Regiodivergent synthesis of functionalized indene derivatives via Pt-catalyzed Rautenstrauch reaction of propargyl carbonates

Article abstract of DOI:10.1021/ol300158d

A regiodivergent synthesis of functionalized indene derivatives from a Pt-catalyzed Rautenstrauch reaction of propargyl carbonate is described. A one-pot Rautenstrauch/Tsuji-Trost reaction delivering 2-indanones was realized efficiently using this methodo

Full text of DOI:10.1021/ol300158d

ORGANIC
LETTERS
2012
Vol. 14, No. 7
1668–1671
Regiodivergent Synthesis of
Functionalized Indene Derivatives via
Pt-Catalyzed Rautenstrauch Reaction
of Propargyl Carbonates
Jinbo Zhao and Daniel A. Clark*
1-014 Center for Science and Technology, Department of Chemistry,
Syracuse University, Syracuse, New York 13244, United States
daclar01@syr.edu
Received January 19, 2012
ABSTRACT
A regiodivergent synthesis of functionalized indene derivatives from a Pt-catalyzed Rautenstrauch reaction of propargyl carbonate is described.
A one-pot Rautenstrauch/TsujiÀTrost reaction delivering 2-indanones was realized efficiently using this methodology.
Indene is a privileged structural motif in natural
product,1 pharmaceutical,2 and materials chemistry3 and
has found use as a ligand for transition metal complexes.4
Despite numerous reports on the synthesis of indenes,5
methods for the selective syntheses of heteroatom function-
alized indene derivatives, especially 2-indanones, are still
lacking.6,7 The goal of this study was to ascertain a selective
method to access both regioisomers of 2-substituted in-
denes using Rautenstrauch methodology.
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r
10.1021/ol300158d
Published on Web 03/14/2012
2012 American Chemical Society

The Rautenstrauch reaction has recently garnered in-
creased attention because it provides straightforward
access to cyclopentanones from readily available 1,4-
enynes.8 Functionalized indenes were later found to be
readily accessed using this reaction when an aryl group was
used in place of the alkene moiety.9 The aryl group
participated in the pentannulation process, presumably
through an electrophilic metalation of the carbenoid spe-
cies Int1 generated through a 1,2-acyloxy shift10,11 induced
by a transition metal (Scheme 1). Recently, Sarpong has
reported a regioselective synthesis of functionalized in-
denes using ester-activated propargyl acetates.9b However,
the use of terminal propargyl acetate derivatives, as re-
ported by Ohe and coworkers, provided a mixture of
regioisomers with poor selectivities. The product ratios
reported by Ohe typically favor of the 1-methyl-1H-indene
B (Scheme 1).9c They attributed this regioselectivity issue
to the nonselective proton shift steps after protonation of
Int2 in the catalytic cycle. This unsolved challenge of
regioselectivity has limited the application of this reaction
in organic synthesis.12 Herein, we report an efficient solu-
tion to this problem: the use of a propargyl carbonate
allows a highly regioselective synthesis of 1-alkyl-1H-
indenes, while its regioisomer 3-alkyl-1H-indenes may be
accessedby aPt-catalyzedRautenstrauchreaction/catalyt-
ic isomerization sequence.
Scheme 2. Tandem Rautenstrauch/TsujiÀTrost Reactions for
the Synthesis of 2-Indanones
To this end, allyl carbonate 1a was synthesized using
standard conditions from the commercially available alco-
hol and allylchloroformate. We explored a variety of metal
salts that are known to work well in the Rautenstrauch
reaction (Table 1). Under the original Rautenstrauch con-
ditions, almost no conversion was observed (entry 1,
Table 1).8 While AuCl3 led to complete conversion, the
yield of the products was low (entry 2, Table 1). AuCl-
(PPh3) did not effect good conversion; the addition of silver
salts to form the corresponding cationic gold(I) species led
to complete decomposition of the starting material regard-
less of solvent (entries 4À6, Table 1). The reactions cata-
lyzed by Pt(II) and Pt(IV) gave the best results, giving
excellent yields in toluene. Other catalysts such as ruthe-
nium, copper, and rhodium complexes were ineffective.14
The reaction catalyzed by PtCl4 afforded isomer 2a
highly selectively, albeit in moderate yield (entry 11,
Table 1). Lowering the reaction temperature to 0 °C led
exclusively to product 2a (entry 12, Table 1). A scrutiny of
the crude reaction mixture revealed a byproduct, which
appeared to be an unidentified decarboxylative dimeriza-
tion product.15 To minimize potential bimolecular pro-
cesses, the effect of concentration was examined. Thus, the
yield of 2a greatly improved to 91% (86% isolated yield)
when the reaction concentration was decreased from 0.1 to
0.02 M (entries 11 and 13, Table 1). Of note, even under these
dilute conditions, the reaction was complete in <5 min.
