Selective N-Formylation of Amines with H2 and CO2 Catalyzed by

Mar 1, 2017 - Tailor-made Molecular Cobalt Catalyst System for the Selective Transformation of Carbon Dioxide to Dialkoxymethane Ethers. Benjamin G. S...
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Selective N-Formylation of Amines with H2 and CO2 Catalyzed by Cobalt Pincer Complexes Prosenjit Daw, Subrata Chakraborty, Gregory Leitus, Yael Diskin-Posner, Yehoshoa Ben-David, and David Milstein ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00116 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Catalysis

Selective N-Formylation of Amines with H2 and CO2 Catalyzed by Cobalt Pincer Complexes Prosenjit Daw,† ‡ Subrata Chakraborty,† ‡ Gregory Leitus,δ Yael Diskin-Posner, δ Yehoshoa Ben-David, † and David Milstein*† Department of †Organic Chemistry and δChemical Research Support, Weizmann Institute of Science, Rehovot, 76100, Israel ABSTRACT: N-formylation of amines utilizing CO2 in the presence of reducing agents constitute an important methodology in organic synthesis. Presented herein is selective N-formylation of amines with CO2 and H2 catalyzed by complexes of earth-abundant cobalt. A wide range of amines were converted to their corresponding formamides under CO2 and H2 pressure catalyzed by Co-PNP pincer complex generating water as the sole by product. KEYWORDS: Cobalt, formylation, CO2, hydrogenation, pincer.

Formamides are a class of compounds widely used in organic synthesis, including synthesis of valuable heterocycles, biological intermediates and pharmaceuticals. Formamides are also used as Lewis base organocatalysts in hydrosilylation reactions and other transformations. Moreover, the formyl group is a useful protecting group of the amine functionality in peptide synthesis.1-6 Traditionally, N-formamide synthesis involves the use of stoichiometric amounts of coupling reagents like chloral, formic acid, formaldehyde or formate, generating copious waste and resulting in poor atom-economy.7 The direct catalytic synthesis of N-formamides from amines and carbonyl sources like formates,7, 8a methanol,8b-gCO28h-j in the presence of a catalyst has become a field of much interest due to high atom efficiency. In this regard an attractive green route for the Nformylation of amines is the use of a mixture of CO2 and H2 as a formylating agent; CO2 is a non-toxic and abundant C1 building block and hydrogen gas is the cleanest and most atom-economical reducing agent; water is the only by-product generated in this reaction. In addition capturing the greenhouse gas CO2 and its utilization for the production of value-added chemicals is highly desirable.9 However, catalytic N-formylation of amines using CO2 is challenging due to the stability of CO2.10 Moreover, selectivity is a crucial issue in this methodology, since methylated amines may also form via over reduction of the formed formamides.11 Despite this, the first homogeneously catalysed preparation of DMF using H2 ,CO2 and dimethylamine was reported by Haynes12 in 1970. Subsequently several other reports appeared for the synthesis of DMF from CO2, H2 and dimethylamine.13 However, this methodology is mostly limited to DMF synthesis. The first example of N-formylation of a wide range of amines with CO2 and H2 was reported by Ding using highly efficient Ru-pincer catalysts.14 The development of catalytic systems based on low toxicity, earth abundant base metals is an important goal in homogeneous catalysis.15 Appreciable progress has been made in recent years employing complexes of base metals (Fe, Co, Mn, Ni) in various (de)hydrogenation reactions.16-19 Homogeneous cobalt catalysts have been reported in hydrogenation reactions of olefins,18a-e ketones,18a,f imines,18a CO2,18g-i N-heterocycles18k and carboxylic acids18l; dehydrogenation of alcohols,19a N-heterocycles,19b N-alkylation of amines with alcohols,19c-e N-alkylation of amines with amines,19f C-alkylation of

unactivated esters and amides with alcohol.19g We have reported ester hydrogenation20a and nitrile hydrogenation20b catalyzed by a pyridinebased PNNH pincer Co complex, and very recently, using this catalyst, we have exploited the dehydrogenative coupling of diols and amines to selectively form functionalized 1,2,5-substituted pyrroles liberating water and hydrogen gas as the sole by-products.21 N-formylation of secondary amines using CO2 and H2 catalyzed by complexes of base metals (Fe and Co) was reported by Beller for the synthesis of DMF, diethylformamide and N-formylpiperidine.22 To our knowledge, formylation of primary amines, using CO2 and H2 catalyzed by basemetal complexes was not reported.

