2013, 355, 2227–2231; h) L.-
J. Wang, H.-T. Zhu, A.-Q. Wang, Y.-F. Qiu, X.-Y. Liu,
Y.-M. Liang, J. Org. Chem. 2014, 79, 204–212; i) Y. Xi,
Q. Wang, Y. Su, M. Li, X. Shi, Chem. Commun. 2014,
50, 2158–2160; j) E. Rettenmeier, M. M. Hansmann, A.
Ahrens, K. Rübenacker, T. Saboo, J. Massholder, C.
Meier, M. Rudolph, F. Rominger, A. S. K. Hashmi,
Chem. Eur. J. 2015, 21, 14401–14409; k) Z. Wang, H.
Zheng, J. Yang, X. Xie, X. She, Adv. Synth. Catal. 2015,
357, 2082–2088; l) M. Petrovic, D. Scarpi, B. Fiser, E.
Gómez-Bengoa, E. G. Occhiato, Eur. J. Org. Chem.
2015, 3943–3956; m) A. Ahlers, T. de Haro, B. Gabor,
A. Fürstner, Angew. Chem. 2016, 128, 1428–1433;
Angew. Chem. Int. Ed. 2016, 55, 1406–1411.
[5] a) Y. Yu, W. Yang, F. Rominger, A. S. K. Hashmi,
Angew. Chem. 2013, 115, 7735–7738; Angew. Chem.
Int. Ed. 2013, 52, 7586–7589; b) Y.-L. Ji, J.-H. Lin, J.-C.
Xiao, Y.-C. Gu, Eur. J. Org. Chem. 2014, 7948–7954;
for examples of allenoate-trapping in cycloaddition re-
action, see: c) C. H. Oh, J. H. Kim, B. K. Oh, J. R.
Park, J. H. Lee, Chem. Eur. J. 2013, 19, 2592–2596.
[6] a) M. Bandini, A. Eichholzer, Angew. Chem. 2009, 121,
9697–9701; Angew. Chem. Int. Ed. 2009, 48, 9533–9537;
b) M. Bandini, A. Eichholzer, A. Gualandi, T. Quinto,
D. Savoia, ChemCatChem 2010, 2, 661–665; c) M. Ban-
dini, M. Monari, A. Romaniello, M. Tragni, Chem. Eur.
J. 2010, 16, 14272–14277; d) M. Bandini, A. Gualandi,
M. Monari, M. Tragni, D. Savoia, J. Organomet. Chem.
2011, 696, 338–347; e) M. Chiarucci, M. Locritani, G.
Cera, M. Bandini, Beilstein J. Org. Chem. 2011, 7,
1198–1204; f) M. Bandini, A. Bottoni, M. Chiarucci, G.
Cera, G. Miscione, J. Am. Chem. Soc. 2012, 134, 20690–
20700; g) M. Chiarucci, R. Mocci, L.-D. Syntrivanis, G.
Cera, A. Mazzanti, M. Bandini, Angew. Chem. 2013,
125, 11050–11053; Angew. Chem. Int. Ed. 2013, 52,
10850–10853; h) M. M. Mastandrea, N. Mellonie, P.
Giacinto, A. Collado, S. P. Nolan, G. P. Miscione, A.
Bottoni, M. Bandini, Angew. Chem. 2015, 127, 15098–
15102; Angew. Chem. Int. Ed. 2015, 54, 14885–14889.
[7] For general reviews on the use of allylic alcohols in cat-
alytic processes, see: a) M. Bandini, M. Tragni, Org.
Biomol. Chem. 2009, 7, 1501–1507; b) M. Bandini,
Angew. Chem. 2011, 123, 1026–1027; Angew. Chem. Int.
Acknowledgements
The University of Bologna is kindly acknowledged for finan-
cial support. Dr. Stefano Grilli (UNIBO) is kindly acknowl-
edged for the assistance during the NMR analysis.
References
[1] For reviews on the topic, see: a) N. Marion, S. P. Nolan,
Angew. Chem. 2007, 119, 3046–3058; Angew. Chem.
Int. Ed. 2007, 46, 2988–3000; b) R. K. Shiroodi, V. Ge-
vorgyan, Chem. Soc. Rev. 2013, 42, 4991–5001; c) D. Te-
jedor, G. MØndez-Abt, L. Cotos, F. García-Tellado,
Chem. Soc. Rev. 2013, 42, 458–471; d) C. Obradors,
A. M. Echavarren, Acc. Chem. Res. 2014, 47, 902–907;
e) G. Henrion, F. Gagosz, Gold-promoted sigmatropic
rearrangements, in: The Chemistry of Organogold Com-
pounds, Pt. 1, (Eds.: Z. Rappoport, I. Marek, J. F. Lieb-
man, S. Patai), Wiley, 2014, pp 149–234.