We reasoned that 3-methyl-1H-indene 3a should be the
thermodynamic product since it bears a tetrasubstituted
double bond in the cyclopentene moiety. Thus, 2a might be
isomerized to 3a in the presence of a suitable base. The
tautomeric rearrangement of simple alkyl-substituted in-
denering systems studiedbyWeidler lended supporttothis
proposal.16 In accordance with this hypothesis, 2a under-
went smooth isomerization to 3a. On the basis of the ease
of use and overall yield, 20 mol % of DBU was selected as a
base for the isomerization process.14 We observed cleaner
Our interest in this field stems from our intention to
synthesize enantioenriched 2-indanone derivatives. We
envisonedthat allyl propargylcarbonates1 would undergo
a regioselective Rautenstrauch reaction to form 3-alkyl-
1H-indene 3,13 which bears the requisite functionality for a
TsujiÀTrost reaction to afford 2-indanone 4 with an all-
carbon quarternary stereocenter (Scheme 2).
Scheme 1. Regioselectivity Issue in Transition-Metal-Catalyzed
Pentannulation of Propargyl Esters
(11) For selected reviews on metalocarbenoid generated from pro-
pagyl esters, see: (a) Soriano, E.; Marco-Contelles, J. Acc. Chem. Res.
2009, 42, 1026. (b) Marco-Contelles, J; Soriano, E. Chem.;Eur. J. 2007,
13, 1350.
(12) Zhang, L.; Xie, X.; Liu, J.; Qi, J.; Ma, D.; She, X. Org. Lett. 2011,
13, 2956.
(8) (a) Rautenstrauch, V. J. Org. Chem. 1984, 49, 950. (b) Caruana,
P. A.; Frontier, A. J. Tetrahedron 2007, 63, 10646.
(9) (a) Miki, K.; Ohe, K.; Uemura, S. J. Org. Chem. 2003, 68, 8505. (b)
Prasad, B. A. B.; Yoshimoto, F. K.; Sarpong, R. J. Am. Chem. Soc. 2005,
127, 12468. (c) Nakanishi, Y.; Miki, K.; Ohe, K. Tetrahedron 2007,63, 12138.
(d) Peng, L.; Zhang, X.; Zhang, S.; Wang, J. J. Org. Chem. 2007, 72, 1192.
(13) For reports on transition-metal-initiated 1,2-shift of carbonates,
see: (a) Wang, S.; Zhang, L. Org. Lett. 2006, 8, 4585. (b) Wang, S.; Zhang,
L. J. Am. Chem. Soc. 2006, 128, 14274. (c) Harrak, Y.; Biaszykowski, C.;
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Benard, M.; Cariou, K.; Mainetti, E.; Mouries, V.; Dhimane, A.-L.;
Fensterbank, L.; Malacria, M. J. Am. Chem. Soc. 2004, 126, 8656.
(14) See the Supporting Information for more detailed information.
(15) For the observation of alkyne dimerization in a Pt(II)-catalyzed
aryl alkynes leading to indene formation, see:Yang, S.; Li, Z.; Jian, X.;
He, C. Angew. Chem., Int. Ed. 2009, 48, 3999.