Figure 1. Co-based pincer complexes explored in this study. Herein we present cobalt catalyzed N-formylation of a wide range of primary- and secondary amines under relatively mild CO2 and H2 pressure. We also describe the key intermediate of the active catalyst. The novel paramagnetic Co(II) chloride complex CoII(iPr-PNHP)Cl2 1 was synthesised by treating our previously reported iPr-PNHP pincer ligand23 with one equivalent of CoCl2 at room temperature in THF in 93% yield (Figure 1, see also ESI). Single crystals of complex 1 suitable for an X-ray diffraction study were obtained by slow diffusion of pentane into a saturated solution of THF at -30 °C. The molecular structure is shown in Figure 2. Complexes 2-6 were prepared according to literature procedures.20a, 24, 25

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higgh yields of N-heexylformamide (entries ( 6 and 7)). In the absencee of basse, using 5 mol% % NaHBEt3 and 5 mol% of 1, 73% % yield of N- hexxylform mamide was obttained (entry 8),, whereas using 20 mol% NaHB BEt3 andd 5 mol% of 1, NN hexylformamidde was obtained in 95% yield (enntry 9). On the other haand, in the absencce Tabble 2. Substrate scope for the N N-formylation off amines using C CO2 andd H2 catalyzed byy complex 1. Figure 2. X-ray sstructure of compplex 1. Thermal ellipsoids are draawn at 50% probability level. Selected hhydrogen atoms are omitted for clarity. c For selected bonnd lengths and anngles see SI. Table 1. Optimization of the reaction conditionss for the N-formyylation of hexylamine.

Entrya

C Cat

1c 2 3d 4e 5f 6 7 8 9 10 11 12 13 14 15g 16 17 18 19

1 1 1 1 1 1 1 1 1 1 1 1 CooCl2 1 2 3 4 5

20

6

NaHBEt3 (mol%) 5 5 5 5 5 5 5 5 20

25 5 5 5 5 5 5

Base (mol%) tBuOK (55) tBuOK (55) tBuOK (55) tBuOK (55) tBuOK (55) NaOEt (55) KHMDS (5) tBuOK (55) tBuOK (25) tBuOK (255) tBuOK (55) tBuOK (55) tBuOK (55) tBuOK (55) tBuOK (55) tBuOK(5)

Yieldb (%) 60 99 80 40 45 90 92 73 95 00 00 00 00 00 99 87 89 85 00

tBuOK (55)

00

h

21

1--Cl

-

tBuOK (55)

96

22h

1--Cl

5

-

93

a

Conditions: hexyylamine (0.5 mmoll), catalyst (5 moll%), NaHBEt3, base, and dry toluene (2 mL L), heated in an auttoclave at 150 °C ((bath temperature)) for 36 h. bIsolated yield. c120 °C. dthe reacttion was carried ouut for 24 h. eTHF ssolvent. f dioxane solvent. gtthe reaction was caarried out in the prresence of 50 equivv. Hg. h See mechanistic paart.

Our initiall attempts were ffocused on assesssing the catalyticc activity of complexess 1-6 in the presence of one equivv. of NaHBEt3 (reelative to Co) and tBuOK as a base, inn the N-formylaation of hexylamine by mmol) in the preesence CO2 and H2. Thhus, reaction of hhexylamine (0.5 m of CO2 (30 bar)) and H2 (30 bar)) using NaHBEtt3 (5 mol%), tBuO OK (5 mol%) and compplex 1 (5 mol%) in toluene resultted in the formattion of N-hexylformamiide in 60% yieldd at 120 °C and iin 99% yield at 1150 °C after 36 h (Tablle 1, entries 1 annd 2). Analysis of the crude reaaction

mixture by GC C revealed selecttive formation of the correspoonding formamide as a sole product. NN methylation of the amine via v further hydrogenattion of the formaamide was not obbserved. Changing the solvent to THF or 1,4-dioxane resulted in poor yyields under anallogous condition (Tablle 1, entries 4 annd 5). Exploring the effect of thee bases KHMDS and NaOEt under simiilar reaction condditions also resullted in

Entrya

Aminees

Formam mides

C Conv.(%)b

Yieeld (% %)c

1

99

99

2

99

[ 95[d]

3

85

80

4

74

60

5

75

70

6

99

96

7

95

88

99

90

99

99

99

92

99

90

12

99

93

13

77

75

14[e][f]

99

[ 99[a][d]

O

8

N CH HO N CHO

9

10

11

N CH HO N N CHO O

a Coonditions: amine (00.5 mmol), catalysst (5 mol%), NaHB BEt3 (5 mol%) tBuOK (5 m mol%), CO2 (30 bbar), H2 (30 bar),, and dry toluene (2 mL), heated inn an autooclave at 150 °C baath temperature foor 36 h. bconversionns determined by G GCMSS based on amine cconsumption. cIsolaated yield. d GC yield. e For TON callculatioon: dimethylaminee (6 mmol, 3 mL L of 2M in THF), catalyst (0.4 moll%), NaH HBEt3 (0.4 mol%) tBuOK (0.4 mol% %), CO2 (30 bar), H2 (30 bar), heatedd in an autoclave a at 150 °C C bath temperature for 36 h (Yield 118%, TON 45). f saame as [[e] in presence of molecular m sieve (Yield 54%, TON 1300).

