Synthesis of indenes
The reaction of 2-(chloromethyl)phenylboronic acid with alkynes in the presence of a Rh(I) catalyst gave indene derivatives in high yields. The regioselectivity depends on the steric nature of the substituent on the alkynes.
M. Miyamoto, Y. Harada, M. Tobisu, N. Chatani, Org. Lett., 2008, 10, 2975-2978.
In the presence of a catalytic amount of FeCl3, a broad range of N-benzylic sulfonamides smoothly react with internal alkynes to afford various functionalized indene derivatives with extremely high regioselectivity through FeCl3-catalyzed cleavage of sp3 carbon-nitrogen bonds to generate benzyl cation intermediates.
C.-R. Liu, F.-L. Yang, Y.-Z. Jin, X.-T. Ma, D.-J. Cheng, N. Li, S.-K. Tian, Org. Lett., 2010, 12, 3832-3835.
Readily available substituted phenols are used as starting materials in a reaction sequence composed of Pd-catalyzed Suzuki coupling and Ru-catalyzed ring-closing metathesis for the controlled construction of functionalized indene derivatives in excellent yields. This approach is also utilized for the synthesis of substituted indenes selectively deuterated in position 3.
A. Jana, K. Misztal, A. Żak, K. Grela, J. Org. Chem., 2017, 82, 4226-4234.
Cyclization of electron-rich 2-alkyl-1-ethynylbenzene derivatives was catalyzed by TpRuPPh3(CH3CN)2PF6 in hot toluene to form 1-substituted-1H-indene and 1-indanone products. The cyclization mechanism involves a 1,5-hydrogen shift of an initial metal-vinylidene intermediate.
A. Odedra, S. Datta, R.-S. Liu, J. Org. Chem., 2007, 72, 3289-3292.
Air-stable alkynophilic metal salts, such as PtCl2, PtCl4, and [RuCl2(CO)3]2, catalyze the cycloisomerization of 1-alkyl-2-ethynylbenzenes to produce substituted indenes. Electrophilically activated alkynes can be intercepted by simple benzylic C-H bonds at primary, secondary, and tertiary carbon centers.
M. Tobisu, H. Nakai, N. Chatani, J. Org. Chem., 2009, 74, 5471-5475.
A number of indenes have been prepared in good yields by treatment of o-bromobenzyl zinc bromide with various terminal and internal alkynes in the presence of a nickel catalyst. The nickel-catalyzed carboannulation reaction was successfully extended to the synthesis of indanes using acrylates and styrene instead of an alkyne.
R. Deng, L. Sun, Z. Li, Org. Lett., 2007, 9, 5207-5210.
A regio- and stereoselective rhodium-catalyzed reaction of propargylic alcohols with boronic acids enables the synthesis of trisubstituted allylic alcohols. The utility of these synthons is demonstrated in a convenient synthesis of indenes and quinolines.
J. Panteleev, R. Y. Huang, E. K. J. Lui, M. Lautens, Org. Lett., 2011, 13, 5314-5317.
A Brønsted acid catalyzed cyclization of diaryl- and alkyl aryl-1,3-dienes allows the synthesis of substituted indenes in good to excellent yields under mild conditions in the presence of a catalytic amount of trifluoromethanesulfonic acid.
D. Eom, S. Park, Y. Park, T. Ryu, P. H. Lee, Org. Lett., 2012, 14, 5392-5395.
A chemoselective iodocarbocyclization of allenyl arenes provides 2-iodoindenes in high yields in the presence of NIS as source of iodine cations in acetonitrile or nitromethane. Variations of the allenic skeletons revealed the high 5-endo selectivity and some competitive pathways of cyclization. Pd- and Cu-catalyzed postfunctionalization reactions of the carbon-iodine bond enable the synthesis of substituted indenes in good yields.
C. Grandclaudon, V. Michelet, P. Y. Toullec, Org. Lett., 2016, 18, 676-679.
Cp*Co(CO)I2 catalyzes a weakly coordinating, carbonyl-assisted C-H activation of aromatic systems with α,β-unsaturated ketone and subsequent aldol condensation to provode indenes under mild reaction conditions. The reaction shows excellent functional group compatibility.
D. H. Dethe, N. C B, A. A. Bhat, J. Org. Chem., 2020, 85, 7565-7575.
The use of i-PrOH as environmentally benign hydrogen surrogate enables a ligand-free Co-catalyzed chemoselective reductive cyclization cascade of enone-tethered aldehydes. The selectivity between Michael-Aldol cycloreduction cascade and oxa-Michael cascade can be adjusted by the addition of TEMPO as a steric Lewis bases to provide 1H-indenes and dihydroisobenzofurans, respectively.
S.-S. Ma, B.-L. Jiang, Z.-K. Yu, S.-J. Zhang, B.-H. Xu, Org. Lett., 2021, 23, 3873-3878.
Lithium hexafluorophosphate is an effecient catalyst for a Nazarov-type cyclization of 1-arylbuta-2,3-dien-1-ols into benzofulvenes, which are valuable building blocks for functional materials and bioactive compounds.
M. Sai, S. Matubara, Synlett, 2014, 25, 2067-2071.
The methylenindene scaffold can be prepared from readily available gem-dibromoolefins using an efficient, highly modular palladium-catalyzed tandem intermolecular Suzuki/intramolecular Heck reaction under mild conditions. The choice of ligand was found to be crucial to control the selectivity of the reaction. Isolation of intermediates under different conditions provides insight into the mechanism.
C. S. Bryan, M. Lautens, Org. Lett., 2010, 12, 2754-2757.
Arylboronate esters bearing a pendant Michael-type acceptor olefin undergo a transmetalation with a rhodium-based catalytic complex. These organorhodium intermediates can cyclize onto nonterminal acetylenes in good to excellent yields.
M. Lautens, T. Marquardt, J. Org. Chem., 2004, 69, 4607-4614.
A Pt-catalyzed Rautenstrauch reaction of propargyl carbonate enables the regiodivergent synthesis of functionalized indenes. A one-pot Rautenstrauch/Tsuji-Trost reaction delivering 2-indanones was realized efficiently using this methodology.
J. Zhao, D. A. Clark, Org. Lett., 2012, 14, 1668-1671.
The preparation of indanones by the intramolecular acylation of 3-arylpropanoic acids or halides requires the use of noncatalytic acid promoters. In the presence of a catalytic amount FeCl3, in situ generated dimethyl acetals of (E)-2-alkylcinnamaldehydes cyclize to 1-methoxy-2-alkyl-1H-indenes in good-to-high yields. The 1-methoxyindenes were converted in high yield into 2-alkylindanones.
G. B. Womack, J. G. Angeles, V. E. Fanelli, C. A. Heyer, J. Org. Chem., 2007, 72, 7046-7049.
A simple and atom-economical, metal-free Brønsted acid promoted two-component reaction between cinnamaldehydes and sulfonamides provides a range of functionalized indenes from easily available starting materials. The resulting N-indenylsulfonamides can be converted into the corresponding indenylenamines or indanones.
Y. Fan, B. Zhou, K. Chen, B. Wang, X. Li, X. Xu, Synlett, 2017, 28, 1657-1659.
An efficient and straightforward synthesis provides 1-amino-1H-indenes from 1,2-bis(boronates) via a Suzuki-Miyaura coupling/Petasis reaction sequence. Starting from the same monoboronic ester intermediates, an intermolecular version of this reaction afforded (Z)-α,β-unsaturated amino esters in moderate yields.
V. Jayaram, T. Sridhar, G. V. M. Sharma, F. Berrée, B. Carboni, J. Org. Chem., 2017, 82, 1803-1811.