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Synthesis of 1,3-oxazoles and benzoxazoles

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Van Leusen Oxazole Synthesis

Recent Literature

Complementary  methods for direct arylation of  with high regioselectivity at both C-5 and C-2 have been developed for a wide range of aryl and heteroaryl bromides, chlorides, iodides, and triflates. Using task-specific phosphine ligands, palladium-catalyzed C-5 arylation of oxazoles is preferred in polar solvents, whereas C-2 arylation is preferred by nonpolar solvents.
N. A. Strotman, H. R. Chobanian, Y. Guo, J. He, J. E. Wilson, Org. Lett., 2010, 12, 3578-3581.

N. A. Strotman, H. R. Chobanian, Y. Guo, J. He, J. E. Wilson, Org. Lett., 2010, 12, 3578-3581.

A quaternary ammonium hydroxide ion exchange resin catalyzes the reaction of p-tolylsulfonylmethyl isocyanide (TosMIC) with aromatic aldehydes to give 5-aryloxazoles. The base and the p-tolylsulfinic acid byproduct can be removed by simple filtration, resulting in oxazoles in high yield and purity.
B. A. Kulkarnia, A. Ganesan, Tetrahedron Lett., 1999, 40, 5637-5638.

An improved one-pot van Leusen oxazole synthesis using tosylmethyl isocyanide (TosMIC), aliphatic halides and various aldehydes in ionic liquids allows the preparation of 4,5-disubstituted oxazoles in high yields. The recovered ionic liquids could be reused as solvent for six runs without significant loss of yields.
B. Wu, J. Wen, J. Zhang, J. Li, Y.-Z. Xiang, X.-Q. Yu, Synlett, 2009, 500-504.

Pd(PPh3)4 efficiently catalyses both direct arylation and alkenylation of oxazoles. The method is regio- and stereospecific with respect to bromoalkenes and tolerates a wide range of functional groups.
F. Besseličvre, S. Lebrequier, F. Mahuteau-Betzer, S. Piguel, Synthesis, 2009, 3511-3512.

The hypervalent iodine reagent PhI(OTf)2, generated in situ, has been successfully utilized in a metal-free intramolecular oxidative cyclization of N-styrylbenzamides. In remarkably short reaction times, the desired 2,5-disubstituted oxazoles were isolated in high yields.
C. Hempel, B. J. Nachtsheim, Synlett, 2013, 24, 2119-2123.

A metal-free annulation of alkynes, nitriles, and O-atoms, using PhIO as oxygen source, in the presence of TfOH or Tf2NH enables a regioselective assembly of 2,4-disubstituted and 2,4,5-trisubstituted oxazole compounds. The present reaction could be applied to a facile synthesis of an anti-inflammatory drug.
A. Saito, A. Taniguchi, Y. Kambara, Y. Hanzawa, Org. Lett., 2013, 15, 2672-2675.

Ring expansion of keto aziridines provides the corresponding 2,5-diaryl oxazoles in the presence of dicyclohexyl carbodiimide and iodine in refluxing acetonitrile. A plausible mechanism is proposed.
H. A. Samimi, S. Mohammadi, Synlett, 2013, 24, 223-225.

ZnI2 and FeCl3 mediate a direct approach to the selective and regiocontrolled synthesis of 2-oxazolines and 2-oxazoles in very good yields under mild reaction conditions by cyclization of acetylenic amides. Various functionalities were well tolerated.
G. C. Senadi, W.-P. Hu, J.-S. Hsiao, J. K. Vandavasi, C.-Y. Chen, J.-J. Wang, Org. Lett., 2012, 14, 4478-4481.

A broad range of functionalized oxazoles were synthesized in good yields from enamides via phenyliodine diacetate (PIDA)-mediated intramolecular cyclization. The oxidative carbon-oxygen bond formation process is heavy-metal-free.
Y. Zheng, X. Li, C. Ren, D. Zhang-Negrerie, Y. Du, K. Zhao, J. Org. Chem., 2012, 77, 10353-10361.

A highly efficient method for the synthesis of oxazole derivatives from simple amides and ketones proceeds through a C-N bond formation followed by a C-O bond formation closing the ring. Because of the simple and readily available starting materials, easy operation, and high bioactivity of oxazoles, this strategy can be broadly applied to medicinal chemistry.
M. Zhang, L. Huang, H. Huang, X. Li, W. Wu, H. Jiang, Org. Lett., 2014, 16, 5906-5909.

