doi:10.1002/adsc.201501150 palladium-catalyzed tandem ......6pd(pph3)4 na2co3 thf 80 9 7pd(pph3)4...

DOI: 10.1002/adsc.201501150

Palladium-Catalyzed Tandem Reaction of Alkyne-BasedAryl Iodides and Salicyl N-Tosylhydrazones to Construct theSpiro[benzofuran-3,2’’-chromene] Skeleton

Xue Song Shang,a Nian Tai Li,a Deng Yuan Li,a and Pei Nian Liua,*a Shanghai Key Laboratory of Functional Materials Chemistry, Key Lab for Advanced Materials and Institute of Fine

Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, PeopleÏs Republic ofChinaFax: : (++86)-21-6425-0552; e-mail: [email protected]

Received: December 17, 2015; Revised: February 29, 2016; Published online: April 15, 2016

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501150.

Abstract: A convenient palladium-catalyzedtandem reaction of aryl iodides and salicyl N-tosyl-hydrazones has been achieved to afford a series ofcompounds containing the novel spiro[benzofuran-3,2’-chromene] scaffold in moderate to good yields.This efficient catalytic reaction, which tolerates var-ious functional groups, combines alkyne-based 5-exo-dig cyclization, palladium(II) carbene migratoryinsertion and intramolecular cyclization, generatingthree new bonds in one reaction.

Keywords: cyclization; migratory insertion; palladi-um; spirocyclic skeletons; tandem reactions

The spirocyclic skeleton is a structural motif found inmany pharmaceuticals, biologically active compoundsand natural products.[1] For example, spiro[[2]benzo-furan-1,4’-piperidine] derivatives serve as novel s1 re-ceptor ligands and promising agents for labeling s1 re-ceptors intracellularly.[2] Other spirocyclic compoundscontaining the benzofuran moiety show antifungal,[3]

anticonvulsant,[4] antihypertensive,[5] and antiviral ac-tivities.[6]

In view of the significant biological and pharma-ceutical activities of spirocyclic compounds, numerousresearchers have focused on constructing related skel-etons. To date, several synthetic methods have beenestablished, including radical cyclization,[7] N-hetero-cyclic carbene annulation,[8] TuÏs semipinacol rear-rangement,[9] [3++2] cycloaddition promoted byPPh3,

[10] and transition metal-catalyzed cyclization.[11]

Although an array of spirocyclic scaffolds has beenconstructed using different methods, only few exam-ples of spirocyclic compounds containing the spiro-[benzofuran-3,2’-chromene] skeleton have been de-

scribed.[12] Therefore, developing an efficient synthesisof the novel spiro[benzofuran-3,2’-chromene] skeletonwould expand the library of spirocyclic compoundswith potentially valuable biological and pharmaceuti-cal activities.

Many recent studies have focused on palladium-cat-alyzed cross-coupling of diazo compounds with arylor vinyl halides, since this approach can generate vari-ous polycyclic aromatic compounds and their hetero-cyclic analogues. The research groups of Vald¦s,[13]

Wang[14] and Liang[15] have reported numerous deli-cate palladium-catalyzed reactions involving alkyl,vinyl, aryl, alkynyl and acyl group migratory insertion.In these reactions, the useful synthon N-tosylhydra-zone usually serves as the diazo precursor to generatea Pd(II) carbene. When vinyl migratory insertion isinvolved, h3-allylpalladium intermediates are formedthat can be trapped with various nucleophiles, yield-ing a variety of heterocyclic compounds in an intra-molecular manner.[16]

Based on those pioneering studies and motivatedby our ongoing interest in heterocycle construction,[17]

we aimed to develop a concise palladium-catalyzedreaction of aryl iodides and salicyl N-tosylhydrazonesin order to generate the interesting spiro[benzofuran-3,2’-chromene] scaffold. Here, we describe our suc-cessful efforts through a process in which three newbonds are formed in a single reaction. This tandem re-action combines alkyne-based 5-exo-dig cyclization,[18]

palladium(II) carbene migratory insertion and intra-molecular cyclization, delivering a new class of com-pounds in satisfying yields with good tolerance forvarious functional groups.

We began to investigate the tandem reaction usingpalladium catalysts and the model substrates 1-iodo-2-[(2-methyl-4-phenylbut-3-yn-2-yl)oxy]benzene 1aand N’-(2-hydroxybenzylidene)-4-methylbenzenesul-fonohydrazide 2a (Table 1). Screening ofseveral palla-

