Selective Single-Step Oxidation of Amine to Cross-Azo Compounds

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Selective Single-Step Oxidation of Amine to Cross-Azo Compounds with an Unhampered Primary Benzyl Alcohol Functionality Sayan Sarkar, Piyali Sarkar, and Pradyut Ghosh* School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Kolkata 700032, India

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S Supporting Information *

ABSTRACT: This is the first report of a single-step synthesis of primary benzyl alcohol containing different cross-azo compounds (14 examples) by Cu(II) in the presence of a newly synthesized amino-ether heteroditopic macrobicycle cage. Interestingly, even with extreme conditions, the benzyl alcohol remains unoxidized by the Cu(II) catalyst due to the protective etherial pocket of the cage.

T

macrobicyclic cage as a host that is suitable for forming a Cu(II) complex as well as cascade complexation toward the syntheses of a series of cross-azo compounds containing BOH functionality (Scheme 1b). To catalyze a selective group in the presence of different labile groups, an appropriately designed structured framework could be very useful as a selective host for a specific group(s), which eventually determines the reactivity of a particular group toward catalysis. Thus, it may be articulated that employment of a cagetype 3D moiety, having its well-defined functional groups, can surround a guest with a greater proximity and greater binding capability.9 Such a concept has indeed been utilized previously by different groups toward catalysis.10 Also, the capability of the cage toward protection of buried functions provides a general approach to selective reactions.11 Keeping all these facts in mind here, we have designed a new oxy-ether tris-amino heteroditopic macrobicyclic cryptand (L) that consists of (i) tris-amine functionality as the metal-chelating site, (ii) three parallel benzene moieties for aromatic π−π stacking interactions for the incoming aromatic guest molecules, and (iii) an oxy-ether pocket for hydrogen bonding interactions. In fact, the tetradentate metal chelating site (N4 site) of L can easily engulf a transition metal ion such as Cu(II),12 which is prone to forming a trigonal bipyramidal (TBP) structure; thus, this can further allow coordination of another monodentate guest molecule.13 Here, in our present cage design, an aromatic amine guest can be chosen to occupy the vacant coordination site of the metal center in the Cu(II) complex of L. On the other hand, the oxy-ether pocket can act as a well-defined 3D cavity which can also participate in hydrogen bonding interaction with the BOH group of the substrate.14 The L is synthesized in five steps starting from oxy-ether (1) formation by reacting 4-hydroxybenzaldehyde with 2-(2chloroethoxy)ethanol followed by stepwise modifications of

he development of synthetic strategies for selective oxidation of a particular group among multiple oxidizable functionalities is always a challenging task. In this direction, syntheses of primary benzyl alcohol containing cross-azo compounds are important in biology and chemistry.1−5 For example, azo functionality itself plays a very important role in photosensitive biological systems due to their cis−trans isomerization,1 and the hydroxymethyl group incorporation eases the progress of azo compounds toward the formation of self-assembled monolayers,2 chain-shattered or end-capped for self-immolative polymers,3 copolymers,4 dendritic organogels,5 etc. In the literature, many of the synthetic strategies for benzylic−OH (BOH) containing azo-benzene (BOCAB) formations involve oxidation of one amine to a nitroso group followed by the condensation with another amine moiety (Scheme 1a).1h,6 Generally, direct single-step syntheses of these Scheme 1. Comparative BOCAB Synthesis

compounds could not be achieved, as most of the oxidative catalysts (metal as well as organic catalysts) responsible for N N bond formation from amines7 also oxidize the benzyl alcohols either to aldehydes or to acids.8 To the best of our knowledge, selective single-step oxidation of amine to azo compound, keeping the primary BOH substitution intact, has not been reported to date. To achieve such partial oxidation by the Cu(II) catalyst here, we introduce a new amino-ether heteroditopic © XXXX American Chemical Society

Received: September 5, 2018

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DOI: 10.1021/acs.orglett.8b02829 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditiona

the leaving group to achieve 3 via 2. Then, the reaction between 3 and phloroglucinol provides the benzene platform-based trialdehyde 4. In the final step, L is prepared in good yield (65%) by reacting 4 with tris(2-aminoethyl)amine in dichloromethane−methanol solvent system via a high-dilution technique followed by NaBH4 reduction (Scheme 2a). Characterization of L is carried out by 1H NMR, 13C NMR, DOSY, COSY NMR, and HRMS (Figures 9S−12S). The ROESY spectrum (Figure 11S) justifies the 3D spatial arrangement of L.

entry

catalyst

solvent

temp (°C)

time (h)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12

CuSO4 Cu(NO3)2 Cu(OTf)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 Cu(ClO4)2 L-Cu(II) cryptate

DMF DMF DMF DMF DMSO CH3CN C2H5OH DMF DMF DMF DMF DMF

100 100 100 80 80 80 80 70 80 90 100 80

24 12 12 2.5 8 12 12 24 8 12 24 2.5

45 59 64 73 61 58 34 66 72 70 70 74

Scheme 2. Synthesis of New Heteroditopic Macrobicycle L

a

Reaction was performed with 4-aminobenzyl alcohol 5a (1.0 mmol), p-toluidine 6a (1.0 mmol), catalyst (10 mol %), and macrobicycle L (10 mol %) in 2 mL of solvent under aerobic conditions. bIsolated yields.