[2] a) A. S. K. Hashmi, G. J. Hutchings, Angew. Chem.
2006, 118, 8064–8105; Angew. Chem. Int. Ed. 2006, 45,
7896–7936; b) A. Fürstner, P. W. Davies, Angew. Chem.
2007, 119, 3478–3519; Angew. Chem. Int. Ed. 2007, 46,
3410–3449; c) A. S. K. Hashmi, Chem. Rev. 2007, 107,
3180–3211; d) E. JimØnez-NfflÇez, A. M. Echavarren,
Chem. Rev. 2008, 108, 3326–3350; e) A. Arcadi, Chem.
Rev. 2008, 108, 3266–3325; f) R. Skouta, C.-J. Li, Tetra-
hedron 2008, 64, 4917–4938; g) Z. G. Li, C. Brouwer, C.
He, Chem. Rev. 2008, 108, 3239–3265; h) A. Fürstner,
Chem. Soc. Rev. 2009, 38, 3208–3221; i) N. Shapiro,
F. D. Toste, Synlett 2010, 675–691; j) T. De Haro, C.
Nevado, Synthesis 2011, 2530–2539; k) M. Bandini,
Chem. Soc. Rev. 2011, 40, 1358–1367; l) T. C. Boorman,
I. Larrosa, Chem. Soc. Rev. 2011, 40, 1910–1925; m) A.
Corma, A. Leyva-PØrez, M. J. Sabater, Chem. Rev.
2011, 111, 1657–1712; n) A. Leyva-PØrez, A. Corma,
Angew. Chem. 2012, 124, 636–658; Angew. Chem. Int.
Ed. 2012, 51, 614–635; o) A. S. K. Hashmi, M. Rudolph,
Chem. Soc. Rev. 2012, 41, 2448–2462; p) D. Gatineau,
J.-P. Goddard, V. Mouries-Mansuy, L. Fensterbank,
Israel J. Chem. 2013, 53, 892–900; q) E. Merino, C.
Nevado, Chimia 2013, 67, 663–668; r) A. S. K. Hashmi,
Top. Organomet. Chem. 2013, 44, 143–164; s) A. S. K.
Hashmi, Acc. Chem. Res. 2014, 47, 864–876; t) H.-S.
Yeom, S. Shin, Acc. Chem. Res. 2014, 47, 966–977; u) L.
Zhang, Acc. Chem. Res. 2014, 47, 877–888; v) X. Liu,
L. He, Y.-M. Liu, Y. Cao, Acc. Chem. Res. 2014, 47,
793–804; w) C. Obradors, A. M. Echavarren, Chem.
Commun. 2014, 50, 16–28; x) Y.-M. Wang, A. D. Lack-
ner, F. D. Toste, Acc. Chem. Res. 2014, 47, 889–901;
1408
asc.wiley-vch.de
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Adv. Synth. Catal. 2016, 358, 1404 – 1409

COMMUNICATIONS
Catalytic a-Allylation of Enones with Alcohols
Ed. 2011, 50, 994–995; c) M. Bandini, G. Cera, M.
Chiarucci, Synthesis 2012, 504–512.
[8] D. Basavaiah, G. Veeraraghavaiah, Chem. Soc. Rev.
2012, 41, 68–78, and references cited therein.
[9] S. Gaillard, C. S. J. Cazin, S. P. Nolan, Acc. Chem. Res.
2012, 45, 778–787.
[10] N. MØzailles, L. Ricard, F. Gagosz, Org. Lett. 2005, 7,
4133–4136.
[11] a) M. Jia, M. Bandini, ACS Catal. 2015, 5, 1638–1652;
b) B. Ranieri, I. Escofet, A. M. Echavarren, Org.
Biomol. Chem. 2015, 13, 7103–7118.
A. M. Echavarren, Angew. Chem. 2006, 118, 5581–
5585; Angew. Chem. Int. Ed. 2006, 45, 5455–5459.
[13] P. Mauleon, F. D. Toste, in: Modern Gold Catalyzed
Synthesis, Wiley-VCH, Weinheim, 2012, chapter IV, 75–
135.
[14] J. Wang, W. Huang, Z. Zhang, X. Xiang, R. Liu, X.
Zhou, J. Org. Chem. 2009, 74, 3299–3304.
[15] CCDC 1444079 [(Z,E)-4ae] contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[16] No product was formed when a stoichiometric amount
of pivalic acid was utilized in the absence of gold cata-
lyst.
[12] a) C. Nieto-Oberhuber, S. López, A. M. Echavarren, J.
Am. Chem. Soc. 2005, 127, 6178–6179; b) E. Herrero-
Gómez, C. N. Oberhuber, S. López, J. Benet-Buchholz,
Adv. Synth. Catal. 2016, 358, 1404 – 1409
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