FOCUS REVIEW

DOI: 10.1002/ajoc.201300012

Catalytic Methods for Imine Synthesis

Rajendra D. Patil and Subbarayappa Adimurthy*[a]

Asian J. Org. Chem. 2013, 2, 726 – 744 726 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

Abstract: This Focus Review describes different methods that have been reported for the synthe- sis of imines. It is organized according to the methods used for imine synthesis starting with metal catalysis, including Ru, Au, V, Cu, Mn, Co and Pd catalysis. Other methods, such as photo- catalysis, electrocatalysis, , and so on, are also emphasized. Ample information on the condensation of carbonyl compounds/ with and direct oxidation of amines to give imines is discussed. Furthermore, among various metal-catalyzed reactions, specific atten- tion has been paid to copper-catalyzed imine synthesis, as copper is less toxic than other heavy metals, comparatively inexpensive, and is easily accessible.

Keywords: amines · carbonyl compounds · catalysis · imine synthesis · oxidation

1. Introduction Much information on the synthesis and chemistry of imines is scattered throughout the literature.[5,6,14–28] Howev- An imine has the general formula R’RC=NR’’, in which R, er, to our knowledge, there has not been a specific and R’, and R’’ can be atoms, groups or comprehensive review on imine synthesis to date. In contin- groups. If R’’ is alkyl or aryl group (not hydrogen) then the uation of our research interest in the development of effi- imine functionality is known as a “”, named cient and sustainable methods for imine synthesis,[29–31] as after Hugo Schiff who discovered them in 1864.[1] There are well as for the wider interest of the scientific community, other similar functional groups which slightly differ from an overview of imine chemistry is presented. It is hoped the definition of an imine at the center, for exam- that by assembling a comprehensive survey of the widely ple, when R’’=NR2 as in and R’’=OH as in scattered information on imine synthesis, it will focus the , and these compounds are not included in this Focus attention of a broad readership because of the potential ap- Review. In the present article, the term imines mainly plications of these compounds. This Focus Review collates refers to Schiff bases. much of the information that is available in the literature Imines are important intermediates in the synthesis of on methods and catalysts used for the synthesis of imines to various biologically active N-heterocyclic compounds and date. in industrial synthetic processes.[2–4] Imines react reversibly Significant progress has been made in recent years in the with amines and under particular reaction condi- synthesis of imines, which have been prepared by various tions under thermodynamic control so that initially formed, methods from aldehydes and/or amines and their chemical kinetically competitive intermediates are replaced by ther- equivalents. As depicted in Scheme 1, these methods in- modynamically stable products over time. For this funda- mental reason, the formation of a dynamic covalent imine bond (dynamic covalent bond refers to the influence of re- active substrates, reagents, and particular reaction condi- tions) is an emerging and versatile method with various ap- plications. Formation of imines underlies a discipline known as dynamic covalent chemistry (DCC), which is now used widely in the construction of exotic molecules and ex- tended structures, such as rotaxanes, catenanes, and so on.[5,6] Imines can act as electrophiles in a number of reac- tions, including reductions, additions, condensations, and cy- cloadditions.[7,8] The presence of the lone pair of electrons on the nitrogen atom of the imine group enables coordina- tion to numerous metals, especially when the imine func- tionality is located at the ortho position of aromatic hetero- cycles, such as . Such molecules are used for inter- esting applications as in homogeneous catalysis.[9,10] Prochiral imines have been widely used for the synthesis of chiral amines.[11–13]

[a] R. D. Patil, S. Adimurthy Central Salt & Marine Chemicals Research Institute (CSIR) G.B. Marg, Bhavnagar 364002, Gujarat (India) Fax : (+91)0278-2567562 Scheme 1. Various synthetic methods that have been reported for imine E-mail: [email protected] synthesis.

Asian J. Org. Chem. 2013, 2, 726 – 744727 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

clude condensation of aldehydes/ (A) with amines the reverse reaction, so that azeotropic distillation by (B, method I), addition of aryl halides and liquid Dean–Stark apparatus is necessary to push the reaction in to aldehydes/ketones (method II), hydroamination of al- the forward direction to favor the imine formation.[32, 33] kynes (method IV), oxidative coupling of amines (B) to There are various factors which influence the equilibrium give imines (method V), oxidative coupling of alcohols and between the imine and the starting and . amines (method VI), dehydrogenation of secondary amines These factors include concentration, steric and electronic (method VII), coupling of aldehydes/ketones with nitro effects, pH, temperature, and solvents. Condensation reac- compounds (method VIII), and the reaction between chem- tions between carbonyl compounds and amines have been ical equivalents of aldehydes/ketones (X and Y) and carried out in the presence of various catalysts, such as [34] [35] ACHTUNGRE [36] ACHTUNGRE [37] amines (method III). TiO2, CeCl3·H2O, Cu(NO3)2, Er(OTf)3, P2O5/ [38] [39] [40] ACHTUNGRE [41] Al2O3, P2O5/SiO2, NaHSO4·SiO2, Mg(ClO4)2, mo- [42–44] [45–49] lecular sieves, TiCl4, MgSO4–pyridinium p-toluene- [50] [51] [52] [53] [54] 2. Imine Synthesis from Amines and Aldehyde sulfonate, ZnCl2, alumina, Ti(OR)4, CuSO4, and and Ketones montmorillonite K-10 clay.[55,56] In such reactions, these cat- alysts act as Lewis acids to catalyze the nucleophilic attack The synthesis of imines originally reported by Schiff in- of the amine on the and also serve as dehy- volves condensation of a carbonyl compound with an drating agents through irreversible binding with water to fa- amine.[32] Such reactions proceed by cilitate the removal of water in the final step. The use of to give a (<-C> C(OH)(NHR)ACHTUNGRE <-C>) inter- dehydrating solvents, such as tetramethyl orthosilicate[57] mediate, then the elimination of water provides the imine and trimethyl orthoformate,[56, 58] were reported to avoid (Scheme 2). The equilibrium in this reaction usually favors azeotropic distillation. In the past two decades, researchers have shown remark- able interest in developing sustainable processes because of environmental concerns, for example, the synthesis of imines with microwaves,[56, 59–64] ultrasound,[65] and IR[66] as energy sources. Furthermore, imine synthesis has also been Scheme 2. Equilibrium in the synthesis of an imine from an aldehyde reported under solvent free conditions.[35,38,39,67,68] Recently, and an amine. ethyl lactate as a tunable solvent has been reported for aryl aldimine synthesis.[59,69] Ethyl lactate can be tuned with a co-solvent to create polarity conditions that are ideal for the synthesis of aryl aldimines, which crystalize directly out Dr. S. Adimurthy was born in 1972 in Ra- of solution in minutes in high yields.[69] Simple, water-medi- mojipalli, Karnataka State, in India. He re- ated procedures for the synthesis of various imines that re- ceived his B.Sc. and M.Sc. degrees in Chemistry from Bangalore University in quire neither catalyst nor any additive were also report- [70–72] 1994 and 1997, respectively. From 2000 to ed. date, he has worked as a Scientist at the In 1962, a review by Layer[73] on imines synthesis focused Central Salt & Marine Chemicals Research on the condensation of carbonyl compounds and amines.[74] Institute, Bhavnagar. He received his Ph.D. in 2005 from Bhavnagar University, India. However, these classical methods have some general limita- He took up a postdoctoral position at the tions. For example, the condensation of primary aliphatic University of Hohenheim, Stuttgart, Germa- aldehydes and amines does not lead to the desired imines, ny, (2007-2008) with Professor U. Beifuss. but instead provides polymeric materials with unreacted He has published over 40 papers and holds amines.[73] Reactions between aliphatic aldehydes and ali- six US patents. His research interests in- clude the synthesis of heterocycles through phatic amines do not easily give imines. Similarly, ketones CH activation, sustainable halogenation, react with amines very slowly and generally require harsh and the development of new oxidative reaction conditions. Moreover, the efficiency of the report- methods. ed procedures is limited to the reaction of highly electro-

Rajendra D. Patil was born in 1983 in Var- philic carbonyl compounds and strongly nucleophilic dhane, India. He received his B.Sc. and amines. Therefore an alternative and efficient strategy with M.Sc. in organic chemistry in 2004 and a broad scope of imine products is highly desirable. Oxida- 2006, respectively, from North Maharashtra tive dehydrogenation of amines (ODH) to give imines has University, India. He joined the Central Salt that potential. and Marine Chemicals Research Institute, Bhavnagar, India in 2007 and received his Ph.D. degree in 2012 under the supervision of Dr. S. Adimurthy. Currently he is work- ing as a Research Fellow at the School of Chemical and Biomedical Engineering, Na- nyang Technological University, Singapore.

Asian J. Org. Chem. 2013, 2, 726 – 744728 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

3. Imines Synthesis through Oxidative Dehydrogenation (ODH) of Amines

Oxidative dehydrogenation (ODH) of amines to give imines is a fundamental approach. However, comparatively little attention has been paid to the oxidation of primary amines to give imines, probably because the intermediates formed from the corresponding primary amines may rapid- ly dehydrogenate into because of the second a- Scheme 4. -catalyzed aerobic oxidation of amines to give [75–80] amino hydrogen atom. Dehydrogenation of amines to imines. give imines in the laboratory was first reported by Ritter in 1933.[81] Many efforts are underway to develop catalytic sys- protocol is useful for oxidation of aromatic amines as well tems that use sustainable oxidants, mainly oxygen or air, as for aliphatic amines but with a slower reaction rate.[83] for the synthesis of imines from primary or secondary Murahashi et al. reported the catalytic oxidation of sec- amines. ondary amines to give imines by using diruthenium com- ACHTUNGRE plex [Ru2(OAc)4Cl] (4 mol%) in a toluene under mild reac- tion conditions (1 atm. O and 508C).[84] However, under 3.1. Imines Synthesis through ODH of Amines with 2 similar reaction conditions, oxidation of af- Transition-metal Catalysts forded the corresponding benzonitrile.[84] Other ruthenium- Oxygen transfer to primary amines can result in a variety catalyzed aerobic oxidations of primary and secondary of oxidized products, such as imines, nitriles, aldehydes, and amines with N-methylmorpholine N-oxide (NMO)[85] and so on, depending on the oxidants and the reaction condi- tert-butyl [86] have been reported. The catalyt- ACHTUNGRE 2+ tions (Scheme 3). However, a number of transition-metal ic system [Ru(bpy)2(NO)Cl] (bpy= bipyridyl) reacts with based catalytic systems are well-known for selective oxida- benzylamine to produce mainly benzylimine and PhCN as tion of amines to give imines. oxidation products.[87] As oxidation products are generated even in the absence of oxygen, a mechanism in which the nitrosyl acts as an oxidant was proposed.[87] Bckvall and co-workers described an elegant aerobic catalytic system for the generation of aldimines and keti- mines by ruthenium-catalyzed dehydrogenation of amines that involves a biomimetic catalytic system.[88–90] The design Scheme 3. Oxidative coupling of amines to give imines. of the oxidation system was inspired by the biological oxi- dation of secondary alcohols in which the ruthenium com- plex acts as a substrate-selective catalyst instead of NAD+ , 3.1.1. Ruthenium Catalysts the ubiquinone (Q) was replaced by another electron-rich m ACHTUNGRE Bailey and James reported an aerobic oxidative dehydro- quinone, and co-catalyst ML ([Co(salen)] or MnO2)was genation of amines to give imines in 1996 by using a dioxo- used for O2 activation in place of cytochrome-c porphyrin–ruthenium complex.[82] This complex, trans- (Scheme 5).[89, 90] It was predicted that this system could 4+ACHTUNGRE [Ru (tmp)(O)2] (tmp =dianion of 5,10,15,20-tetramesityl- overcome the high energy barriers encountered in the tradi- porphyrin), catalytically dehydrogenates primary and sec- tional oxidation process by allowing reoxidation of the re- ondary amines in the presence of air as an oxidant in ben- duced metal to take place in a series of redox steps. In this zene as solvent and within 24 h. The possible reaction steps system, the quinone acted as a hydrogen acceptor to reduce involve a disproportionation reaction that generates a Ru2+ the metal for the next catalytic cycle. Further, the reduced intermediate, as shown by the isolated bis(benzylamine) quinone was subsequently reoxidized by molecular oxygen 2+ACHTUNGRE ACHTUNGRE complex [Ru (tmp)(PhCH2NH2)2] which was characterized by crystallographically. In another report by Albrecht and co-workers, a series of “[RuACHTUNGRE 2+(ACHTUNGREh6-arene)(NHC)]”ACHTUNGRE complexes (NHC= 1,2,3-triazolylidene, imidazolidene) were prepared and tested for the homocoupling of amines to give imines (Scheme 4).[83] In their report, the loading of catalysts 1–3 was 5 mol% in the absence of an auxiliary base and the re- action was carried out at 1508C. The normal NHC complex catalyst 3 was more active than 1 and the reaction reached full conversion after 12 h.[83] In contrast, the carbonate-con- taining complexes 2 were inactive for this transformation. Scheme 5. Ruthenium-catalyzed aerobic oxidation of amines by using This may be a result of exchange of the carbonate ligand by a biomimetic coupled catalytic system.[88] Yields are based on 1HNMR an amine, which is thermodynamically disfavored.[83] This spectroscopy.