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dium catalysts in THF showed that Pd(dppf)Cl2 didnot catalyze the reaction at 80 88C (entry 1). However,Pd(PPh3)2Cl2 and Pd2(dba)3 worked well, affordingproduct 3a in respective yields of 80% and 70% (en-tries 2 and 3). Satisfyingly, product 3a was obtained in83% yield in the presence of 5 mol% Pd(PPh3)4 andK2CO3 (entry 4). The structure of 3a was confirmedby single-crystal X-ray diffraction analysis(Figure 1).[19] Performing the reaction in the absenceof palladium catalyst led to no desired product(entry 5). We then screened several bases, and foundthat Na2CO3, Cs2CO3 and KOH led to substantiallylower yields of 3a (entries 6–8), while t-BuOK gaveonly trace amounts of 3a (entry 9). No product 3a wasobtained in the absence of base (entry 10). THF gavebetter results than any other solvent that we screened(entries 11–15). Lowering the reaction temperature to60 88C sharply reduced the yield of 3a (entry 16). How-

ever, increasing the reaction temperature from 80 88Cto 100 88C and shortening the reaction time to 12 hnegligibly affected the yield of 3a (entry 17). Theamount of catalyst proved crucial to the reaction of1a with 2a : decreasing the load of Pd(PPh3)4 to2.5 mol% reduced the yield of 3a (entry 18). Finally,we tried to use 1-bromo-2-[(2-methyl-4-phenylbut-3-yn-2-yl)oxy]benzene or 1-chloro-2-[(2-methyl-4-phe-nylbut-3-yn-2-yl)oxy]benzene to react with 2a, butonly trace amounts of 3a were obtained.

Using the optimized reaction conditions [1a :2a=1:2, 5 mol% Pd(PPh3)4, 1.5 mL THF, 80 88C, 15 h], weexplored the scope and limitations of this tandem re-action (Table 2). First, we examined the ability of var-ious salicyl N-tosylhydrazones 2 to react with 1-iodo-2-[(2-methyl-4-phenylbut-3-yn-2-yl)oxy]benzene 1aand generate different spiro[benzofuran-3,2’-chro-mene] derivatives. Salicyl N-tosylhydrazones with anelectron-donating OMe or Me group positioned parato the hydroxy group reacted well with 1a to give thecorresponding products 3b and 3c in respective isolat-ed yields of 81% and 79%, which were higher thanthe 78% isolated yield of 3a. Salicyl N-tosylhydrazonesubstituted with F gave substantially lower yield of3d, even after prolonging the reaction time to 20 h.Similarly, salicyl N-tosylhydrazone bearing a Cl groupreacted with 1a to afford the target product 3e in50% yield, even after the reaction temperature wasincreased to 100 88C. Salicyl N-tosylhydrazone substi-tuted with NO2 reacted with 1a to give a complexmixture of products, demonstrating the negativeeffect of electron-withdrawing substituents. Salicyl N-tosylhydrazones containing electron-donating OMe orNEt2 positioned para to the imine group gave theproducts 3f and 3g in respective yields of 72% and57%. Salicyl N-tosylhydrazone substituted with 3-me-thoxy or 3,5-di-tert-butyl groups also worked, deliver-ing products 3h and 3i in respective yields of 72% and

Table 1. Optimization of conditions for the palladium-cata-lyzed tandem reaction of aryl iodide (1a) and salicyl N-tosyl-hydrazone (2a).[a]

En-try

[Pd] Base Solvent Temp[oC]

Yield[%][b]

1 Pd(dppf)Cl2 K2CO3 THF 80 ND2 Pd(PPh3)2Cl2 K2CO3 THF 80 803[c,d] Pd2(dba)3 K2CO3 THF 80 704 Pd(PPh3)4 K2CO3 THF 80 83 (78)5 none K2CO3 THF 80 ND6 Pd(PPh3)4 Na2CO3 THF 80 97 Pd(PPh3)4 Cs2CO3 THF 80 68 Pd(PPh3)4 KOH THF 80 329 Pd(PPh3)4 t-BuOK THF 80 trace10 Pd(PPh3)4 none THF 80 ND11 Pd(PPh3)4 K2CO3 toluene 80 trace12 Pd(PPh3)4 K2CO3 1,2-DCE 80 trace13 Pd(PPh3)4 K2CO3 DMF 80 1814 Pd(PPh3)4 K2CO3 MeCN 80 4215 Pd(PPh3)4 K2CO3 dioxane 80 6516 Pd(PPh3)4 K2CO3 THF 60 1817[e] Pd(PPh3)4 K2CO3 THF 100 8118[d] Pd(PPh3)4 K2CO3 THF 80 41

[a] Reaction conditions: 1a (0.20 mmol), 2a (0.40 mmol),[Pd] (0.01 mmol, 5 mol%), base (1.0 mmol, 5.0 equiv.),solvent (1.5 mL), 15 h, under N2, unless otherwise noted.

[b] Yield of 3a determined by 1H NMR using PhSiMe3 as theinternal standard. Isolated yield is shown in the parenthe-ses. ND= not detected.

[c] PPh3 (0.03 mmol, 15 mol%) was added.[d] [Pd] (0.005 mmol, 2.5 mol%).[e] 12 h.

Figure 1. The ORTEP diagram of 3a with ellipsoids shownat the 30% probability level.