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of NaHBEt3 no conversion was observed o under the t same reaction conditions even upoon increase of thee base loading too 25 mol% (entriies 1011). The absencce of both tBuOK K and NaHBEt3 also did not shoow any conversion of thhe amine (entry 12). The aforem mentioned experiiments indicated that booth NaHBEt3 annd tBuOK play crucial roles in geenerating the catalyticcally active speciees. However usinng only NaBEt3H and tBuOK (25 moll% each) withouut any Co sourcee did not result in any formamide form mation (entry 133). Using CoCl2 as pre-catalyst in the presence of NaB BEt3H and tBuO OK under the sam me conditions reesulted in no conversionn of hexylamine (entry 14). Moreover, a reactionn using pre-catalyst 1 unnder same condiition in the preseence of 50 equivvalents of Hg gave quanntitative conversiion to the N-hexyylformamide (Taable 1, entry 15), indiccating that Co nnanoparticle form mation is unlikely ly, and supporting the hhomogeneity of tthe reaction. Nexxt, the catalytic activity a of our previouslly reported dihallo Co-PNP, Co--PNN and Co-P PNNH complexes 2-6 (Figure 1) wass explored.[20a],[224,25] Indeed, thhe CoPNPiPr compleex 2, Co-PNPtBuu complex 3 annd Co-PNNBy 4 also catalyze the reacction in 85-89% yields (Table 1, entries 16-18) under similar conditionns. Surprisingly, Co-PNNH and Co-PNNEt, com mplexes 5 and 6, did nnot show any cattalytic activity (T Table 1, entries 119 and 20) under analoogous conditionss. The best perfoorming complex 1 was chosen for furtheer substrate scoppe studies. A An array of prim mary and secondary amines weree then examined in the N-formylation reaction r using ccomplex 1 (5 m mol%), NaHBEtt3 ( 5 mol%), tBuOK (5 mol%), 30 bbar CO2 and 30 bar H2 at 150 °C. ° As shown in Table 2, the N-formylaation reaction prroceeded smoothhly for primary aliphatiic and benzylic amines a to selecttively afford the corresponding formaamides in good to excellent 60--99% yields (Taable 2, entries 1-6). Cyyclohexylamine gave N-cyclohexxylformamide inn 88% yield (Table 2, entry 7). Cyclicc secondary aminnes yielded the corremamides selectivvely in excellent yyields, (Table 2, eentries sponding N-form 8-12). The acycllic secondary am mine N-methyl beenzylamine yieldded Nmethyl-N-benzyylformamide in 775% yield (Table 2, entry 13). Hoowever the less nucleopphilic aromatic amines a aniline annd 3,4-dimethylaaniline were completelyy inactive underr the optimized protocol. Finallly, the industrially impoortant DMF was also obtained inn 99% yield from dimethylamine, CO2 and H2 (Table 22, entry 14). Usinng lower catalystt loading (0.4 mol% ccatalyst, 6 mmoll dimethylamine,,) the TON valuue was 45 (18% yield), likely because thhe large excess oof the generatedd water with respect to tthe catalyst hamppers its activity. A Addition of 500 mg of 4Å molecular siieves resulted inn TON increasess to 130 (54% yyield). Thus, in large sccale experimentss addition of mollecular sieves is advana tageous (Table 22, entry 14e,f, andd see SI) Regarding thee nature of the aactive cobalt cataalyst, we previouusly reported that treattment of the pinccer complex Co--(PNNH)Cl2 witth one equiv of NaHB BEt3 at room temperature gaave the paramaagnetic (PNNH)CoICl,20a which was prroposed as an acctive species in hhydrogenation and dehhydrogenation reactions.20,21 Sim milarly, we preparred the paramagnetic Coo(I) complex [(iiPr-PNHP)CoICll], 1-Cl (Schemee 1) by treatment of com mplex 1 with onee equivalent of N NaHBEt3 at room m temperature (see ESSI). Complex 1-Cl was characterized by X-ray crystalc lography (Figurre 3, and see ESSI). It adopts an uncommon disstorted tetrahedral geom metry and hence it is paramagnetic. Complex 1-C Cl was tested in the form mylation reaction of hexylamine with a mixture oof CO2 and H2. Using 5 mol% of 1-Cl annd 5 mol% of tBuuOK at 150 °C yyielded 96% of N-hexylfoormamide after 336 h (Table 1, enttry 21), which is compac rable to the resultts obtained using the CoII complexx 1 (Table 1, entryy 2), in line with it being involved in the geeneration of the actual a Co(I) catalyyst.