Asian J. Org. Chem. 2013, 2, 726 – 744729 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

typical route for oxidation under aqueous conditions with supported gold nano- particles requires careful washing of the solid to remove the cations and halo- gen ions.[101] The presence of such ions may be responsible for reducing the activity of the catalysts.[101] Further- more, it was necessary to prepare the gold nanoparti- cles in a separate step by chemical reduction or ther- mal decomposition. Howev- er, Baiker and co-workers demonstrated that a highly active gold catalyst could be prepared without following the typical routes men- tioned.[99] The supported gold nanoparticles (Au/ Scheme 6. Ruthenium/quinone-catalyzed dehydrogenation of amines to give imines.[90] Yields are based on CeO2) were generated in situ 1 H NMR spectroscopy. by simple addition of a gold precursor and the support [90] [99] by the co-catalyst. The major advantage of this systems is into an organic solvent. AuCl3,HAuCl4·3H2O, and ACHTUNGRE that aldimines and ketimines can be efficiently prepared Au(OAc)3 were the best gold precursors. In terms of TOFs, (Scheme 6); however, the use of 1.5 equivalents of quinone the acetate-based catalysts were 2–3 times more active than is the concern from the sustainable chemistry point of alumina-supported catalysts and 7000 times active than view.[90] bulk gold.[79,93,95,97–100] Oxidation of amines to give imines by [102] [103] [104] The tetranuclear ruthenium complex {[(PCy3)(CO)RuH]4 using gold supported on TiO2, CeO2, graphite, ACHTUNGRE ACHTUNGRE ACHTUNGRE [105] (m4-O)(m3-OH)(m2-OH)} is a useful catalyst for dehydrogen- porous coordination polymers, and that produced by ation of amines.[91] A -based pincer ruthenium com- sputtering techniques was also investigated.[106] plex was reported for amine coupling to give imines (ruthe- The notable variance between gold- and ruthenium-cata- nium pincer complex (1 mol%), toluene, argon atmosphere, lyzed oxidation of amines is that gold-catalyzed oxidation at 1158C).[92] of primary amines provides imines as the major products, whereas ruthenium-catalyzed oxidation gives nitriles as the 3.1.2. Gold Catalysts major products (Scheme 7). The first gold catalyzed synthesis of imines from amines It has been suggested that the mechanisms of the rutheni- was reported by Zhu and Angelici in 2007.[93] Bulk gold um- and gold-catalyzed reactions proceed through b-hy- powder (ca. 103 nm particle size) was an active catalyst for dride elimination to provide similar imine intermediates in the ODH of secondary imines to give imines under mild step 1 (Scheme 7).[105] In the subsequent step for ruthenium- conditions (1 atm O2 at 60–1008C) in acetonitrile or toluene catalyzed reactions, second b-hydride elimination may give as the solvent. The gold powder was prepared by the reduc- the corresponding as the predominant product. How- [94] tion of HAuCl4 with hydroquinone. Zhu et al. continued ever, in the case of gold catalysis, the elimination of the their study on gold catalysis and found that gold supported second b-hydride would be slow or energetically unfavora- on alumina nanoparticles, Au/Al2O3 (20–150 nm), was sig- ble. Moreover, in gold-catalyzed reactions, the pathway for nificantly more active than bulk gold powder.[95] The cata- the imine intermediate to couple with another amine could lytic activity of 5 mg of gold from the Au/Al2O3 catalyst be fast and yield energetically favorable dibenzylimine as was more active than 1 g of bulk gold powder. The support- the predominant product. There has still not been a satisfac- ed gold catalyst was prepared by the incipient wetness im- tory explanation for the discrepancies between the poten- pregnation method. In a recent report, Angelici and co- tial pathways of the gold- and ruthenium-catalyzed reac- workers reported that aliphatic amine N-oxides are effec- tions and it could be important to investigate this. tive oxidants for Au-catalyzed ODH of amines and alco- 3.1.3. Vanadium Catalysts hols.[96] ACHTUNGRE Baiker and co-workers reported the use of gold nanopar- An oxovanadium complex VO(Hhpic)2 (H2hpic =3-hydrox- ticles for the aerobic oxidation of secondary amines.[97–100] A ypicolinic acid) was successfully used as a catalyst for selec-

Asian J. Org. Chem. 2013, 2, 726 – 744730 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

When BrOH was used for the oxidation of benzyla- mine, a small yield of imine was obtained under aqueous conditions (7% at RT and 17% at reflux in water). Other halogenated species, Scheme 7. Possible mechanistic pathways for ruthenium- versus gold-catalyzed oxidation of amines. such as N-bromosuccinimide (NBS), HBr/H2O2, HCl/

H2O2, and iodine were also tive oxidation of to obtain the corresponding studied for imine synthesis. benzylimines under aerobic conditions (Scheme 8).[107] In an The results obtained with the bromide- and chloride-based ACHTUNGRE ionic liquid, the VO(Hhpic)2 catalysts were reusable. reagents under similar experimental conditions were not significant. However, the reaction with iodine gave a 68% yield of imine.[29] Iodine is a Lewis acid and was effective for oxidation of benzylamines. Based on these outcomes, we hypothesized that a related transition-metal halide spe- cies may be highly selectivity for imine synthesis under non-aqueous conditions. To our delight, we found that copper chloride functioned well in this context.[29] After ex- Scheme 8. Vanadium-catalyzed oxidation of amines. tensive screening of various copper catalysts and various experimental conditions, 0.5 mol% copper(I) chloride at

Vanadium pentoxide (V2O5) was also reported as a cata- 1008C in atmospheric air were the optimum reaction condi- lyst for efficient oxidation of benzylamines to give imines tions (Scheme 9).[29] This system is very general and is appli- [108] with H2O2 as an environmentally benign oxidant. Inter- cable for a wide range of primary and secondary amines, in- estingly, reactions with benzylamines that have electron- cluding heteroaromatic and cyclic amines (Scheme 9). This withdrawing substituents, such as F, Cl, Br, and COOC2H5, reaction is also efficient for the synthesis of unsymmetrical provide good to quantitative yields of the corresponding imines (Table 1). Under these conditions, the strongly elec- imines; however, electron-donating substituents 4-Me, 4- tron-withdrawing 3-nitroaniline combined with aromatic OMe, 3-OAc, and 1-naphthylamine failed to provide the amine substrates produced only symmetrical imines, possi- corresponding imines under these conditions. bly because of the competitive nucleophilicity of the corre- Mixed vanadium and molybdenum complexes were also sponding amines. reported for ODH of amines.[109–111] An important aspect of In copper(I)-catalyzed oxidation of amines (Scheme 9 these catalysts is their inherent stability under strongly oxi- and Table 1); a small amount of the corresponding alde- dizing conditions. Among different vanadium mixed com- hyde byproduct was formed. To overcome byproduct for- plexes, NPV6Mo6 in particular, is a good catalyst in terms mation, copper powder was used for selective imine synthe- of yield and selectivity.

3.1.4. Copper Catalysts Table 1. Synthesis of unsymmetrical imines. In synthetic organic chemistry, aerobic oxidations of amines has been mainly studied with ruthenium and gold catalysts. The limited availability of these metals and their high price makes it highly desirable to search for more economical al- 2 ternative metal catalysts. The easily and abundantly avail- R Yield [%] unsym./sym. able copper and its complexes are emerging as alternative 78 23:77 catalysts. Various copper complexes have been reported for [112–114] ODH of primary amines to give imines. Recently, our 86 86:14 group developed an environmentally benign brominating reagent for diverse applications.[115–118] This reagent is a com- 82 17:83 bination of a 2:1 mole ratio of bromide/bromate salts, which, upon acidification, generates active species BrOH 93 33:67 [Eq. (1)]. 97 0:100

þ 2Br þ BrO3 þ 3H ! 3 BrOH ð1Þ 94 0:100 BrOH has been explored in organic synthesis for applica- 78 56:44 tions that include oxidation and oxybrominations.[119–122]

Asian J. Org. Chem. 2013, 2, 726 – 744731 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

aliphatic amines, heteroaromatic amines, and unsymmetri- cally coupled imines. The advantages of copper powder over copper chloride are a lower cost, higher stability under aerobic conditions, a lower environmental impact, mild re- action conditions, and easy isolation of products without chromatography. Therefore, after completion of the reac- tion, the reaction mixture was filtered through filter paper (cellulose paper sheet) and washed with a minimum quanti- ty of diethyl . After removal of the solvent from the filtrate, pure imine was obtained. Mechanisms for Copper-catalyzed Imine Synthesis Similar to ruthenium and gold, copper is not well-recog- nized as a hydride transfer catalyst; therefore reactions cat- alyzed by copper may proceed through a different reaction mechanism, as shown in Scheme 10. In the proposed mech- anism the first step is the formation of a copper(II)–amine complex (Scheme 10A) through the oxidative addition of copper to the amine. Further, reductive elimination of copper from the copper(II) complex results the imine inter- mediate I. Reaction of intermediate I with another amine leads to the final imine (Path 1, Scheme 10B). On the other hand, the presence of water partially hydrolyses intermedia- te I to give an aldehyde (Path 2). In the final step, conden- sation of the aldehyde and amine leads to the desired imine. Similar reactions for the aerobic oxidation of amines to Scheme 9. Copper(I)-chloride-catalyzed aerobic oxidation of amines to give imines were reported with a CuBr2/TEMPO/oxygen [123] give imines. Ratios are that of imine to aldehyde. Numbers in parenthe- system in aqueous acetonitrile at room temperature. The ses are the yields of crude product as determined by 1H NMR spectros- TEMPO-catalyzed reaction of benzylamines was carried copy. out with different catalytic systems, such as CuBr2, CuCl2, ACHTUNGRE Cu(CH3COO)2 and FeCl3, which resulted in 86%, 58%, and 62% conversions and no reaction, respectively. sis under neat aerobic conditions.[30] Under these conditions, 3.1.5. Manganese Catalysts good to excellent yields of the corresponding imines were obtained with a wide substrate scope, such as benzylamines Manganese dioxide,[124,125] potassium permanganate[126] with electron-donating and electron-withdrawing substitu- either in acidic or neutral medium, and manganese sul- ents, secondary amines, cyclic secondary amines, primary fate[127] were used for the oxidation of amines to give the

Scheme 10. Possible mechanism for the copper-catalysed aerobic oxidation of primary amines to give imines.