The use of iodosobenzene (PhIO) as an oxidant realizes an intermolecular oxidative C(sp2)-O bond formation between enamines and various carboxylic acids, including N-protected amino acids. This direct β-acyloxylation of enamine compounds tolerates a wide range of functional groups and furnishes various β-acyloxy enamines that can be conveniently converted to oxazoles via cyclodehydration.
X. Liu, R. Cheng, F. Zhao, D. Zhang-Negrerie, Y. Du, K. Zhao, Org. Lett., 2012, 14, 5480-5483.

A straightforward route allows the synthesis of 2-(hetero)arylated and 2,5-di(hetero)arylated oxazoles through regiocontrolled palladium-catalyzed direct (hetero)arylation of ethyl oxazole-4-carboxylate with iodo-, bromo-, and chloro(hetero)aromatics.
C. Verrier, T. Martin, C. Hoarau, F. Marsais, J. Org. Chem., 2008, 73, 7383-7386.

C. Verrier, T. Martin, C. Hoarau, F. Marsais, J. Org. Chem., 2008, 73, 7383-7386.

An efficient intermolecular reaction of gold carbene intermediates generated via gold-catalyzed alkyne oxidation using nitriles as both the reacting partner and the reaction solvent offers a generally efficient synthesis of 2,5-disubstituted oxazoles with broad substrate scope. The overall reaction is a [2 + 2 + 1] annulation of a terminal alkyne, a nitrile, and an oxygen atom from an oxidant.
W. He, C. Li, L. Zhang, J. Am. Chem. Soc., 2011, 133, 8482-8485.

A facile one-pot, transition-metal-free process enables the synthesis of various polysubstituted oxazoles via t-BuOOH/I2-mediated domino oxidative cyclization from readily available starting materials under mild conditions.
H. Jiang, H. Huang, H. Cao, C. Qi, Org. Lett., 2010, 12, 5561-5563.

In a practical and simple synthesis of 2,5-disubstituted oxazoles via an iodine-catalyzed tandem oxidative cyclization, a wide range of common commercial aromatic aldehydes can be used as reaction substrates, which displayed excellent functional group compatibility.
C. Wan, L. Gao, Q. Wang, J. Zhang, Z. Wang, Org. Lett., 2010, 12, 3902-9305.

A simple and efficient Cs2CO3-mediated reaction of aromatic and unsaturated primary amides with 2,3-dibromopropene allows the synthesis of 2-aryl-5-alkyl-substituted oxazoles in a single step in good yields.
N. Yasmin, J. K. Ray, Synlett, 2009, 2825-2827.

A copper(II)-catalyzed oxidative cyclization of enamides gives oxyzoles via vinylic C-H bond functionalization at room temperature. Various 2,5-disubstituted oxazoles bearing aryl, vinyl, alkyl, and heteroaryl substituents could be synthesized in good yields. This reaction protocol is complementary to a previously reported iodine-mediated cyclization of enamides to afford 2,4,5-trisubstituted oxazoles.
C. W. Cheung, S. L. Buchwald, J. Org. Chem., 2012, 77, 7526-7537.

A modular and practical synthesis of highly substituted oxazoles consists of a sequential copper-catalyzed amidation of vinyl halides followed by cyclization promoted by iodine. A wide variety of functionalized oxazoles and polyazoles can be obtained in a selective manner from simple and easily accessible precursors.
R. Martín, A. Cuenca, S. L. Buchwald, Org. Lett., 2007, 9, 5521-5524.

An efficient one-pot propargylation/cycloisomerization tandem process provides a rapid and efficient access to substituted oxazoles from propargylic alcohols and amides with use of p-toluenesulfonic acid monohydrate (PTSA) as a bifunctional catalyst.
Y.-M. Pan, F.-J. Zheng, H.-X. Lin, Z.-P. Zhan, J. Org. Chem., 2009, 74, 3148-3151.

Zn(OTf)2 catalyzed the cyclization of propargyl alcohols with anilines and phenols in toluene at 100°C without additive and gave various indole and benzofuran products with different structures. The cyclization of propargyl alcohols and amides gave oxazoles. Mechanisms for the different substituation patterns are discussed.
M. P. Kumar, R.-S. Liu, J. Org. Chem., 2006, 71, 4951-4955.