To ascertain the loading of the Cu(II)−L mixture in the reaction of 5a and 6a, various amounts of a 1:1 mixture of Cu(II) and L were attempted (Figure 14S). An effective percentage of the mixture shows a linear increase in yield up to 10 mol %. Further increase in the loading does not show any improvement in the yield. Employment of the acyclic analogue, L′ (condensation product of 1 and tris(2-aminoethyl)amine, Scheme 2b), in place of L under the optimized reaction condition does not produce any trace of 7a. This was noticed from TLC and more importantly from the 1H NMR of the crude, which shows no characteristic peak of the benzylic proton of 7a (Figure 45S). It is probably due to unbound orientations of the arms which are incapable of providing protection to the benzylic−OH group. Also, when Cu(II) salt alone is used as a catalyst (Scheme 3a), cross-coupled product with oxidation of the BOH group is obtained as the minor product (∼10%) along with predominant homocoupled products (overall ∼77%) obtained from HPLC

To establish the role of L in the oxidative coupling reaction, we have chosen 4-aminobenzyl alcohol (5a) and p-toluidine (6a) as model substrates. We have utilized L as a host having a defined cavity as well as functionalities to perform the reaction of 5a and 6a in the presence of Cu(II) catalyst. Interestingly, in this case, cross-coupled product with an unperturbed BOH group is obtained as the major product (60−73% yields, depending on the counteranions). Thus, this approach could activate the amine group by keeping the BOH group intact, which eventually results in the synthesis of the product BOCAB, 7a, in high yield, even in different experimental conditions (Table 1). The catalytic activity of various Cu(II) salts such as CuSO4, Cu(OTf)2, Cu(NO3)2, and Cu(ClO4)2 was screened with different time and temperature (Table 1). Due to relatively noninteracting nature of the counteranions for Cu(OTf)2 and Cu(ClO4)2, better throughputs of yields (60−70%) have been achieved (Table 1, entries 3 and 4). When different solvent systems (Table 1, entries 4−7) are explored, DMF provides the highest yield (73%). Interestingly, even in extreme conditions, such as at higher temperature and with greater reaction time (Table 1, entries 9−11), benzylic alcohol functionality remains intact with comparable yields. The catalytic activity of the isolated L− Cu(II) cryptate is also investigated, but no mentionable improvement in the yield is observed (Table 1, entry 12), indicating that the in situ formed Cu(II) cryptate is equally efficient to catalyze the above reaction. Due to the presence of a single metal binding site (N4 cavity) in L, an equimolar mixture (1:1) of Cu(II) and L is used as a catalyst in the above studies.

Scheme 3. Oxidative Coupling Using (a) Cu(II) Salt as Catalyst and (b) Cu(II) Salt in the Presence of L as Catalyst

B

DOI: 10.1021/acs.orglett.8b02829 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 4. List of Synthesized Compoundsa,b

analysis. These results eventually prove that the cage moiety of L is essential for this selective oxidation. The beauty of the reaction here is the formation of the crossazo compound as the major product over the corresponding homo products, which is established by HPLC data of the crude product for the reaction of 5a and 6a, using hexane as eluent (Figure 1). The second peak from the left corresponds to the

Figure 1. HPLC data of the crude product for the reaction of 5a and 6a.

hetero product, and other two peaks represent the homo products, which indicates that the formation of a cross-coupled product highly outweighs the homo products in the presence of L. At the optimized reaction conditions, several aromatic amines and aminobenzyl alcohols have been explored to produce 14 various unsymmetrical azo derivatives. Each of those products is characterized by 1H NMR, 13C NMR, IR, and HRMS spectroscopy (Figures 15S−42S), which clearly shows the presence of the BOH functionality. Scheme 4 clearly demonstrates that this reaction method is favorable for a library of aniline systems containing different substituents. Electron-donating substituents in the nucleophilic amine part advance yields of desired products (7a, 7b, 7j, 7l, and 7m), whereas, for electron-withdrawing groups, the yields are reduced diminutively (7c−7i, 7k, and 7n). Diversely substituted aminobenzyl alcohols also provide excellent yields, which demonstrate the stability of meta-BOH and substituted BOH in the cavity of L. Interestingly, we have successfully crystallized compounds 7e and 7n. The single-crystal X-ray structures of these two compounds (Figure 43S) confirm the presence of the unhampered BOH moiety in this class of molecules. Here, we propose a plausible mechanism for this oxidative coupling, taking 4-aminobenzyl alcohol (5a) and p-toluidine (6a) as model substrates (Scheme 5). When L and Cu(ClO4)2 are mixed in DMF, the UV−vis data recorded after 5 min of mixing show a d−d band at 560 nm (Figure 2a, spectrum 1), which indicates the formation of a L−Cu(II) complex. A characteristic peak of [L−Cu(II) + ClO4−]+ at m/z 962.31 (Figure 2b) in the ESI-MS study confirms the above complex formation. Upon addition of 4-aminobenzyl alcohol (PABA) and p-toluidine into L−Cu(II), the UV−vis spectrum (taken after 10 min, Figure 2a, spectrum 4) of the reaction mixture shows a new peak at around λmax 438 nm along with the peak at 560 nm. Comparison of this spectrum with a 1:1:1 mixture of (i)