Asian J. Org. Chem. 2013, 2, 726 – 744732 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

corresponding imines. A manganese–porphyrin com- plex in combination with tert-butyl hydroperoxide as an oxidant in various organic Scheme 11. Transfer from amines to to produce imines and . solvents, such as dichlorome- thane,[128] acetonitrile,[129] and toluene,[130] was reported to catalyze this reaction. Ji and dium black.[138] In these reports, imines were generated as co-workers demonstrated that a manganese porphyrin/ stable intermediates through ODH of amines with the pal- [137,138] tBuOOH catalytic system is also effective in an aqueous ladium catalyst. Palladium catalyst PdCl2/PPh3 is medium.[131] useful for dehydrogenation of secondary benzylamines but not for primary benzylamines under aerobic conditions.[139] 3.1.6. Cobalt Catalysts ACHTUNGRE Dirhodium caprolactamate [Rh2(cap)4] is another effective Cobalt(II) catalytic systems that consist of salen ligands catalyst for dehydrogenation of secondary amines with ACHTUNGRE [132] [140] [Co(salen)]/O2 catalyze the oxidation of primary and tBuOOH as oxidant in acetonitrile at room temperature. secondary amines.[133] The reactivity of the catalyst and the Other transition metals used for the oxidation of amines to efficiency of the reaction depends on the structure of the give imines include iridium,[141] zinc,[142] and mercury.[143] ligand. Several [Co2+(salen)]ACHTUNGRE catalytic systems with different ligand structures were studied[134] and their order of reactiv- 3.2. Photocatalysis ity with tBuOOH as the oxidant follows the order: 2+ 5 2+ 1 2+ 2 2+ 3 2+ 4 Co (L )>Co (L )>Co (L )> Co (L )>Co (L )> The photocatalytic properties of TiO2 were discovered by Co2+(L6) [Figure 1].[134] However, with oxygen as an oxidant Fujishima and Honda in 1967 and published in 1972.[144] Co2+(L5) was inactive. The reactivity of Co2+(L) may be at- Later, in 1985, photocatalytic formation of imines from pri- tributed to the susceptibility of the CoO bond in mary amines on a platinized TiO2 suspension in acetonitrile [CoACHTUNGRE 3+(L)(OOACHTUNGRE tBu)] to homolytic cleavage. with the elimination of ammonia and hydrogen was report- ed (Scheme 12).[145]

Scheme 12. Photocatalytic oxidation of amines to give imines catalyzed

by TiO2-supported Pt.

Recently Zhao and co-workers reported aerobic oxida-

tion of amines to give imines on the surface of TiO2 in an inert solvent under UV irradiation with high selectivity.[146]

+ Most photocatalytic organic reactions that have been re- Figure 1. [CoACHTUNGRE 2 (salen)]ACHTUNGRE ligand structures. ported for synthetic transformations are performed under UV irradiation and it is difficult to perform such reactions with visible light. Generally, reactions with visible light re- 3.1.7. Other Transition-metal Catalysts quire doping of the TiO2 surface with noble-metal com-

A NiSO4/K2S2O8 catalytic system was effective for the oxi- plexes or nanoparticles. Zhao and co-workers discovered dation of secondary amines to give imines.[135] Garcia and that a series of benzylic amines adsorbed on the surface of [147] co-workers developed a Ni-catalyzed strategy for in situ TiO2 can absorb light in the visible region. This property from amines to alkynes to produce was used for the aerobic oxidation of amines in atmospher- [136] [147] imines and alkenes as products (Scheme 11). The secon- ic air on an anatase surface of TiO2. ACHTUNGRE dary amines were dehydrogenated by the [(dppe)Ni(m-H)] The reaction mechanism for TiO2-based photocatalysis catalyst (dppe= 1,2-bis(diphenylphosphino)ethane, was explained as being similar to a semiconductor-type phe- 0.5 mol%) with simultaneous hydrogen transfer from the nomenon. This mechanism is different from the ODH amines to the alkynes. mechanism that was reported for transition-metal catalysts In 1973, Murahashi and co-workers reported the synthe- as discussed above. In this case, the formation of imines sis of unsymmetrical secondary and tertiary amines by proceeds through oxygenation of amines to give aldehydes using a palladium catalyst (palladium black).[137] Similarly, as intermediates, rather than the primary imine RC=NH in 1983 the same group reported catalytic alkyl exchange which is generated through oxidation and dehydrogenation reaction between primary and secondary amines with palla- of the amines (Scheme 13).

Asian J. Org. Chem. 2013, 2, 726 – 744733 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

a wide substrate scope for various benzylamines, including heterocyclic and cyclic amines (Scheme 15). Moreover, the system was extended for cascade one-pot synthesis of other

Scheme 13. Oxidation of amines to give imines on a TiO2 surface under visible-light (vis) irradiation.

Under photochemical conditions, an electron is liberated from the TiO2 surface and creates a hole and a liberated electron localized within the TiO2 framework (Scheme 14). The created hole initiates the activation of the amine sub- strate and generates an radical. The radical then combines with dioxygen to form a peroxy intermediate, which subsequently dissociates to give an aldehyde.

Scheme 15. Oxidation of various amines to give imines photocatalyzed by visible light with nitride. Numbers in parentheses indicate yield of crude product as determined by 1H NMR spectroscopy.

heterocycles, such as benzoxazoles, benzimidazoles, and benzothiazoles, in high yields. The reaction is initiated by electron (e) and hole (h+) pairs that are photogenerated

by irradiation of mpg-C3N4 with visible light (Scheme 16).

Scheme 14. Proposed mechanism for the oxidation of amines to give [147] imines on a TiO2 surface under visible-light irradiation.

Scheme 16. Proposed mechanism for the coupling of amines to give [149] Apart from TiO2, other photocatalytic systems have also imines photocatalyzed by visible light with carbon nitride. been reported. Nb2O5 does not absorb visible light directly; however it binds with amine substrates to form a complex that absorbs at long wavelengths in the visible region. The liberated electron reduces molecular oxygen to pro- * Therefore Nb2O5 is catalytically active with high selectivity duce O2 , which was confirmed by ESR analysis. In paral- under visible light (l> 390).[148] The reaction mechanism for lel, benzylamine also loses an electron to form amine spe- *+ *+ the Nb2O5 photocatalytic system is different from that of cies PhCH2NH . The collision between PhCH2NH and * TiO2. Amine oxidation over Nb2O5 with visible light may O2 species leads to PhCH= NH, which combines with an- be attributed to direct electron transfer from a 2p orbital of other molecule of benzylamine to give an imine. However, the amine nitrogen atom that is bound to Nb2O5. Various high oxygen pressure (0.5 MPa) and trifluorotoluene as the amines, including primary, secondary, and cyclic amines, solvent are necessary to obtain good yields of products.[149] were converted into the corresponding imines in excellent Recently Son and co-workers developed phenothiazine- yields by using Nb2O5 under atmospheric pressure and at based organic dyes for visible-light-driven, photocatalytic room temperature. organic transformations.[150] The oxidative coupling of ben-

Mesoporous graphite carbon nitride (mpg-C3N4)as zylamines to give imines with a 3,7,-disubstituted pheno- a stable, reusable, heterogeneous photocatalytic system that thiazine catalyst (0.5 mol%) under visible light irradiation is free from metals and organic oxidizing agents for the oxi- from a blue LED in acetonitrile under ambient conditions dation of amines was reported in 2011.[149] This system has (1 atm. air, room temperature) was reported.[150]

Asian J. Org. Chem. 2013, 2, 726 – 744734 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

Metal organic frameworks (MOFs) are crystalline materi- aminophenol products. The aminophenol is further oxidized als that consist of metal ions and organic molecules that are to regenerate the staring quinone co-factor with the elimi- joined together to form one, two, or three dimensional net- nation of ammonia and the cycle continues. In synthetic works and are well known for catalysis. N-Hydroxyphthali- chemistry, a similar path to the biological cycle may lead to mide (NHPI) incorporated into the intracrystalline space of an aldehyde, which is not suitable for mimicking imine syn- iron-based MOF NHPI/Fe(BTC)ACHTUNGRE (BTC =1,3,5-benzenetri- thesis. However, in the absence of water an imine inter- carboxylate) is a good catalytic system for imine synthesis mediate can undergo transamination, which results in the from various primary and secondary amines under neat desired imine product instead of an aldehyde. Consequent- conditions.[151] The [NHPI/Fe(BTC)]ACHTUNGRE catalyst is heterogene- ly, biomimetic approaches which aim to mimic the models ous in nature and can be recovered and reused. Lin and co- of amine dehydrogenases/oxidases enzymes for oxidation of workers studied the MOF for applications in solar energy amines have been widely studied.[157–161] Several synthetic harvesting and subsequently for organic photocatalysis.[152] models of naturally occurring quinones were developed for 1 Singlet oxygen ( O2) is generated through excitation of the oxidation of amines to give imines under metal-free molecular oxygen usually with the help of photosensitizer. conditions. In the elegant catalytic system reported by Singlet oxygen is highly reactive and, hence, readily reacts Largeron and Fleury, 3,4-iminoquinone acts as a catalyst with most of molecules because it has the same quantum for autorecycling oxidation of benzylamine through the state as most of other molecules. A variety of secondary transamination process of amine oxidase cofactors benzylic amines were oxidized to give imines by singlet (Scheme 18).[162] The 3,4-iminoquinone was electrogenerat- oxygen generated from oxygen and meso-tetraphenylpor- ed from its starting precursor 3,4-aminophenol by anodic [153] phyrin photosensitizer H2TPP. The imines were formed controlled-potential electrolysis at a platinum electrode in in situ and treated with and carboxylic acids in deuterated methanol. As alkylimines are unstable, they are an Ugi-type reaction to synthesize C1- and N-functionalized converted into the corresponding 2,4-dinitrophenylhydra- amines (Scheme 17). Berlicka and Kçnig also studied pho- zone derivatives by using 2,4-dinitrophenylhydrazine during the work up and isolation process.