Hexafluorophosphoric acid promotes the formation of 1,3,4-oxadiazolium hexafluorophosphate salts from N′-acyl-N-aroyl-N-arylhydrazides or N′-acyl-N-acyl-N-arylhydrazides under mild conditions. A subsequent reaction with cyanamide in propan-2-ol in the presence of triethylamine generates 1,5-disubstituted 3-amino-1H-1,2,4-triazoles in good yields.
B. Wong, A. Stumpf. D. Carrera, C. Gu, H. Zhang, Synthesis, 2013, 45, 1083-1093.

A series of propargylic amides were transformed to the corresponding alkylideneoxazolines by a gold(I) catalyst. A subsequent autoxidation to hydroperoxides bearing the heteroaromatic oxazoles followed by reduction to the corresponding alcohols with sodium borohydride enables a highly efficient, and atom-economic access to a series of functionalized 2,5-disubstituted oxazoles.
A. S. K. Hashmi, M. C. B. Jaimes, A. M. Schuster, F. Rominger, J. Org. Chem., 2012, 77, 6394-6408.

Palladium-catalyzed direct alkynylations of heteroarenes with inexpensive gem-dichloroalkenes as user-friendly electrophiles set the stage for a modular, step-economical synthesis of diversely decorated heteroaryl alkynes.
L. Ackermann, C. Kornhaass, Y. Zhu, Org. Lett., 2012, 14, 1824-1826.

An expedient and mild method for the direct conversion of aldehydes to 2,4-disubstituted oxazoles relies on the oxidation of an oxazolidine formed from the condensation of serine with an aldehyde and proceeds through a 2,5-dihydrooxazole intermediate. In contrast to standard methods that start from carboxylic acids, the use of aldehydes as starting materials does not require intermediate purification.
T. H. Graham, Org. Lett., 2010, 12, 3614-3617.

3-Oxazoline-4-carboxylates as easily available synthetic intermediates can be oxidized to yield oxazole-4-carboxylates. Furthermore, derivatization of 3-oxazoline-4-carboxylates with Grignard reagents enables a facile preparation of 4-keto-oxazole derivatives.
K. Murai, Y. Takahara, T. Matsushita, H. Komatsu, H. Fujioka, Org. Lett., 2010, 12, 3456-3459.

A highly efficient copper-catalyzed tandem oxidative cyclization gives polysubstituted oxazoles from readily available starting materials under mild conditions. This is an attractive alternative method for the synthesis of oxazole derivatives.
C. Wang, J. Zhang, S. Wang, J. Fan, Z. Wang, Org. Lett., 2010, 12, 2338-2341.

A small library of compounds with oxazole and thiazole scaffolds and structural diversity in both positions 2 and 5 has been synthesized. Double acylation of a protected glycine affords intermediate α-amido-β-ketoesters, which in turn can be dehydrated to afford 1,3-oxazoles or reacted with Lawesson’s reagent to furnish 1,3-thiazoles.
J. F. Sanz-Cervera, R. Blasco, J. Piera, M. Cynamon, I. Ibáńez, M. Murguía, S. Fustero, J. Org. Chem., 2009, 74, 8988-8996.

A tandem reaction of a vinyliminophosphorane with various acyl chlorides gives unexpectedly 2,4,5-trisubstituted oxazoles in a one-pot fashion.
H. Xie, D. Yuan, M.-W. Ding, J. Org. Chem., 2012, 77, 2954-2958.

Substituted oxazol-5-ylethanones can be synthesized in a consecutive three-component sequence starting with amidation of propargylamine with an acid chloride followed by cross-coupling with another acid chloride. Therefore, this diversity-oriented one-pot approach to substituted oxazoles can be considered as an amidation-coupling-cycloisomerization (ACCI) sequence.
E. Merkul, O. Grotkopp, T. J. J. Müller, Synthesis, 2009, 502-507.

The reaction of 1-(methylthio)acetone with different nitriles in the presence of triflic anhydride gave 2-substituted 5-methyl-4-methylthio-1,3-oxazoles in good yield. The methylthio group at the C4 position can easily be removed with Raney nickel. 4-Methylsulfonyl derivatives were prepared by the oxidation of the MeS group with m-CPBA.
A. Herrera, R. Martinez-Alvarez, P. Ramiro, D. Molero, J. Almy, J. Org. Chem., 2006, 71, 3026-3032.