(10) For mechanistic studies on Pt- and Au-catalyzed rearrangement
ꢀ
of propargyl derivatives, see: (a) Faza, O. N.; Lopez, C. S.; Alvarez, R.;
ꢀ
de Lera, A. R. J. Am. Chem. Soc. 2006, 128, 2434. (b) Correa, A.;
Marion, N.; Fensterbank, L.; Malacria, M.; Nolan, S. P.; Cavallo, L.
Angew. Chem., Int. Ed. 2008, 47, 718.
(16) Weidler, A.-M. Acta Chem. Scand. 1963, 17, 2724.
Org. Lett., Vol. 14, No. 7, 2012
1669

which conditions B gave the thermodynamic product in
slightly lower (92/8) selectivity; this was probably due to the
A
1,3 strain of the two methyl groups in product 3b. Under
Table 1. Reaction Optimization
standard conditions, substrates with R2 being linear alkyl
(entries 9 and 11, Table 2), branched alkyl (entry 10,
Table 2), and benzyl (entry 12, Table 2) groups all afforded
both isomers in good yields and high regioselectivities.
entry
catalyst
solvent (temp/°C) yieldb/% (2a/3a)b
1c
2
3
4
5
6
PdCl2(CH3CN)2
AuCl3
MeCN (60)
Tol (80)
DCM (25À40)
CH3NO2 (25)
DCM (25)
Tol (80)
Tol (110)
Tol (100)
Tol (80)
Tol (60)
Tol (25)
Tol (0)
Tol (25)
0 (À)
∼16 (∼32/68)
À (À)
Table 2. Substrate Scope Studies of Pt-Catalyzed Regioisomeric
Synthesis of Functionlized Indenesa
AuCl(PPh3)
AuCl(PPh3)/AgSbF6
AuCl(PPh3)/AgSbF6
AuCl(PPh3)/AgSbF6
PtCl2(PPh3)2
PtCl2(PPh3)2/2PhIO
PtCl2
PtI2
PtCl4
PtCl4
À (À)
À (À)
<5 (À)
7
>27 (70/30)
63 (84/16)
70 (83/17)
74 (42/58)
53 (96/4)
45 (>99/1)
91 (94/6)
8d
9
10
11
12
13e PtCl4
conditions A
conditions B
a Reaction conditions: 1a (0.20 mmol), catalyst (5 mol % metal), and
solvent (2 mL) unless otherwise specified. b On the basis of 1H NMR of
crude reaction mixture with mesitylene as internal standard. c Concen-
tration: 0.7 M. d 10 mol % [Pt], 20 mol % PhIO, 0.2 M. e Concentration:
0.02 M, isolated yield: 86%.
entry
R1/R2
H/Me (1a)
2-Me/Me (1b)
4-Cl/Me (1c)
4-Ph/Me (1d)
yieldb/%
2/3c yieldb/% 2/3c
1
2
3
4
5
6
7
8
86 (2a)
80 (2b)
75d (2c)
80d (2d)
96/4
96/4
95/5
94/6
76 (3a) <1/99
78 (3b) 8/92
63 (3c) <1/99
63 (3d) <1/99
3,5-di-MeO/Me (1e) 83e (2e/3e) 67/33 73 (3e) <1/99
4-Br/Me (1f)
4-CF3/Me (1g)
4-i-Pr/Me (1h)
H/Et (1i)
H/i-Pr (1j)
H/n-Bu (1k)
H/Bn (1l)
87d (2f)
37e (2g/3g) 79/21 72 (3g) <1/99
80 (2h)
82 (2i)
91 (2j)
89 (2k)
78 (2l)
95/5
85 (3f) <1/99
pentannulation reactions of propargyl carbonates with
PtI2 compared to PtCl2 for the synthesis of the thermo-
dynamic product 3. Although no further optimization was
done on catalyst loading, it was found that the loadings of
PtI2 and DBU could be reduced to 1 and 10 mol %,
respectively, on a 1 mmol scale using 1a, with the reaction
affording product 3a in 84% yield.