Scheme 17. Synthesis of functionalized amines by photocatalytic oxida- tion. tocatalytic oxidation of amines to give imines by using sin- glet oxygen and a 2,7,12,17-tetrapropylporphycene

(H2TPrPc) photocatalyst along with blue light-emitting diodes (LEDs).[154] Photooxidation of benzylamine under photoirradiation in the presence of 9-mesityl-10-methylacri- + dium perchlorate ((Acr –Mes)ClO4 ) with molecular oxygen affords the corresponding imine.[155] Recently Sadow and co-workers reported oxidant-free conversation of amines into imines under photocatalytic conditions.[156] The merits associated with photocatalytic aerobic oxidation Scheme 18. Mechanism of catalytic oxidation of primary aliphatic amines mediated by electrogenerated 3,4-iminoquinone model cofactor of organic molecules are efficient conversation and a sus- 1ox. tainable nature, as well as reusability and durability of the catalyst. Later, the same group extended the scope of their study on the biomimetic electrocatalytic system to the oxidation 3.3. Electrocatalysis (Biomimetic Approach) of amines.[163–166] The above electrocatalytic method is effi- Many efforts have been made to mimic the biological activ- cient for the oxidation of unactivated primary aliphatic ities of amine dehydrogenases/oxidases for the oxidation of amines to give imines, which is difficult to achieve by other amines. In biological systems, the amine is not dehydrogen- synthetic methods, particularly in the absence of a metal. ated but reacts with a carbonyl group of a quinone cofactor, However, these electrocatalytic systems are poorly selective which leads to an imine intermediate. Upon , the with a-branched primary amines, and secondary amines did imine is converted into the corresponding aldehyde and not react. Okimoto et al. also reported electrochemical oxi-

Asian J. Org. Chem. 2013, 2, 726 – 744735 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

dation of benzylic amines to give imines by using a catalytic A recent development in aerobic oxidation of amines to amount of KI in methanol.[167] It was suggested that iodide give imines involves the use of graphite oxide as catalyst.[196] ions also play the catalytic role of electron carrier, thus The efficiency of the reaction is attributed to the high sur- iodine acts as an electrocatalyst. Unlike iminoquinone, face area of the catalyst and the reaction was performed at iodide ions as electrocatalysts failed to facilitate oxidation 1008C without any co-solvent. Because graphite oxide is of alkyl amines effectively. thermally and mechanically stable, it was reused five times. A notable improvement in synthesizing imines from amines was the use of “on water” reactions.[197] By heating a suspen- 3.4. Organocatalysis sion of an amine and water to reflux under one atmosphere Recently an azobisisobytyronitrile (AIBN)-catalyzed of oxygen pressure and without any additives; an imine (7 mol%) oxidative coupling of primary amines to give formed.[197] A possible reaction pathway for “on water” oxi- imines with oxygen (1 bar) as the oxidant in at dation of amines is one in which the amino group forms hy- 808C was reported.[168] Organic compounds that are capable drogen bonds with water.[197] This hydrated amine species is of reversible redox functions may be used as organocata- further oxidized by atmospheric oxygen to form a peroxo lysts in oxidation reactions. Wendlandt and Stahl reported complex, which, after subsequent reactions, provides an the biomimetic aerobic oxidation of primary and secondary imine intermediate with release of H2O2 (Scheme 19). The amines with a quinone species as an organocatalyst and imine intermediate reacts with another molecule of amine oxygen as the oxidant in acetonitrile at room tempera- to give the final imine. ture.[169] Nitta and co-workers extensively studied the syn- thesis, properties, and redox ability of organocatalysts.[170–172] Conducting polymers, such as polyanilines, have the ability to interconvert into various oxidation states based on their conjugate structure, which makes it variable redox system.[173] Hirao et al. reported that p-conjugated polyani- lines serve as synthetic metal catalysts with reversible redox Scheme 19. Possible reaction mechanism for water-mediated oxidation properties in the presence of oxygen to induce dehydrogen- of amines.[197] ative oxidation of benzylamines and give the corresponding imines.[174–176] It is known that doped polyaniline emeraldine (PANI-ES) synthesized by a self-stabilized dispersion 4. Imine Synthesis through Oxidative Coupling of method has a nanoporous structure. This nanoporous conju- Alcohols and Amines gated polymer with a high surface area was effectively used for the dehydrogenation of amines in the presence or ab- The coupling of alcohols and amines in the presence of oxi- sence of oxygen.[177] dants is also a commonly used procedure for imine synthe- The oxidation of amines to give imines can be achieved sis. Alcohols are desirable starting materials because they by Swern oxidation[178] and with activated DMSO.[179,180] are readily available, inexpensive, and theoretically produce However, the recovery and reuse of DMSO has not been only hydrogen or water as a byproduct. The selectivity of accomplished. Therefore, modification of the DMSO cata- the reaction can be controlled with the catalyst. Many stud- lytic system with polymer supports is beneficial. Polymer- ies have described transient generation of imine intermedi- supported sulphoxide (PSS)[181] and polymer-supported per- ates in situ, which are rapidly hydrogenated to give secon- ruthenate (PPP) are reusable and recyclable catalysts for dary amines; however, this section is focused on the con- this transformation.[182] The process was also extended for trolled and selective synthesis of the imine product. the synthesis of [2, 1-c][1,4]benzodiazepineACHTUNGRE antibiot- The direct alkylation of amines with alcohols has been ics by the oxidation of a cyclic secondary amine with cata- known since 1909, when Sabatier reported the N-alkylation lytic amounts of tetra-n-propylammonium perruthenate of amines with alcohols to form secondary amines as final (TPAP) and N-methylmorpholine N-oxide (NMO) as a co- products.[198] In 2001, Blackburn and Taylor reported imine oxidants.[183] Nicolaou et al. developed a new reagent, 2-io- synthesis through stepwise formation of imines from alco- [184,185] [199] doxybenzoic acid, for oxidations of secondary amines. hols and amines with MnO2 as the oxidant. The oxidant and reductant used in this procedure were then removed by filtration and the desired products were isolated by simple 3.5. Miscellaneous Catalysts evaporation of the solvent. However, the use of large [186–188] Selenium and its complexes were reported for oxida- excess of MnO2 (10 equiv. with respect to the sub- tion of amines to give imines. A variety of other oxidants, strate) and inefficiency of the oxidation with unactivated al- such as hypervalent iodine,[189] Frmy’s salt,[190] arylsulphon- cohols are shortcomings of the method.[199] Another manga- yl peroxide,[191] N-tert-butylphenylsulfinimidoyl chloride,[192] nese-catalyzed process that produces imine directly from sulfurane,[193] and di-tert-butyliminoxyl[194] are also available amines and alcohols with manganese octahedral molecular for the oxidation of amines to give imines.[195] sieves was also reported.[200]

Asian J. Org. Chem. 2013, 2, 726 – 744736 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

Recently, new dehydrogenative reactions have been de- The rigidity of the pincer–metal interaction provides high veloped for the synthesis of imines through the coupling of thermal stability for complexes that are capable of activat- alcohols and amines, in which hydrogen gas is liberated ing inert bonds. These methods produce minimum waste without the use of stoichiometric additives (Scheme 20). and are, therefore, considered more environmentally benign protocols.[203,204] However, these processes can be improved by replacing the organic solvent and lowering the temperature. Impregnated ruthenium on magnetite[205] and supported ruthenium hydroxide[206] catalysts were intro- Scheme 20. Dehydrogenative synthesis of imines. duced recently. In the first report, the use of a stoichiomet- ric amount of base was the drawback, whereas the latter method was limited to activated alcohols.[206] A convenient For example, reactions catalyzed by ruthenium and iridium synthesis of a,b-unsaturated imines from allylic alcohols complexes proceed through imine intermediates.[201,202] In and amines catalyzed by ruthenium is the most recent de- 2010, Milstein and co-workers reported a reaction with a ho- velopment in this field.[207] mogenous dearomatized 2-(di-tert-butylphosphinomethyl)- Cao and co-workers reported the use of gold nanoparti- 6-(diethylaminomethyl)pyridine (PNN) pincer-type rutheni- cles supported on hydroxyapatite (Au/HAP) for the synthe- um complex in an argon atmosphere that stops at the imine sis of imines through coupling of alcohols and amines stage without converting the imine into a secondary under solvent-free conditions at 608C.[208] The catalyst is amine.[203] Similarly, osmium pincer complexes have also heterogeneous and reusable. Later, gold nanoparticles sup- been used for this transformation under anaerobic condi- ported on TiO2 with oxygen as the oxidant were investigat- tions.[204] Pincer ligands have made a tremendous impact on ed and the reactions performed at room temperature.[209] homogenous catalysis. Especially, methods catalyzed by This type of reaction catalyzed by Pd/AlO(OH) in an metal pincer complexes deserve extra attention from re- oxygen atmosphere yields imines, whereas in a hydrogen at- searchers because of their ability to efficiently oxidize non- mosphere it results in secondary amines.[210,211] The latest activated aliphatic and cyclic alcohols, which is challenging improvement, which involves Pd/DNA in water at 508C, task (Table 2).[203,204] needs more attention, particularly for a mechanistic study.[212]

Table 2. Selective examples of imine synthesis from unactivated alcohols and amines with pincer complexes.

R1 R2 Product Yield [%] Reference

58[a] [203]

86[a] [203]

94[b] [204]

65[a] [203]

57[a] [203]

37[b] [204]

30[b] [204]

20[c] [203]

55[b] [204]

65[b] [204]

[a] Yield of isolated product. [b] Yield based on 1H NMR spectroscopy. [c] Yield based on GC analysis.

Asian J. Org. Chem. 2013, 2, 726 – 744737 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