The reaction of aldehydes and ketones, including aliphatic and aromatic ones, with amides of α-isocyano-β-phenylpropionic acid in toluene in the presence of lithium bromide gives 2,4,5-trisubstituted oxazoles in good to excellent yield.
G. Cuny, R. Gamez-Montano, J. Zhu, Tetrahedron, 2004, 60, 4879-4885.

A simple, green, and efficient method enables the synthesis of benzoxazoles and benzothiazoles from o-amino(thio)phenols and aldehydes using samarium triflate as a reusable acid catalyst under mild reaction conditions in aqueous medium.
P. B. Gorepatil, Y. D. Mane, V. S. Ingle, Synlett, 2013, 24, 2241-2244.

Cyclization reactions of 2-aminophenols with β-diketones catalyzed by a combination of Brřnsted acid and CuI give various 2-substituted benzoxazoles. Different substituents such as methyl, chloro, bromo, nitro, and methoxy on 2-aminophenol are tolerated under the optimized reaction conditions.
M. S. Mayo, X. Yu, X. Zhou, X. Feng, Y. Yamamoto, M. Bao, J. Org. Chem., 2014, 79, 6310-6314.

The reaction of ortho-substituted anilines with functionalized orthoesters yields benzoxazole, benzothiazole, and benzimidazole derivatives in an efficient and connective methodology. The versatility of this approach enables the development of new libraries of heterocycles containing multifunctional sites.
G. Bastug, C. Eviolitte, I. E. Markó, Org. Lett., 2012, 14, 3502-3505.

Efficient and convenient three-component couplings of aryl halides, amino alcohols and tert-butyl isocyanide under palladium catalysis provide a range of oxazolines in excellent yield. The use of 1,2-amino phenols instead of amino alcohols enables the synthesis of benzoxazoles.
P. J. Boissarie, Z. E. Hamilton, S. Lang, J. A. Murphy, C. J. Suckling, Org. Lett., 2011, 13, 6184-6187.

An experimentally simple, general, efficient, and ligand-free synthesis of substituted benzimidazoles, 2-aminobenzimidazoles, 2-aminobenzothiazoles, and benzoxazoles via intramolecular cyclization of o-bromoaryl derivatives is catalyzed by copper(II) oxide nanoparticles in DMSO under air. The heterogeneous catalyst can be recovered and recycled without loss of activity.
P. Saha, T. Ramana, N. Purkait, M. A. Ali, R. Paul, T. Punniyamurthy, J. Org. Chem., 2009, 74, 8719-8725.

A divergent and regioselective synthesis of either 3-substituted benzisoxazoles or 2-substituted benzoxazoles from readily accessible ortho-hydroxyaryl N-H ketimines proceeds in two distinct pathways through a common N-Cl imine intermediate: (a) N-O bond formation to form benzisoxazole under anhydrous conditions and (b) NaOCl mediated Beckmann-type rearrangement to form benzoxazole, respectively.
C.-y Chen, T. Andreani, H. Li, Org. Lett., 2011, 13, 6300-6303.

A copper(II)-catalyzed conversion of bisaryloxime ethers to 2-arylbenzoxazoles involves a cascade C-H functionalization and C-N/C-O bond formation under oxygen atmosphere.
M. M. Guru, M. A. Ali, T. Punniyamurthy, Org. Lett., 2011, 13, 1194-1197.

An efficient method for the transformation of N-benzyl bisarylhydrazones and bisaryloxime ethers to functionalized 2-aryl-N-benzylbenzimidazoles and 2-arylbenzoxazoles involves a copper(II)-mediated cascade C-H functionalization/C-N/C-O bond formation under neutral conditions. Substrates having either electron-donating or -withdrawing substituents undergo the cyclization at moderate temperature.
M. M. Guru, M. A. Ali, T. Punniyamurthy, J. Org. Chem., 2011, 76, 5295-5308.

Lawesson’s reagent is an efficient promoter in the solvent-free microwave-assisted synthesis of 2-substituted benzoxazoles and benzothiazoles from carboxylic acids and 2-aminophenol or 2-aminothiophenol, respectively. Various aromatic, heteroaromatic and aliphatic carboxylic acids react under the conditions developed with good yields.
J. A. Seijas, M. P. Vázquez-Tato, M. R. Carballido-Reboredo, J. Crecente-Campo, L. Romar-López, Synlett, 2007, 313-316.