a

All the reactions were carried out using amines (1 mmol each), Cu(ClO4)2 (10 mol %), and L (10 mol %) in DMF (2.0 mL) at 80 °C for 2−3 h under aerobic conditions. bIsolated yields.

L/Cu(ClO4)2/PABA (λmax ∼ 440 nm, Figure 2a, spectrum 2) and (ii) L/Cu(ClO4)2/p-toluidine (λmax ∼ 412 nm, Figure 2a, spectrum 3) reveals that the λmax of L/Cu(ClO4)2/PABA matches well with the spectrum of the L/Cu(ClO4)2/PABA/ptoluidine mixture. This indicates the formation of a new fivecoordinated Cu(II) complex13 preferably with PABA in the reaction mixture. The preference of PABA over the p-toluidine could be due to possible hydrogen bonding interactions between the BOH group and the oxy-ether pocket of L. Further, the ESIMS spectrum of this reaction mixture shows a peak at m/z 1085.4, which strongly supports this 1:1:1 ternary complex [II] formation (Figure 2c). After the ternary complex formation, one e− transfer takes place from the amine of PABA to the Cu(II) center that produces a radical cation [III],7b,c,g,i,j and subsequently Cu(II) is converted to Cu(I). Then Cu(I) oxidizes to Cu(II) by donating an electron to aerial O2 to form superoxide radical (O2•−). Simultaneously, nucleophilic attack of p-toluidine to the radical cation [III] generates 3e− σ bond species [IV].15 Then 1e− transfer from 3e− σ bond to O2•− generates a peroxide anion (O22−) which takes up two protons from species [IV] that produces the hydrazine derivative [V] along with H2O2. Finally, hydrazine [V] is oxidized via similar two-electron oxidation procedure by aerial oxygen,7b,i,j which C

DOI: 10.1021/acs.orglett.8b02829 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

In conclusion, we have developed a new oxidative coupling route for the synthesis of the unaffected primary benzylic alcohol group containing cross-azo compounds from aromatic amines, by using Cu(ClO4)2 in the presence of a well-designed cryptand having both an oxy-ether moiety and a metal chelating site as the catalyst. The selectivity of the particular oxidation of the amine part is imparted by the oxy-ether moiety of macrobicyclic cryptand through H-bonding with benzylic alcohol. Moreover, the cross-selectivity is also maintained via the same macrobicycle. Formation of ternary copper complex tuned the formation of unsymmetric azo compounds with reliable yields. This novel approach of regulation of selectivity using a macrobicycle moiety may improve modern synthetic chemistry in the near future.

Scheme 5. Proposed Mechanism for Single-Step Catalytic Aerial Oxidation

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02829. Experimental procedures and spectroscopic data (PDF) Accession Codes

CCDC 1860527−1860528 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pradyut Ghosh: 0000-0002-5503-6428 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, for his fellowship (SRF). P.S. thanks the SERB project PDF/2016/001529 for funding. The authors acknowledge Somnath Bej for his suggestion during the initial stage of work. The authors also thank IACS for the Instrumentation Facility.

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Figure 2. (a) UV−vis spectra of L−Cu(ClO4)2 (pink); 1:1:1 mixture of L, Cu(ClO4)2, and 4-aminobenzyl alcohol (PABA) (brown); 1:1:1 mixture of L, Cu(ClO4)2, and p-toluidine (green); 1:1 mixture of PABA and Cu(ClO4)2 (orange); PABA (blue) at 1 × 10−3 M each in DMF solvent and crude reaction mixture (violet) after 10 min. (b) ESI-MS of L−CuII(ClO4)2. The inset picture depicts the enlarged portion of the region for the corresponding monopositive ion (blue) and its isotopic distribution pattern (black). (c) ESI-MS of the 1:1:1 ternary complex, [L−Cu(II) + PABA + ClO4−]+. (d) ESI-MS of the crude reaction mixture after 30 min of advancement of the reaction.

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DOI: 10.1021/acs.orglett.8b02829 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.8b02829 Org. Lett. XXXX, XXX, XXX−XXX