The coupling of amines with alcohols or deaminative self-coupling of primary amines have been successfully ach- ieved by means of a heterogeneous Pt–Sn bimetallic/g- [213] Al2O3 (0.5 wt% Pt, Pt/Sn molar ratio =1:3) catalyst. In Scheme 21. NaOH-catalyzed aerobic oxidative coupling of amines and the absence of oxygen, imines are hydrogenated to give alcohols. [213] amines through a borrowing hydrogen strategy. TiO2 coated onto platinum nanoparticles promotes one-pot syn- thesis of imines from amines and alcohols under UV irradi- (Scheme 23). In transition-metal-catalyzed oxidative cou- ation (l> 300) at room temperature.[214] An iridium-thioeth- pling of alcohols and amines, a likely mechanism is b-hy- 3 5 er-dithiolate complex [Cp*Ir (h -tpdt)] (Cp*= h -C5Me5, dride elimination from the alcohol by the transition metal ACHTUNGRE tpdt= S(CH2CH2S )2) was used for imine formation from to generate a metal hydride intermediate, which subse- activated as well as unactivated aliphatic alcohols, but only quently provides aldehyde. The generation of metal hy- moderate yields of the corresponding imines were obtained, drides by using simple alkali bases is rare. Moreover, the even at 1108C and with prolonged reaction times (45 h).[215] reaction of benzyl alcohol in the absence of amines does Therefore, this method is no more efficient than those cata- not lead to aldehydes (Scheme 24). Therefore, it was pro- lyzed by ruthenium and iridium pincer complexes.[203,204,215] posed that activation of the alcohol by NaOH in the pres- It is necessary to explore potential catalysis with common ence of an amine and oxygen likely results in the genera- metals, such as copper and iron. The homogenous copper tion of an aldehyde as an intermediate (Scheme 23).[31] The catalyst reported by Kang and Zhang[216] and an impregnat- aldehyde then reacts with the amine to give the imine. ed copper catalyst are such examples for imine synthesis.[217] This method has a wide scope for various benzyl alcohols A noteworthy example is the synthesis of imines from alco- and amines; however, it has some limitations for aliphatic hols and amines at room temperature catalyzed by copper alcohols. Tang and co-workers obtained similar results for iodide (1 mol%).[218] imine synthesis by using KOH as a reagent.[219] In contrast The best way to achieve efficient tandem transformations to our conditions (10 mol% NaOH as the catalyst and no is with simple laboratory chemicals, for example, alkali bases as catalysts and air/oxygen as an oxidant. In view of this, we have devel- oped a strategy for imine synthesis through aerobic ox- idative coupling of amines and alcohols catalyzed by sodium hydroxide in an air atmosphere without any co- solvent or transition metal.[31] Extensive screening of various conditions showed that NaOH (10 mol%) at 1008C in air are the opti- mized reaction conditions for this transformation (Scheme 21). Under these conditions, a range of struc- turally diverse primary ben- zylic alcohols and amines were examined. NaOH is highly active for catalyzing the transformation of benzyl- ic and heteroatom-contain- ing alcohols and amines into imines (Scheme 22). Based on experimental data and literature reports, a probable mechanism was proposed for this base-cata- Scheme 22. Scope of imine synthesis from alcohols and amines. Reaction conditions: alcohol (6 mmol), aniline lyzed imine synthesis (2 mmol), 1008C, open to air. Numbers in parentheses refer to yield based on GC area%.

Asian J. Org. Chem. 2013, 2, 726 – 744738 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

catalyze intermolecular hydroamination of alkynes with amines to provide Markownikov addition products. Anti- Markownikov addition of amines to alkynes has also been frequently reported and generally leads to aldimines as products.[247–251] The hydroamination of alkynes is not an ideal pathway for imine synthesis as it is associated with the some drawbacks. For example, when unsymmetrical alkynes are used as substrates, regioisomers are obtained. Also, it is challenging to achieve hydroamination reactions under mild conditions without use of precious-metal-based cata- lysts.

Scheme 23. Probable mechanism for NaOH-catalyzed coupling of amines and alcohols. 6. Miscellaneous Imine Synthesis

Various methods which differ from the above mentioned methods have been studied for imine synthesis (Scheme 26). These methods include reductive imination of Scheme 24. Deduction of the mechanism of NaOH-catalyzed coupling of nitro compounds,[252,253] decarboxylative amine coupling,[254] amines and alcohols. addition of organometallic reagents to nitriles[255,256] and N- silylate co-solvent)[31] these reactions were performed with toluene as the solvent and a stoichiometric amount of KOH as a re- agent.[219]

5. Imine Synthesis Through Hydroamination of Alkynes with Amines

The direct addition of amines to alkynes is a facile, effi- cient, and atom economical route for synthesizing aldi- mines/ketimines.[220–224] Addition of amines across terminal alkynes (when R2 = H) or internal alkynes (when R2 =aryl, alkyl etc.) takes place in Markownikov and/or anti-Mar- kownikov fashion, and the former gives ketimines while latter gives aldimes/ketimines as products (Scheme 25). In case of unsymmetrical terminal alkynes, Markownikov ad- dition is predominant.

Scheme 25. Hydroamination of alkynes to aldimines/ketimines. Scheme 26. Various routes for imine synthesis. BBN =9-borabicyclo- [3.3.1]nonane.ACHTUNGRE

In early 1939, Loritsch and Vogt used mercuric oxide and boron trifluoride in stoichiometric amounts for the hydroa- d-/N-alkyl-/N-arylformamides,[257] arylation of nitriles,[258] mination of alkynes to give ketimines.[225] Since then, coupling of aldimines with boronates,[259] addition of arenes a number of homogenous and heterogeneous metal-based or boronic acids to nitriles,[260–262] coupling of aryl halides catalysts, such as titanium,[226–229] gold,[230,231] copper,[232] with isonitriles and organometallic reagents,[263] reduction of silver,[233] ruthenium,[234,235] palladium,[236] zinc,[237] rhodi- secondary ,[264] coupling of aldehydes with alkyl bro- um,[238,239] tantalum,[240] zirconium,[241,242] mercury,[243] thalli- mides and ammonia,[265] coupling of gem-dibromomethylar- um[244] lanthanides,[245] and actinides,[246] have been used to yl compounds and primary amines,[266] addition of isocya-

Asian J. Org. Chem. 2013, 2, 726 – 744739 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

Focus Review describes how the field of imine chemistry has developed since its in- ception by Hugo Schiff up until today. Nowadays, many efficient methods for the synthesis of imines are scat- tered throughout the litera- ture. The direct synthesis of imines through oxidative condensation of amines as well as through oxidative coupling of amines and alco- hols without any metal cata- lysts or by using simple cata- lysts, such as copper, under mild conditions provides a broad scope. As described above, pincer ligands have made tremendous impact in homogenous catalysis be- cause of their oxidation reac- tions with non-activated ali- phatic and cyclic alcohols. These types of reactions remain challenging. The report that describes oxidative imination of tol- uenes by using a palladium catalyst certainly intensifies our interest in activating other unactivated chemical [270] Scheme 27. Oxidative imination of toluene. Yields in a) are based on GC area%; Yield in b) are yields of equivalents for imine synthe- isolated products. sis. To date, most of the reac- tions were carried out at a temperature between 60– nides to electron-rich arenes,[267,268] and coupling of vinyl 1108C in organic solvents and have product selectivity bromide with amines.[269] issues. In the future, methods that are energy efficient (and The oxidative imination of toluenes through CH activa- possibly proceed at room temperature), use readily avail- tion is the latest development in imine synthesis able reagents and catalysts, avoid organic solvents, and ach- (Scheme 27).[270] The reaction is catalyzed by a heterogene- ieve high conversion as well as selectivity may lead to ideal ous and reusable Au/Pd bimetallic catalyst. It is the first ex- conditions for imine synthesis. The systematic experimental ample of its kind for imine synthesis through primary CH and theoretical study of mechanisms will help to achieve activation of toluenes and its applications extend to hetero- this goal. It is our hope that this review will provide vital cycle synthesis. In the near future, toluenes or any other un- information about the various methods and reagents avail- activated substrates that are chemical equivalents of amines able for imine synthesis to date, which will be useful to and aldehydes/ketones could be activated to form imines a wider scientific community. without heavy, precious-metal catalysts under simple and mild conditions. Such findings or methods will deserve true recognition from the scientific community. [1] M. Nic, J. Jirat, B. Kosata, “Schiff base” IUPAC Compendium of Chemical Terminology, 2006. [2] S. I. Murahashi, Y. Imada in Transition Metals for Organic Synthe- 7. Summary and Outlook sis Vol. 2, 2nd ed. (Eds.: M. Beller, C. Bolm), Wiley-VCH, Wein- heim, 2004, p. 497. [3] S. I. Murahashi, Angew. Chem. 1995, 107, 2670 –2693; Angew. Research in the field of imine synthesis has always been Chem. Int. Ed. Engl. 1995, 34, 2443 –2465. and remains one of the major and important topics in or- [4] J. Gawronski, N. Wascinska, J. Gajewy, Chem. Rev. 2008, 108, ganic synthesis because of their diverse applications. This 5227– 5252.

Asian J. Org. Chem. 2013, 2, 726 – 744740 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