Bassed on the aforem mentioned observvations a plausiblee mechanism is ppropossed in Scheme 1. 1-Cl is generated upon treatmennt of complex 1 with w onee equivalent NaB BEt3H (Scheme 1). A highly reacctive, coordinativvely unssaturated intermeediate A is expecteed to be formed upon u reaction of 1-Cl 1 withh tBuOK, which under H2 generaates the Co(I) hyydride B. CO2 inssertionn into the Co-H bond forms the η1-formato complex C. The presennce of excess e amine resuults in formate saalt formation, likeely by deprotonattion of the t coordinated amine of the pinncer ligand, as suuggested by Bellerr.22c The formate salt liberates water form ming the formamiide13b and regeneratmediate A. Direcct nucleophilic atttack on the form mato ingg the active interm com mplex C by aminee followed by libeeration of H2O frrom the metal cenntre cannnot be excluded aat this stage.

Figgure 3. Moleculaar structure of coomplex 1-Cl. Theermal ellipsoids are draawn at 50% probbability level. Sellected hydrogen atoms are omittted for clarity. For seleccted bond lengthhs and angles see SI. To support innvolvement of inttermediates B andd C, the Co(I) coomplexx 1-Cl was reacteed with one equivvalent of NaBEt3H (Scheme 1), forrmingg a paramagnetic species, probablyy a Co(I) hydridee intermediate B. In thee IR spectrum a w weak signal at 1995 cm-1 was asssigned to the Coo-H streetching frequencyy of B. However, a crystal structurre of B could nott be obttained. When B w was used as catalysst in the absence oof any base, quanttitativee conversion of hexylamine h to heexylformamide (93%) was observved (Taable 1, entry 22).. Moreover, a shaarp peak at 1595 cm,-1 which is in the rannge of νC-O vibratioon of a formate complex,26 was nooticed in the IR sppectrum m when B was treeated with one baar CO2 at room teemperature, suppoortingg formation of inteermediate C (see ESI). Treatment of 1-Cl with sodiium form mate in THF at rooom temperaturee resulted in a form mate complex exhhibitinng this IR stretch aas well (see SI). An alternative possiibility involving thhe generation of free formic acid folwed by reaction wi with the amine cann be excluded (seee SI). Performing the low cataalytic reaction unnder the conditioons of Table 2, buut in the absencee of am mine substrate did not generate any formic acid. Schheme 1. Proposed mechanism forr the N-formylatiion of amines ussing CO O2 and H2 catalyzeed by Co-PNP coomplex. H N

R

N H

P iPr2

CHO

NH3+ HCOO O-

P Co iPr2 Cl Cl 1

1-Cl

Cl

N P iPr2

R

Co

PiPr2

H2 PiP Pr2

A

NH2 H N P iPr2

H N

NaHBEt 3

tBuOK

- H2O R

PiPr2

Co

Co

H N PiPr2

P iPr2

O OCHO

C

PiPr2

Co

B

H

NaHBEt 3

CO2

H N P iPr2

Co Cl

PiPr2

1-Cl

c the ssynthesis of a widde range of N-forrmamides catalyzzed In conclusion, by a base metal com mplex in good to excellent yields using both prim mary mines (for the firsst time) and secoondary amines unnder relatively m mild am

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CO2 and H2 pressure (30 bar each) was accomplished. The reaction is catalyzed by a complex of low-toxicity, earth-abundant cobalt. The active Co(I) species is generated using catalytic NaHBEt3 and tBuOK. A plausible mechanism is proposed. We believe that this environmentally benign protocol is attractive, considering the wide scope, excellent yields, earth-abundant metal, and H2O as the sole by-product.

ASSOCIATED CONTENT   Experimental procedure, crystallographic data of 1 and 1-Cl, GC-MS, NMR spectra of products “This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION  Corresponding Author  *E-mail: [email protected].

Author Contributions  ‡These authors contributed equally.

Notes  The authors declare no competing financial interest.

ACKNOWLEDGMENT   This research was supported by the Israel Science Foundation and by the Kimmel Center for Molecular Design. D.M. holds the Israel Matz Professorial Chair. P.D. is thankful to the Planning and Budgeting Committee (PBC) for a fellowship. S.C. thanks the Swiss Friends of the Weizmann Institute of Science for a generous postdoctoral fellowship.

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