95/5
94/6
98/2
97/3
95/5
82 (3h) <1/99
9
84 (3i)
83 (3j)
<1/99
<1/99
10
11
12
76 (3k) <1/99
82 (3l) <1/99
a Reaction scale: 0.2 mmol. b Isolated yields. c On the basis of 1H
NMR of crude reaction mixture. d Slow addition of substrate over 2 h.
e 1H NMR yield versus mesitylene as internal standard.
With optimized conditions in hand, the substrate scope
of this reaction was examined using substitution on the aryl
group (R1) and propargylic substitution (R2, Table 2).
Using Pt(IV) catalysis (conditions A), the reaction delivers
kinetic products 2 in good to excellent yields with g95:5
regioselectivities, when R1 is alkyl (entries 2 and 8, Table 2),
halogen (entries 3 and 6, Table 2), or aryl (entry 4, Table 2).
It is noted that in the presence of an ortho-methyl group, the
reaction time was 1 h, and it afforded the corresponding
product 2b in 80% yield (entry 2, Table 2). However, the
reaction of 3,5-dimethoxyl-substituted propargyl carbo-
nate 1e afforded a mixture of isomers 2e and 3e (entry 5,
Table 2) in a ratio of 67/33 in 80% yield. The same reaction
conducted at a lower temperature (0 °C) did not affect the
selectivity. Similarly, the presence of a strong electron-
withdrawing group such as para-trifluoromethyl gave di-
minished selectivity (79/21) and 37% yield (entry 7, Table 2).
For substrates with electron-deficient aryl moieties, slow
addition of substrate to the catalyst solution was used to
reduce the undesired bimolecular byproduct (entries 3, 4,
and 6, Table 2). Using Pt(II) (conditions B), all substrates
underwent smooth cyclization and catalytic isomerization
to exclusively afford the thermodynamic isomer 3. Even
substrates 1g, which bears a CF3 group, worked well under
elevated temperature and delivered the product 3g in good
yield and excellent selectivity (entry 7, Table 2). The ortho-
methyl-substituted substrate 1b was the only substrate for
In order to clarify the unique regioselectivity in the PtCl4-
catalyzed conditions, several derivatives of the propargyl
alcohol (1mÀ1r) were prepared and tested under both
conditions A and B. The results in Table 3 show that esters
(entries 1À3, Table 3) gave moderate selectivities, while
carbonates (entries 4À6, Table 3) gave very high regioselec-
tivities. This unambiguously demonstrates that the carbo-
nate functionality is necessary for obtaining high selectivity.
It is noted that for the Boc-substituted starting material 1p,
the yield of the product under PtCl4 catalysis was very low
(entry 4, Table 3). Instead, most of the material was
converted to a bimolecular decarboxylative product. This
indicates the participation of the butyoxyl oxygen in sta-
bilizing cationic intermediates, making the t-Bu group more
labile as previously observed.17 This might be the reason for
the high regioselectivity observed in the kinetic products
obtained with conditions A. Under conditions B, however, all
variants afforded the thermodynamic products in good
yields with very high selectivities, demonstrating the general-
ity of the catalytic isomerization process.
The pentannulation of secondary acetate derivatives is
known to be challenging;9c we found that the same is true
(17) Buzas, A. K.; Istrate, F. M.; Gagosz, F. Tetrahedron 2009, 65, 1889.
Org. Lett., Vol. 14, No. 7, 2012
1670

Table 3. Functional Group Effects on Reactivity and Selectivity
Scheme 3. Reactivity of Related Substrates
conditions A
conditions B
entry
FG
yielda/%
2/3b
yielda/%
2/3b
1
2
3
4
5
6
Ac (1m)
Bz (1n)
42 (2m/3m)
72 (2n/3n)
77 (2o/3o)
22 (2p/3p)
66 (2q)
83/17
79/21
72/28
92/8
75 (3m)
82 (3n)
78 (3o)
55 (3p)
50 (3q)
73 (3r)
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
Piv (1o)
Boc (1p)
Cbz (1q)
CO2Et (1r)
95/5
66 (2r)
>94/6
a Isolated yields. b Ratio determined by 1H NMR of crude product.
for secondary carbonates. The proclivity for pentannulation
was examined with secondary allyl carbonate 1s. Carbonate
1s required a higher temperature (110 °C) to give indene 2s in
moderate 44% yield (Scheme 3a). Pentannulation of the
corresponding acetate, reported by Ohe, provided 17% yield
of the corresponding indene using PtCl2.9c Elevated tempera-
tures were also required for allyl propargyl substrate 1t.