[5] M. E. Belowich, J. F. Stoddart, Chem. Soc. Rev. 2012, 41, 2003 – [43] C. K. Z. Andrade, S. C. S. Takada, L. M. Alves, J. P. Rodrigues, 2024. P. A. Suarez, R. F. Brand¼o, V. C. D. Soares, Synlett 2004, 2135 – [6] C. D. Meyer, C. S. Joiner, J. F. Stoddart, Chem. Soc. Rev. 2007, 36, 2138. 1705– 1723. [44] K. Taguchi, F. H. Wesheimer, J. Org. Chem. 1971, 36, 1570 –1572. [7] F. Heaney, Eur. J. Org. Chem. 2012, 3043– 3058. [45] M. Selva, P. Tundo, C. A. Marques, Synth. Commun. 1995, 25, [8] J. P. Adams, J. Chem. Soc. Perkin Trans. 1 2000, 125–139. 369– 378. [9] E. L. Dias, M. Brookhart, P. S. White, Chem. Commun. 2001, 423– [46] R. Carlson, U. Larsson, L. Hansson, Acta Chem. Scand. 1992, 46, 424. 1211– 1214. [10] E. L. Dias, M. Brookhart, P. S. White, J. Am. Chem. Soc. 2001, [47] W. B. Jennings, C. J. Lovely, Tetrahedron Lett. 1988, 29, 3725 – 123, 2442– 2443. 3728. [11] Chiral Amine Synthesis: Methods, Development and Applications [48] W. A. White, H. Weingarten, J. Org. Chem. 1967, 32, 213 –214. (Ed.: T. C. Nugent), Wiley-VCH, Weinheim, 2010. [49] H. Weingarten, J. P. Chupp, W. A. White, J. Org. Chem. 1967, 32, [12] C. Palomo, J. M. Aizpurua, I. Ganboa, M. Oiarbide, Eur. J. Org. 3246– 3249. Chem. 1999, 3223 –3235. [50] B. P. Branchaud, J. Org. Chem. 1983, 48, 3531 –3538. [13] B. Yin, Y. Zhang, L. W. Xu, Synthesis 2010, 3583–3595. [51] J. H. Billman, K. M. Tai, J. Org. Chem. 1958, 23, 535–539. [14] F. Collet, B. Song, F. Rudolphi, L. J. Gooben, Eur. J. Org. Chem. [52] F. Texier-Boullet, Synthesis 1986, 679 –681. 2011, 6486– 6501. [53] J. D. Armstrong, C. N. Wolfe, J. L. Keller, J. Lynch, M. Bhupathy, [15] R. S. Baligar, S. Sharma, H. B. Singh, R. J. Butcher, J. Organomet. R. P. Volante, Tetrahedron Lett. 1997, 38, 1531 –1532. [54] G. Liu, D. A. Cogan, T. D. Owens, T. P. Tang, J. A. Ellman, J. Org. Chem. 2011, 696, 3015–3022. Chem. 1999, 64, 1278–1284. [16] M. Zarei, A. Zarrahpour, Iran J. Sci. Technol. 2011, A3, 235 –242. [55] R. S. Vass, J. Dudas, R. S. Varma, Tetrahedron Lett. 1999, 40, [17] C. M. da Silva, D. L. Da Silva, C. V. B. Martins, M. A. de Resende, 4951– 4954. E. S. Dias, T. F. F. Magalhaes, L. P. Rodrigues, A. A. Sabino, R. B. [56] R. S. Varma, R. Dahiya, S. Kumar, Tetrahedron Lett. 1997, 38, Alves, A. de Fatima, Chem. Biol. Drug Des. 2011, 78, 810– 815. 2039– 2042. [18] Y. M. S. A. Al-Kahraman, H. M. F. Madkour, D. Ali, M. Yasinzai, [57] B. E. Love, J. Ren, J. Org. Chem. 1993, 58, 556–559. Molecules 2010, 15, 660– 671. [58] G. C. Look, M. M. Murphy, D. A. Campbell, M. A. Gallop, Tetra- [19] G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681 –703. hedron Lett. 1995, 36, 2937– 2940. [20] I. A. Mller, F. Kratz, M. Jung, A. Warnecke, Tetrahedron Lett. [59] S. Das, V. K. Das, L. Saikia, A. J. Thakur, Green Chem. Lett. Rev. 2010, 51, 4371–4374. 2012, 1 –18. [21] V. Saggiomo, U. Luning, Tetrahedron Lett. 2009, 50, 4663– 4665. [60] A. Kundu, N. A. Shakil, D. B. Saxena, Pankaj, J. Kumar, S. Walia, [22] C. Godoy-Alcntar, A. K. Yatsimirsky, J. M. Lehn, J. Phys. Org. J. Environ. Sci. Health Part B 2009, 44, 428– 434. Chem. 2005, 18, 979–985. [61] M. Gopalakrishnan, P. Sureshkumar, V. Kanagarajan, J. Thanusu, [23] D. O. Berbasov, I. D. Ojemaye, V. A. Soloshonok, J. Fluorine Res. Chem. Intermed. 2007, 33, 541 –548. Chem. 2004, 125, 603–607. [62] M. Gopalakrishnan, P. sureshkumar, V. Kanagarajan, J. Thanusu, [24] M. Lee, H. Kim, H. Rhee, J. Choo, Bull. Korean Chem. Soc. 2003, R. Govindaraju, Arkivoc 2006, (xiii), 130– 141. 24, 205 –208. [63] J. F. Collados, E. Toledano, D. Guijarro, M. Yus, J. Org. Chem., [25] A. Simion, C. Simion, T. Kanda, S. Nagashima, Y. Mitoma, T. 2012, 77, 5744–5750.. Yamada, K. Mimura, M. Tashiro, J. Chem. Soc. Perkin Trans. [64] R. S. Varma, R. Dahiya, Synlett 1997, 1245 –1246. 1 2001, 2071–2078. [65] K. P. Guzen, A. S. Guarezemini, A. T. G. Orfao, R. Cella, C. M. P. [26] G. Rothenberg, A. P. Downie, C. L. Raston, J. L. Scott, J. Am. Pereira, H. A. Stefani, Tetrahedron Lett. 2007, 48, 1845–1848. Chem. Soc. 2001, 123, 8701–8708. [66] M. A. Vzquez, M. Landa, L. Reyes, R. Miranda, J. Tamariz, F. [27] Z. Zalkin, D. B. Sprinson, J. Biol. Chem. 1966, 241, 1067–1071. Delgado, Synth. Commun. 2004, 34, 2705 –2718. [28] H. Schiff, Ann. Chem. Pharm. 1866, 140, 92 –137. [67] A. Shockravi, M. Sadeghpour, A. Olyaei, Synth. Commun. 2010, [29] R. D. Patil, S. Adimurthy, Adv. Synth. Catal. 2011, 353, 1695– 40, 2531 –2538. 1700. [68] T. R. van den Ancker, G. W. V. Cave, C. L. Raston, Green Chem. [30] R. D. Patil, S. Adimurthy, RSC Adv. 2012, 2, 5119–5122. 2006, 8, 50–53. [31] D. Ramachandra Reddy, R. D. Patil, S. Adimurthy, Eur. J. Org. [69] J. S. Bennett, K. L. Charles, M. R. Miner, C. F. Heuberger, E. J. Chem. 2012, 4457 –4460. Spina, M. F. Bartels, T. Foreman, Green Chem. 2009, 11, 166– 168. [32] H. Schiff, Annals 1864, 131, 118– 119. [70] V. K. Rao, S. S. Reddy, B. S. Krishna, K. R. M. Naidu, C. N. Raju, [33] R. B. Moffett, In Organic Syntheses, Coll. Vol. 4 (Ed.: N. Rab- S. K. Ghosh, Green Chem. Lett. Rev. 2010, 3, 217–223. john), Wiley, New York, 1963, pp. 605 –608. [71] M. S. Singh, A. K. Singh, P. Singh, R. Jain, Org. Prep. Proced. Int. [34] M. Hosseini-Sarvari, Chin. Chem. Lett. 2011, 22, 547 –550. 2005, 37, 173–203. [35] L. Ravishankar, S. A. Patwe, N. Gosarani, A. Roy, Synth. [72] K. Tanaka, R. Shiraishi, Green Chem. 2000, 2, 272– 273. Commun. 2010, 40, 3177 –3180. [73] R. W. Layer, Chem. Rev. 1963, 63, 489– 510. [36] A. Mobinikhaledi, P. J. Steel, M. Polson, Synth. React. Inorg. Met. [74] Please see relavent references for imine synthesis cited in: [73]. 2009, 39, 189–192. [75] M. T. Schmperli, C. Hammonds, I. Hermans, ACS Catal. 2012, 2, [37] R. Dalpozzo, A. D. Nino, M. Nardi, B. Russo, A. Procopio, Syn- 1108– 1117 and references therein. thesis 2006, 1127 –1132. [76] S. Kamiguchi, A. Nakamura, A. Suzuki, M. Odomari, M. Nomura, [38] H. Naeimi, F. Salimi, K. Rabiei, J. Mol. Catal. A 2006, 260, 100 – Y. Iwasawa, T. Chihara, J. Catal. 2005, 230, 204 –213. 104. [77] K. Yamaguchi, N. Mizuno, Angew. Chem. 2003, 115, 1518 –1521; [39] H. Naeimi, H. Shargi, F. Salimi, K. Rabiei, Heteroat. Chem. 2008, Angew. Chem. Int. Ed. 2003, 42, 1480–1483. 19, 43 –47. [78] K. Yamaguchi, N. Mizuno, Chem. Eur. J. 2003, 9, 4353–4361. [40] M. Gopalakrishnan, P. Sureshkumar, V. Kanagarajan, J. Thanusu, [79] K. Mori, K. Yamaguchi, T. Mizugaki, K. Ebitani, K. Kaneda, R. Govindaraju, J. Chem. Res. 2005, 2005, 299– 303. Chem. Commun. 2001, 461– 462. [41] A. K. Chakraborti, S. Bhagat, S. Rudrawar, Tetrahedron Lett. [80] S. E. Diamond, G. M. Tom, H. Taube, J. Am. Chem. Soc. 1975, 97, 2004, 45, 7641–7644. 2661– 2664. [81] J. J. Ritter, J. Am. Chem. Soc. , 55, 3322 –3326. [42] J. Bennett, K. Meldi, C. Kimmell II., J. Chem. Educ. 2006, 83, 1933 [82] A. J. Bailey, B. R. James, J. Chem. Soc. Chem. Commun. 1996, 1221– 1224. 2343– 2344.

Asian J. Org. Chem. 2013, 2, 726 – 744741 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