Dess-Martin periodinane (DMP) efficiently mediates the intramolecular cyclization of phenolic azomethines at ambient temperature leading to substituted benzoxazoles and benzothiazoles. Treatment of the reaction mixtures sequentially with Amberlyst A-26 thiosulfate resin and diisopropylaminomethyl resin (PS-DIEA) removes excess reagent and byproducts, to give pure products.
D. S. Bose, M. Idrees, Synthesis, 2010, 398-402.

A highly efficient Pd/Cu-catalyzed C-H arylation method for a range of heterocycles has been discovered. The efficiency and low loading of a combination of a palladium catalyst and a well-defined copper cocatalyst together with the mild reaction conditions demonstrate this method to be practically useful and mechanistically interesting.
J. Huang, J. Chan, Y. Chen, C. J. Borths, K. D. Baucom, R. D. Larsen, M. M. Faul, J. Am. Chem. Soc., 2010, 132, 3674-3675.

A new method for a direct, copper-catalyzed arylation of heterocycle C-H bonds by aryl iodides allows the conversion of electron-rich five-membered heterocycles and electron-poor pyridine oxides. The best results are obtained by using a combination of lithium tert-butoxide as base and copper iodide as catalyst.
H.-Q. Do, O. Daugulis, J. Am. Chem. Soc., 2007, 129, 12404-12405.

A simple and straightforward method for the direct carboxylation of aromatic heterocylces such as oxazoles, thiazoles, and oxadiazoles using CO2 as the C1 source requires no metal catalyst and only Cs2CO3 as the base. A good functional group tolerance is achieved.
O. Vechorkin, N. Hirt, X. Hu, Org. Lett., 2010, 12, 3567-3569.

A Regel-type transition-metal-free direct C-2 aroylation of (benzo)oxazoles, (benzo)thiazoles and 1,3,4-oxadiazoles with acid chlorides is catalyzed by N,N-dimethyl-4-aminopyridine (DMAP) and affords the corresponding 2-keto­azoles in good yields.
P. Lassalas, F. Marsais, C. Hoarau, Synlett, 2013, 24, 2233-2240.

Readily available, air-stable, recyclable copper(II) ferrite nanopartices serve as a catalyst in a sustainable synthesis of benzoxazoles from substituted N-(2-halophenyl)benzamides. The nanocatalyst can be completely recovered with an external magnet and can be used seven times without significant loss of catalytic activity.
D. Yang, X. Zhu, W. Wei, M. Jiang, N. Zhang, D. Ren, J. You, H. Wang, Synlett, 2014, 25, 729-735.

A general method for the formation of benzoxazoles and benzothiazoles via a copper-catalyzed cyclization of ortho-haloanilides is reported. Optimal conditions for cyclization used a catalyst combination of CuI and 1,10-phenanthroline as ligand. The mechanism is believed to proceed via an oxidative insertion/reductive elimination pathway through a Cu(I)/Cu(III) manifold. The rate of reaction of ortho-haloanilides follows the order I > Br > Cl, consistent with oxidative addition being the rate-determining step.
G. Evindar, R. A. Batey, J. Org. Chem., 2006, 71, 1802-1808.

A versatile one-pot domino acylation annulation reaction of 2-bromoanilines with acyl chlorides in the presence of Cs2CO3, catalytic CuI, and 1,10-phenanthroline under microwave conditions was applied to the synthesis of benzoxazoles. These copper-catalyzed approaches complement existing strategies for benzoxazole synthesis, which typically utilize 2-aminopheonls as precursors.
R. D. Viirre, G. Evindar, R. A. Batey, J. Org. Chem., 2008, 73, 3452-3459.

The direct C-H bond arylation of benzoxazoles and oxazoles with aryl chlorides is catalyzed by a well-defined NHC-Pd(II)-Im complex. Under optimized conditions, various aryl chlorides can be used to yield 2-aryloxazole derivatives in good yields.
X.-B. Shen, Y. Zhang, W.-X. Chen, Z.-K. Xiao, T.-T. Hu, L. X. Shao, Org. Lett., 2014, 16, 1984-1987.