Under PtCl4 catalysis, 1t gave a mixture of indenes 2t and
3t as well as diene 4 in 57% combined yield (Scheme 3b).
Following the conditions reported by Sarpong, a similar yield
and ratio of products was observed for 1t.9b,14 Attempts to
apply base-catalyzed isomerization was not successful for
PtCl4 or the Sarpong protocol.18
Scheme 4. Synthesis of 2-Indanone 5a Bearing an R-Quaternary
Stereogenic Center
Incorporation of an enol carbonate into the indenyl
products presents a unique opportunity to explore allyl
transfer reactions.19 In particular, use of products 3 in such
a reaction would grant access to a benzylic all-carbon
quaternary stereocenter, which is difficult to install in a
selective manner.19d As shown in Scheme 4, a sequential
one-pot transformation to 2-indanone 5a could be
efficiently realized. The process is rapid, high yielding
(>83% per step), and completely regioselective. More-
over, the facile access to indene 3a allowed us to validate
the installation of the required quaternary center asym-
metrically. The asymmetric TsujiÀTrost reaction of 3a
using the standard Trost DACH ligand L1 afforded 5a
in 70% yield with an er of 78:22 (Scheme 4).
shown to be crucial for obtaining high regioselectivity of
the kinetic isomer. The thermodynamic isomer could be
formedby a sequentialRautenstrauchcyclization followed
by base-catalyzed isomerization.
In addition, we demonstrated a tandem one-pot Rauten-
strauch/isomerization/TsujiÀTrost reaction sequence for the
synthesis of 2-indanones that possess an all-carbon quaterna-
ry stereocenter. The asymmetric variant of the TsujiÀTrost
reaction opens a new avenue for obtaining 2-indanones
enantioselectively. These results validate the synthetic useful-
ness of the current methodology in the preparation of highly
functionalized indene derivatives. Future work will focus on
understanding the nature of the propargyl carbonate on
selectivity and use of this method in natural product synthesis.
In conclusion, we have developed a facile synthesis of
regioisomeric functionalized indenes that relies on a pro-
pargyl carbonate Rautenstrauch rearrangement. The use
of Pt(IV) complexes and the carbonate functionality were
Acknowledgment. We thank Syracuse University for
generous start up funds and Professors Chisholm and
Totah (Syracuse University) for sharing chemicals and
equipment, as well as useful discussions.
(18) The substrate with alkyl substitution on the alkyne is acid
sensitive, prone to elimination, and decomposed during flash chroma-
tography purification.
(19) (a) Trost, B. M.; Machacek, M. R.; Aponik, A. Acc. Chem. Res.
2006, 39, 747. (b) Mohr, J. T.; Stoltz, B. M. Chem.;Asian. J. 2007, 2,
1476. (c) Weaver, J. D.; Recio, A., III.; Grenning, A. J.; Tunge, J. A.
Chem. Rev. 2011, 111, 1846. (d) Behenna, D. C.; Mohr, J. T.; Sherden,
N. H.; Marinescu, S. C.; Harned, A. M.; Tani, K.; Seto, M.; Ma, S.;
Supporting Information Available. Procedures for synthe-
sisofstartingmaterialsandproductsaswellascopiesof NMR
spectra of all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
ꢀ
Novak, Z.; Krout, M. R.; McFadden, R. M.; Roizen, J. L.; Enquist,
J. A., Jr.; White, D. E.; Levine, S. R.; Petrova, K. V.; Iwashita, A.; Virgil,
S. C.; Stoltz, B. M. Chem.; Eur. J. 2011, 17, 14199.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 7, 2012
1671