[83] A. Prades, E. Peris, M. Albrecht, Organometallics 2011, 30, 1162 – [120] M. K. Agrawal, S. Adimurthy, B. Ganguly, P. K. Ghosh, Tetrahe- 1167. dron 2009, 65, 2791 –2797. [84] S. I. Murahashi, Y. Okano, H. Sato, T. Nakae, N. Komiya, Synlett [121] G. Joshi, S. Bhadra, S. Ghosh, M. K. Agrawal, B. Ganguly, S. Adi- 2007, 1675– 1678. murthy, P. K. Ghosh, B. C. Ranu, Ind. Eng. Chem. Res. 2010, 49, [85] A. Goti, M. Romani, Tetrahedron Lett. 1994, 35, 6567 –6570. 1236– 1241. [86] S. I. Murahashi, T. Naota, H. Taki, J. Chem. Soc. Chem. Commun. [122] R. D. Patil, S. Bhadra, S. Adimurthy, B. C. Ranu, Synth. Commun. 1985, 613– 614. 2010, 40, 2922–2929. [87] F. Doctorovich, M. Granara, F. D. Salvo, Transit. Metal Chem. [123] Z. Hu, F. M. Kerton, Org. Biomol. Chem. 2012, 10, 1618– 1624. 2001, 26, 505–509. [124] R. J. Highet, W. C. Wildman, J. Am. Chem. Soc. 1955, 77, 4399 – [88] J. S. M. Samec, A. H. Ell, J. E. Bckvall, Chem. Eur. J. 2005, 11, 4401. 2327– 2334. [125] E. F. Pratt, T. P. McGovern, J. Org. Chem. 1964, 29, 1540 –1543. [89] J. S. M. Samec, J. E. Bckvall, Chem. Eur. J. 2002, 8, 2955 –2961. [126] H. Shechter, S. S. Rawalay, M. Tubis, J. Am. Chem. Soc. 1964, 86, [90] A. H. ll, J. S. M. Samec, C. Brasse, J. E. Bckvall, Chem. 1701– 1705. Commun. 2002, 1144– 1145. [127] S. S. Kim, S. S. Thakur, J. Y. Song, K. H. Lee, Bull. Korean Chem. [91] C. S. Yi, D. W. Lee, Organometallics 2009, 28, 947– 949. Soc. 2005, 26, 499 –501. [92] L. P. He, T. Chen, D. Gong, Z. Lai, K. W. Huang, Organometallics [128] S. Tollari, A. Fumagalli, F. Porta, Inorg. Chim. Acta 1996, 247,71– 2012, 31, 5208–5211. 74. [93] B. Zhu, R. J. Angelici, Chem. Commun. 2007, 2157– 2159. [129] S. S. Kim, S. S. Thakur, Bull. Korean Chem. Soc. 2005, 26, 1600 – [94] B. P. Block, Inorg. Synth. 1953, 4, 14–17. 1602. [95] B. Zhu, M. Lazar, B. G. Trewyn, R. J. Angelici, J. Catal. 2008, 260, [130] Q. L. Yuan, X. T. Zhou, H. B. Ji, Catal. Commun. 2010, 12, 202– 1–6. 206. [96] E. R. Klobukowski, R. J. Angelici, L. K. Woo, Catal. Lett. 2012, [131] X. T. Zhou, Q. G. Ren, H. B. Ji, Tetrahedron Lett. 2012, 53, 3369 – 142, 161 –167. 3373. [97] L. Aschwanden, T. Mallat, M. Maciejewski, F. Krumeich, A. [132] P. A. Ganeshpure, A. Sudalai, S. Satish, J. Chem. Sci. 1991, 103, Baiker, ChemCatChem 2010, 2, 666– 673. 741– 745. [98] L. Aschwanden, B. Panella, P. Rossbach, B. Keller, A. Baiker, [133] A. Nishinaga, S. Yamazaki, T. Matsuura, Tetrahedron Lett. 1988, ChemCatChem 2009, 1, 111 –115. 29, 4115 –4118. [99] L. Aschwanden, T. Mallat, F. Krumeich, A. Baiker, J. Mol. Catal. [134] K. Maruyama, T. Kusukawa, Y. Highchi, A. Nishinaga, Chem. A 2009, 309, 57– 62. Lett. 1991, 1093–1096. [100] L. Aschwanden, T. Mallat, J. D. Grunwaldt, F. Krumeich, A. [135] S. Yamazaki, Chem. Lett. 1992, 823– 826. Baiker, J. Mol. Catal. A 2009, 300, 111 –115. [136] A. Reyes-Snchez, F. Canavera-Buelvas, R. Barrios-Francisco, [101] G. C. Bond, C. Louis, D. T. Thomson, Catalysis by Gold, Imperial O. L. Cifuentes-Vaca, M. Flores-Alamo, J. J. Garcia, Organometal- College Press, London, 2006, pp. 72–120. lics 2011, 30, 3340 –3345. [102] A. Grirrane, A. Corma, H. Garcia, J. Catal. 2009, 264, 138– 144. [137] N. Yoshimura, I. Moritani, T. Shimamura, S. I. Murahashi, J. Am. [103] Y. Prez, C. Aprile, A. Corma, H. Garcia, Catal. Lett. 2010, 134, Chem. Soc. 1973, 95, 3038–3039. 204– 209. [138] S. I. Murahashi, N. Yoshimura, T. Tsumiyama, T. Kojima, J. Am. [104] M. H. So, Y. Liu, C. M. Ho, C. M. Che, Chem. Asian J. 2009, 4, Chem. Soc. 1983, 105, 5002–5011. 1551– 1561. [139] J. R. Wang, Y. Fu, B. B. Zhang, X. Cui, L. Liu, Q. X. Guo, Tetrahe- [105] T. Ishida, N. Kawakita, T. Akita, M. Haruta, Gold Bull. 2009, 42, dron Lett. 2006, 47, 8293– 8297. 267– 274. [140] H. Choi, M. P. Doyle, Chem. Commun. 2007, 745– 747. [106] H. Guo, M. Kemell, A. Al-Hunaiti, S. Rautiainen, M. Leskela, T. [141] R. Yamaguchi, C. Ikeda, Y. Takahashi, K. Fujita, J. Am. Chem. Repo, Catal. Commun. 2011, 12, 1260–1264. Soc. 2009, 131, 8410 –8412. [107] S. Kodama, J. Yoshida, A. Nomoto, Y. Ueta, S. Yano, M. Ueshima, [142] B. M. Choudary, N. Narender, V. Bhuma, Synth. Commun. 1996, A. Ogawa, Tetrahedron Lett. 2010, 51, 2450 –2452. 26, 631 –635. [108] G. Chu, C. Li, Org. Biomol. Chem. 2010, 8, 4716–4719. [143] K. Orito, T. Hatakeyama, M. Takeo, S. Uchiito, M. Tokuda, H. Su- [109] R. Neumann, M. Levin, J. Org. Chem. 1991, 56, 5707–5710. ginome, Tetrahedron 1998, 54, 8403 –8410. [110] K. Nakayama, M. Hamamoto, Y. Nishiyama, Y. Ishii, Chem. Lett. [144] A. Fujishima, K. Honda, Nature 1972, 238, 37– 38. 1993, 1699– 1702. [145] B. Ohtani, H. Osaki, S. I. Nishimoto, T. Kagia, Chem. Lett. 1985, [111] J. S. Reddy, A. Sayari, Catal. Lett. 1994, 28, 263– 267. 1075– 1078. [112] L. M. Sayre, W. Tang, K. V. Reddy, D. Nadkarni, Bioinorganic [146] X. Lang, H. Ji, C. Chen, W. Ma, J. Zhao, Angew. Chem. 2011, 123, Chemistry of Copper (Ed.: K. D. Karin, Z. Tyeklar), Chapman & 4020– 4023; Angew. Chem. Int. Ed. 2011, 50, 3934–3937. Hall, New York, 1993, pp. 236–248. [147] X. Lang, W. Ma, Y. Zhao, C. Chen, H. Ji, J. Zhao, Chem. Eur. J. [113] S. Minakata, Y. Ohshima, A. Takemiya, I. Ryu, M. Komatsu, Y. 2012, 18, 2624–2631. Ohshiro, Chem. Lett. 1997, 311–312. [148] S. Furukawa, Y. Ohno, T. Shishido, k. Teramura, T. Tanaka, ACS [114] J. Zhu, C. Mirkin, J. Am. Chem. Soc. 1998, 120, 5126– 5127. Catal. 2011, 1, 1150– 1153. [115] S. Adimurthy, G. Ramachandraiah, A. V. Bedekar, S. Ghosh, B. C. [149] F. Su, S. C. Mathew, L. Mohlmann, M. Antonietti, X. Wang, S. Ranu, P. K. Ghosh, Green Chem. 2006, 8, 916 –922. Blechert, Angew. Chem. 2011, 123, 683 –686; Angew. Chem. Int. [116] S. Adimurthy, S. Ghosh, P. U. Patoliya, G. Ramachandraiah, M. Ed. 2011, 50, 657–660. Agrawal, M. R. Gandhi, S. C. Upadhyay, P. K. Ghosh, B. C. Ranu, [150] J. H. Park, K. C. Ko, E. Kim, N. Park, J. H. Ko, D. H. Ryu, T. K. Green Chem. 2008, 10, 232– 237. Ahn, J. Y. Lee, S. U. Son, Org. Lett. 2012, 14, 5502 –5505. [117] G. Ramachandraiah, P. K. Ghosh, S. Adimurthy, A. S. Mehta, [151] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, ChemCatChem A. D. Jethva, S. S. Vaghela, US Patent No. 6,740,253 dated 25 2010, 2, 1438–1443. May, 2004. [152] C. Wang, Z. Xie, K. E. deKrafft, W. Lin, J. Am. Chem. Soc. 2011, [118] M. Dinda, M. K. Agrawal, M. R. Gandhi, S. C. Upadhyay, S. Adi- 133, 13445– 13454. murthy, S. Chakraborty, P. K. Ghosh, RSC Adv. 2012, 2, 6645 – [153] G. Jiang, J. Chen, J. S. Huang, C. M. Che, Org. Lett. 2009, 11, 6649. 4568– 4571. [119] R. D. Patil, G. Joshi, S. Adimurthy, B. C. Ranu, Tetrahedron Lett. [154] A. Berlicka, B. Kçnig, Photochem. Photobiol. Sci. 2010, 9, 1359 – 2009, 50, 2529–2532. 1366.

Asian J. Org. Chem. 2013, 2, 726 – 744742 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