A copper(I) complex catalyzes a direct alkylation of benzoxazoles using nonactivated secondary alkyl halides in the presence of bis[2-(N,N-dimethylamino)ethyl] ether.
P. Ren, I. Salihu, R. Scopelliti, X. Hu, Org. Lett., 2012, 14, 1748-1751.

Palladium-catalyzed direct alkynylations of heteroarenes with inexpensive gem-dichloroalkenes as user-friendly electrophiles set the stage for a modular, step-economical synthesis of diversely decorated heteroaryl alkynes.
L. Ackermann, C. Kornhaass, Y. Zhu, Org. Lett., 2012, 14, 1824-1826.

The reaction of N-cyano-N-phenyl-p-toluenesulfonamide (NCTS) as nonhazardous electrophilic cyanating agent with various substituted 2-aminophenols and benzene-1,2-diamine enables a facile synthesis of 2-aminobenzoxazole and 2-aminobenz­imidazole derivatives in the presence of lithium hexamethyldisilazide (LiHMDS). This protocol offers operational simplicity, short reaction time, and simple workup.
M. Kasthuri, H. S. Babu, K. S. Kumar, Ch. Sudhakar, P. V. N. Kumar, Synlett, 2015, 26, 897-900.

A Pd-catalyzed aerobic oxidation of o-aminophenols and isocyanides gives 2-aminobenzoxazoles and 3-aminobenzoxazines in good yields and a broad substrate scope. This methodology has the advantages of experimental simplicity, mild reaction conditions, and easily accessible starting materials. Furthermore, the synthesis of other types of useful nitrogen heterocycles has been achieved.
B. Liu, M. Yin, H. Gao, W. Wu, H. Jiang, J. Org. Chem., 2013, 78, 3009-3020.

Visible light efficiently mediates a mild intramolecular oxidative cyclization of o-hydroxy-N-aryl-N,N-dialkylformamidines leading to 2-aminobenzoxazole derivatives in excellent yields in the presence of only 1 mol% tris(2,2′-bipyridine)ruthenium(II) as a photoredox catalyst and air as terminal oxidant.
V. P. Srivastava, L. D. S. Yadav, Synlett, 2013, 24, 2758-2762.

Copper-catalyzed C-H bond activation enables a facile, efficient, and simple protocol for direct oxidative C-H amination of benzoxazoles with primary amines using tert-butyl peroxide (TBP) as oxidant under air. Various substituted aminobenzoxazoles were synthesized with very good yields.
J. Gu, C. Cai, Synlett, 2015, 26, 639-642.

A convenient, copper-catalyzed C-H and C-N bond activation enables an efficient and conceptually new method for oxidative amination of azoles with tertiary amines. This protocol can be performed in the absence of external base and only requires atmospheric oxygen as oxidant.
S. Guo, B. Qian, C. Xia, H. Huang, Org. Lett., 2011, 13, 522-525.

A facile metal-free oxidative amination of benzoxazole by activation of C-H bonds with secondary or primary amines in the presence of catalytic iodine in aqueous tert-butyl hydroperoxide proceeds smoothly at ambient temperature under neat reaction condition to furnish products in high yields. This user-friendly method produces only tertiary butanol and water as byproducts.
M. Lamani, K. R. Prabhu, J. Org. Chem., 2011, 76, 7938-7944.

Simple azole substrates can be converted into N-substituted 2-aminoazole derivatives in the presence of nitrogen nucleophiles, lithium tert-butoxide as the base, and iodine to mediate carbon-nitrogen bond formation. This method proceeds at room temperature under an air atmosphere using a normal benchtop set-up, or can be performed conveniently using microwave irradiation.
S. Yotphan, D. Beukeaw, V. Reutrakul, Synthesis, 2013, 45, 936-942.

Catalytic amounts of tetrabutylammoniumiodide (TBAI), aqueous solutions of H2O2 or TBHP as co-oxidant enabled an efficient transition-metal-free amination of benzoxazoles under mild reaction conditions, to yield highly desirable 2-aminobenzoxazoles in good yields. First mechanistic experiments indicate the in situ iodination of the secondary amine as the putative mode of activation.
T. Froehr, C. P. Sindlinger, U. Kloeckner, P. Finkbeiner, B. J. Nachtsheim, Org. Lett., 2011, 13, 3754-3757.