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74%. A naphthalene-based N-tosylhydrazone gener-ated the product 3j in 76% yield.

Next we investigated the ability of aryl iodides1 with various substituents at the R1 position to reactwith 2a. On the whole, aryl iodides carrying para sub-stitutions at the benzene ring (4-OMe, 4-Me, 4-F, 4-Cl, 4-CF3) reacted with 2a to give the products 3k–3oin moderate to good yields of 58–82%. The corre-sponding substrates 1 substituted at the meta or orthoposition on the benzene ring generated the products3p–3r in yields of 78–86%. The reaction also toleratedother substituents, including naphthyl and thienylmoieties, which gave the respective products 3s and 3tin yields of 64% and 67%. An aliphatic substituentwas also tolerated, affording the product 3u in 82%yield.

To further explore the flexibility of this tandem re-action, we tested the ability of aryl iodides 1 substitut-ed at the R2 and R3 positions to react with 2a. Anaryl iodide bearing a cyclic substituent at the R2 andR3 positions gave the final product 3v in good yield.Notably, aryl iodides carrying methyl and isobutylgroups at the R2 and R3 positions also worked well inthis catalytic reaction, yielding product 3w as a diaste-reoisomer (dr=3:1.35) in 75% yield. In contrast, re-acting aryl iodide carrying an H atom at the R2 or R3

position with 2a did not give the corresponding spiro-cyclic product, but instead generated the b-H elimina-tion product 4 in 38% yield. The structure of 4 wasconfirmed by single-crystal X-ray diffraction analy-sis.[19] This result indicates that under the optimizedreaction conditions, b-H elimination proceeds much

Table 2. Reaction of aryl iodides (1a–o) and various salicyl N-tosylhydrazones (2a–k) in the presence of Pd(PPh3)4.[a,b]

[a] Reaction conditions: 1a–o (0.20 mmol), 2a–k (0.40 mmol), Pd(PPh3)4 (0.01 mmol), K2CO3 (1.0 mmol), THF (1.5 mL),80 88C, 15 h, under N2, unless otherwise noted.

[b] Isolated yield.[c] 20 h.[d] 100 88C.[e] Determined by 1H NMR spectroscopy.

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faster than O-nucleophile-assisted intramolecular cyc-lization.

Based on these results and previous studies on pal-ladium-catalyzed cascade reactions, we propose thereaction mechanism shown in Scheme 1. First, 1a un-dergoes oxidative addition to Pd(0) to afford Pd(II)intermediate A. Then, 5-exo-dig cyclization occurs togive vinyl Pd(II) intermediate B. The diazo com-pound, previously generated in situ from salicyl N-to-sylhydrazone 2a, reacts with B to give Pd(II) carbenespecies C, which undergoes migratory insertion of thevinyl group to generate the intermediate D. The h1-al-lylpalladium complex D isomerizes to a more stablekey h3-allylpalladium species E, in which b-H elimina-tion is avoided. Instead, intramolecular cyclization as-sisted by an O-nucleophile affords the final spirocyclicproduct 3a and simultaneously releases the palladiumspecies F.[16c,d] Finally, the initial Pd(0) catalyst is re-generated after reductive elimination with the aid ofK2CO3.

In summary, we have developed a convenient cata-lytic reaction of aryl iodides and salicyl N-tosylhydra-zones to access the new spiro[benzofuran-3,2’-chro-

mene] skeleton. The reaction involves 5-exo-dig cycli-zation, palladium(II) carbene migratory insertion andintramolecular cyclization in the presence of 5 mol%Pd(PPh3)4 catalyst. A series of new compounds carry-ing a broad array of substituents are obtained in mod-erate to good yields. This appears to be the first timethat this novel scaffold has been constructed ina single reaction. Further studies focused on the con-struction of polycyclic compounds are ongoing in ourlaboratory.

Experimental Section

Typical Experimental Procedure for Product 3a

A mixture of 1a (0.20 mmol), salicyl N-tosylhydrazones 2a(0.40 mmol), Pd(PPh3)4 (11.6 mg, 0.01 mmol) and K2CO3(138.2 mg, 1.0 mmol) in THF (1.5 mL) was stirred at 80 88C(oil bath temperature) in a sealed tube under nitrogen at-mosphere for 15 h (unless otherwise noted). Then, the re-sulting mixture was cooled down to room temperature anddiluted with CH2Cl2. The solvent was evaporated under re-duced pressure and the residue was passed through columnchromatography on silica gel to afford the target product 3a.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Project Nos. 21421004, 21561162003,21372072 and 21190033), the Eastern Scholar DistinguishedProfessor Program, the Programme of Introducing Talents ofDiscipline to Universities (B16017), the NCET (NCET-13-0798), the Basic Research Program of the Shanghai Commit-tee of Science and Technology (Project No. 13M1400802)and the Fundamental Research Funds for the Central Univer-sities.

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