[155] K. Ohkubo, T. Nanjo, S. Fukuzumi, Bull. Chem. Soc. Jpn. 2006, [195] S. Bank, R. Jewett, Tetrahedron Lett. 1991, 32, 303–306. 79, 1489 –1500. [196] H. Huang, J. Huang, Y. M. Liu, H. Y. He, Y. Cao, K. N. Fan, [156] H. A. Ho, K. Manna, A. D. Sadow, Angew. Chem. 2012, 124, Green Chem. 2012, 14, 930– 934. 8735– 8738; Angew. Chem. Int. Ed. 2012, 51, 8607–8610. [197] L. Liu, S. Zhang, X. Fu, C. H. Yan, Chem. Commun. 2011, 47, [157] K. Tabuchi, M. Z. Ertem, H. Sugimoto, A. Kunishita, T. Tano, N. 10148– 10150. Fujieda, C. J. Cramer, S. Itoh, Inorg. Chem. 2011, 50, 1633 –1647. [198] P. Sabatier, A. Mailhe, C. R. Hebd, Seances Acad. Sci. 1909, 148, [158] O. R. Luca, T. Wang, S. J. Konezny, V. S. Batista, R. H. Crabtree, 898. New J. Chem. 2011, 35, 998– 999. [199] L. Blackburn, R. J. K. Taylor, Org. Lett. 2001, 3, 1637– 1639. [159] A. Rinaldi, A. Rescigno, A. Rinaldi, E. Sanjust, Bioorg. Chem. [200] S. Sithambaram, R. Kumar, Y. C. Son, S. L. Suib, J. Catal. 2008, 1999, 27, 253–288. 253, 269 –277. [160] M. Mure, Acc. Chem. Res. 2004, 37, 131– 139. [201] K. Yamaguchi, J. He, T. Oishi, N. Mizuno, Chem. Eur. J. 2010, 16, [161] K. Q. Ling, J. Kim, L. M. Sayre, J. Am. Chem. Soc. 2001, 123, 7199– 7207. 9606– 9611. [202] D. Gnanamgari, E. L. O. Sauer, N. D. Schley, C. Butler, C. D. In- [162] M. Largeron, M. B. Fleury, J. Org. Chem. 2000, 65, 8874 –8881. carvito, R. H. Crabtree, Organometallics 2009, 28, 321– 325. [163] M. Largeron, A. Neudorffer, M. B. Fleury, Angew. Chem. 2003, [203] B. Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem. 2010, 115, 1056– 1059; Angew. Chem. Int. Ed. 2003, 42, 1026– 1029. 122, 1510– 1513; Angew. Chem. Int. Ed. 2010, 49, 1468– 1471. [164] M. Largeron, M. B. Fleury, Angew. Chem. 2012, 124, 5505 –5508; [204] M. A. Esteruelas, N. Honczek, M. Olivan, E. Onate, M. Valencia, Angew. Chem. Int. Ed. 2012, 51, 5409–5412. Organometallics 2011, 30, 2468– 2471. [165] M. Largeron, M. B. Fleury, M. S. Benedetti, Org. Biomol. Chem. [205] R. Cano, D. J. Ramon, M. Yus, J. Org. Chem. 2011, 76, 5547– 5557. 2010, 8, 3796–3800. [206] J. W. Kim, J. He, K. Yamaguchi, N. Mizuno, Chem. Lett. 2009, 38, [166] M. Largeron, A. Chiaroni, M. B. Fleury, Chem. Eur. J. 2008, 14, 920– 921. 996– 1003. [207] J. W. Rigoli, S. A. Moyer, S. D. Pearce, J. M. Schomaker, Org. [167] M. Okimoto, Y. Takahashi, K. Numata, Y. Nagata, G. Sasaki, Biomol. Chem. 2012, 10, 1746– 1749. Synth. Commun. 2005, 35, 1989 –1995. [208] H. Sun, F. Z. Su, J. Ni, Y. Cao, H. Y. He, K. N. Fan, Angew. Chem. [168] L. Liu, Z. Wang, X. Fu, C. H. Yan, Org. Lett. 2012, 14, 5692– 5695. 2009, 121, 4454– 4457; Angew. Chem. Int. Ed. 2009, 48, 4390– [169] A. E. Wendlandt, S. S. Stahl, Org. Lett. 2012, 14, 2850– 2853. 4393. [170] S. Naya, J. Nishimura, M. Nitta, J. Org. Chem. , 70, 9780– 2005 [209] S. Kegnæs, J. Mielby, U. V. Mentzel, C. H. Christensen, A. Riisag- 9788. er, Green Chem. 2010, 12, 1437– 1441. [171] S. Naya, M. Warita, Y. Mitsumoto, M. Nitta, J. Org. Chem. 2004, [210] M. S. Kwon, S. Kim, S. Park, W. Bosco, R. K. Chidrala, J. Park, J. 69, 9184 –9190. Org. Chem. 2009, 74, 2877–2879. [172] S. Naya, T. Tokunaka, M. Nitta, J. Org. Chem. 2004, 69, 4732– [211] L. Jiang, L. Jin, H. Tian, X. Yuan, X. Yu, Q. Xu, Chem. Commun. 4740. 2011, 47, 10833–10835. [173] A. G. MacDiarmid, A. J. Epstenin, Faraday Discuss. Chem. Soc. [212] L. Tang, H. Sun, Y. Li, Z. Zha, Z. Wang, Green Chem. 2012, 14, 1989, 88, 317–332. 3423– 3428. [174] T. Hirao, S. Fukuhara, J. Org. Chem. 1998, 63, 7534– 7535. [213] W. He, L. Wang, C. Sun, K. Wu, S. He, J. Chen, P. Wu, Z. Yu, [175] M. Higuchi, I. Ikeda, T. Hirao, J. Org. Chem. 1997, 62, 1072– 1078. Chem. Eur. J. 2011, 17, 13308–13317. [176] T. Hirao, M. Higuchi, I. Ikeda, Y. Ohshiro, J. Chem. Soc. Chem. [214] Y. Shiraishi, M. Ikeda, D. Tsukamoto, S. Tanaka, T. Hirai, Chem. Commun. 1993, 194– 195. Commun. 2011, 47, 4811 –4813. [177] K. W. Chi, H. Y. Hwang, J. Y. Park, C. W. Lee, Synth. Met. 2009, [215] C. Xu, L. Y. Goh, S. A. Pullarcut, Organometallics 2011, 30, 6499– 159, 26 –28. 6502. [178] D. Keirs, K. Overton, J. Chem. Soc. Chem. Commun. 1987, 1660– [216] Q. Kang, Y. Zhang, Green Chem. 2012, 14, 1016–1019. 1661. [217] J. M. Prez, R. Cano, M. Yus, D. J. Ramon, Eur. J. Org. Chem. [179] A. Kamal, M. V. Rao, Chem. Commun. 1996, 385 –386. [180] L. Gentilucci, Y. Grijzen, L. Thijs, B. Zwaneburg, Tetrahedron 2012, 4548– 4554. Lett. 1995, 36, 4665– 4668. [218] H. Tian, X. Yu, Q. Li, J. Wang, Q. Xu, Adv. Synth. Catal. 2012, [181] A. Kamal, V. Devaiah, K. L. Reddy, N. Shankaraiah, Adv. Synth. 354, 2671– 2677. Catal. 2006, 348, 249– 254. [219] J. Xu, R. Zhuang, L. Bao, G. Tang, Y. Zhao, Green Chem. 2012, [182] B. Hinzen, S. V. Ley, J. Chem. Soc. Chem. Commun. 1997, 1907– 14, 2384 –2387. 1908. [220] T. E. Mller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem. [183] A. Kamal, P. W. Howard, B. S. N. Reddy, B. S. P. Reddy, D. E. Rev. 2008, 108, 3795 –3892. Thurston, Tetrahedron 1997, 53, 3223–3230. [221] R. Severin, S. Doye, Chem. Soc. Rev. 2007, 36, 1407 –1420. [184] K. C. Nicolaou, C. J. N. Mathison, T. Montagnon, J. Am. Chem. [222] T. E. Mller, M. Beller, Chem. Rev. 1998, 98, 675– 703. Soc. 2004, 126, 5192 –5201. [223] R. F. Pohlki, S. Doye, Chem. Soc. Rev. 2003, 32, 104 –114. [185] K. C. Nicolaou, C. J. N. Mathison, T. Montagnon, Angew. Chem. [224] F. Alonso, I. P. Beletskaya, M. Yus, Chem. Rev. 2004, 104, 3079– 2003, 115, 4211 –4216; Angew. Chem. Int. Ed. 2003, 42, 4077 – 3160. 4082. [225] J. A. Loritsch, R. R. Vogt, J. Am. Chem. Soc. 1939, 61, 1462 –1463. [186] E. A. Calderon, Anales Asoc. Quim. Arg. 1947, 35, 149. [226] J. A. Bexrud, C. Li, L. L. Schafer, Organometallics 2007, 26, 6366– [187] J. P. Marino, R. D. Larsen Jr., J. Am. Chem. Soc. 1981, 103, 4642 – 6372. 4643. [227] E. Smolensky, M. Kapon, M. S. Eisen, Organometallics 2007, 26, [188] D. H. R. Barton, A. Billion, J. Boivin, Tetrahedron Lett. 1985, 26, 4510– 4527. 1229– 1232. [228] J. S. Johnson, R. G. Bergman, J. Am. Chem. Soc. 2001, 123, 2923– [189] P. Mller, D. M. Gilabert, Tetrahedron 1988, 44, 7171–7175. 2924. [190] P. A. Wehrli, B. Schaer, Synthesis 1974, 288 –289. [229] B. F. Straub, R. G. Bergman, Angew. Chem. 2001, 113, 4768 –4771; [191] R. V. Hoffman, A. Kumar, J. Org. Chem. 1984, 49, 4011– 4014. Angew. Chem. Int. Ed. 2001, 40, 4632–4635. [192] T. Mukaiyama, A. Kawana, Y. Fukuda, J. Matsuo, Chem. Lett. [230] C. Dash, M. M. Shaikh, R. J. Butcher, P. Ghosh, Inorg. Chem. 2001, 390– 391. 2010, 49, 4972–4983. [193] J. A. Franz, J. C. Martin, J. Am. Chem. Soc. 1997, 119, 583 –591. [231] V. Lavallo, G. D. Frey, B. Donnadieu, M. Soleilhavoup, G. Ber- [194] J. J. Cornejo, K. D. Larson, G. D. Mendenhall, J. Org. Chem. 1985, trand, Angew. Chem. 2008, 120, 5302– 5306; Angew. Chem. Int. 50, 5382 –5383. Ed. 2008, 47, 5224–5228.

Asian J. Org. Chem. 2013, 2, 726 – 744743 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.AsianJOC.org Rajendra D. Patil and Subbarayappa Adimurthy

[232] G. V. Shanbhag, S. M. Kumbar, T. Joseph, S. B. Halligudi, Tetrahe- [252] J. Huang, L. Yu, L. He, Y. M. Liu, Y. Cao, K. N. Fan, Green dron Lett. 2006, 47, 141– 143. Chem. 2011, 13, 2672–2677. [233] N. Lingaiah, N. S. Babu, K. M. Reddy, P. S. S. Prasad, I. Suryanar- [253] Y. Xiang, Q. Meng, X. Li, J. Wang, Chem. Commun. 2010, 46, ayana, Chem. Commun. 2007, 278 –279. 5918– 5920. [234] D. P. Klein, A. Ellern, R. J. Angelici, Organometallics 2004, 23, [254] J. N. Payette, H. Yamamoto, J. Am. Chem. Soc. 2008, 130, 12276 – 5662– 5670. 12278. [235] T. Kondo, T. Okada, T. Mitsudo, J. Am. Chem. Soc. 2002, 124, [255] G. E. P. Smith Jr., F. W. Bergstorm, J. Am. Chem. Soc. 1934, 56, 186– 187. 2095– 2098. [236] A. R. Shaffer, J. A. R. Schmidt, Organometallics 2008, 27, 1259 – [256] R. L. Garner, L. Hellerman, J. Am. Chem. Soc. 1946, 68, 823 –825. 1266. [257] B. L. Feringa, J. F. G. A. Jansen, Synthesis 1988, 184– 186. [237] G. V. Shanbhag, S. B. Halligudi, J. Mol. Catal. A 2004, 222, 223– [258] K. C. Gulati, S. R. Seth, K. Venkataraman, Org. Synth. 1935, 15, 228. 70– 71. [238] E. Kumaran, W. K. Leong, Organometallics 2012, 31, 1068– 1072. [259] Y. J. Park, E. A. Jo, C. H. Jun, Chem. Commun. 2005, 1185– 1187. [239] C. G. Hartung, A. Tillack, H. Trauthwein, M. Beller, J. Org. [260] Y. C. Wong, K. Parthasarthy, C. H. Cheng, Org. Lett. 2010, 12, Chem. 2001, 66, 6339–6343. 1736– 1739. [240] L. L. Anderson, J. Arnold, R. G. Bergman, Org. Lett. 2004, 6, [261] C. Zhou, R. C. Larock, J. Org. Chem. 2006, 71, 3551– 3558. 2519– 2522. [262] C. Zhou, R. C. Larock, J. Am. Chem. Soc. 2004, 126, 2302– 2303. [241] K. Born, S. Doye, Eur. J. Org. Chem. 2012, 764– 771. [263] T. Ishiyama, T. Oh-e, N. Miyaura, A. Suzuki, Tetrahedron Lett. [242] S. Chessa, N. J. Clayden, M. Bochmann, J. A. Wright, Chem. 1992, 33, 4465–4468. Commun. 2009, 797– 799. [264] G. Pelletier, W. S. Bechara, A. B. Charette, J. Am. Chem. Soc. [243] J. Barluenga, F. Aznar, R. Liz, R. Rodes, J. Chem. Soc. Perkin 2010, 132, 12817–12819. Trans. 1 1980, 2732– 2737. [265] J.-M. Huang, J.-F. Zhang, Y. Dong, W. Gong, J. Org. Chem. 2011, [244] J. Barluenga, F. Aznar, Synthesis 1977, 195– 196. 76, 3511 –3514. [245] J. S. Ryu, G. Y. Li, T. J. Marks, J. Am. Chem. Soc. 2003, 125, [266] N. Weibel, L. J. Charbonniere, R. F. Ziessel, J. Org. Chem. 2002, 12584– 12605. 67, 7876 –7879. [246] T. Straub, A. Haskel, T. G. Neyroud, M. Kapon, M. Botoshansky, [267] M. Tobisu, S. Yamaguchi, N. Chatani, Org. Lett. 2007, 9, 3351– M. S. Eisen, Organometallics 2001, 20, 5017 –5035. 3353. [247] K. Sakai, T. Kochi, F. Kakiuchi, Org. Lett. 2011, 13, 3928 –3931. [268] M. Tanaka, T. Sakakura, Y. Tokunaga, T. Sodeyama, Chem. Lett. [248] C. Alonso-Moreno, F. C. Hermosilaa, J. R. Fernandez, A. M. Ro- 1987, 2373– 2374. driguez, A. Otero, A. Antinolo, Adv. Synth. Catal. 2009, 351, 881 – [269] C. V. Reddy, J. V. Kingston, J. G. Verkade, J. Org. Chem. 2008, 73, 890. 3047– 3062. [249] A. Tillack, H. Jiao, I. G. Castro, C. G. Hartung, M. Beller, Chem. [270] X. Cui, F. Shi, Y. Deng, Chem. Commun. 2012, 48, 7586– 7588. Eur. J. 2004, 10, 2409–2420. [250] A. Tillack, V. Khedkar, M. Beller, Tetrahedron Lett. 2004, 45, Received: January 16, 2013 8875– 8878. Revised: February 15, 2013 [251] A. Tillack, I. G. Castro, C. G. Hartung, M. Beller, Angew. Chem. Published online: April 22, 2013 2002, 114, 2646 –2648; Angew. Chem. Int. Ed. 2002, 41, 2541 – 2543.

Asian J. Org. Chem. 2013, 2, 726 – 744744 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim