Molybdenum hexacarbonyl: air stable catalyst for microwave assisted intermolecular [2+2+2] co-trimerization involving propargyl halides

Tetrahedron Letters xxx (2015) xxx–xxx

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Molybdenum hexacarbonyl: air stable catalyst for microwave assisted intermolecular [2+2+2] co-trimerization involving propargyl halides Sambasivarao Kotha ⇑, Gaddamedi Sreevani Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India

a r t i c l e

i n f o

Article history: Received 10 July 2015 Revised 7 September 2015 Accepted 8 September 2015 Available online xxxx

a b s t r a c t Intermolecular [2+2+2] cycloaddition of propargyl halides with 1,6-diynes was achieved with air stable, catalytic amount of Mo(CO)6 and the benzyl halide derivatives were isolated in good yields. The reaction conditions were optimized and better yields were observed under microwave irradiation conditions using acetonitrile as a solvent. Ó 2015 Elsevier Ltd. All rights reserved.

We would like to dedicate this paper to Prof. D. Basavaiah. Keywords: [2+2+2] co-trimerization Propargyl halides Mo(CO)6 Microwave irradiation Benzyl halides 1,6-Diynes

Transition-metal complexes have emerged as indispensible tools in synthetic organic chemistry where a large number of functional groups are tolerated. These compounds enable either difficult or unusual chemical transformations which are not possible by traditional methods. Cycloaddition reactions are powerful tools in synthetic organic chemistry because these reactions allow multiple bond formation exhibiting a high degree of selectivity. In this context, [2+2+2] cycloaddition is found to be an efficient protocol for assembling aromatic compounds which can act as functional materials. This protocol is an atom economic process leading to the formation of unsaturated six-membered, highly substituted carbo- and heterocycles such as benzenes, pyridines, pyridones, and 1,3 cyclohexadienes etc., in a single operation involving catalytic amounts of organometallic complexes of around 15 different transition metals such as Rh,1 Ir,2 Ni,3 Ru,4 Co,5 Pd,6 Nb,7 Fe8 etc., (Fig. 1).9 Reppe pioneered the transition metal-catalyzed trimerization reaction of acetylene in 1948,10 and later the [2+2+2] cycloaddition reaction has expanded in several directions and found diverse applications in organic synthesis.11,12 Now, these protocols reached to a high degree of sophistication where several complex natural prod⇑ Corresponding author. Tel.: +91 22 2576 7160; fax: +91 22 2572 7152. E-mail address: [email protected] (S. Kotha).

ucts and unnatural compounds have been synthesized easily by utilizing the [2+2+2] cycloaddition reaction as a key step.13–16 Although many reports are available related to the [2+2+2] cycloaddition reactions, only a limited number of examples are available where propargyl halides17 are used as co-trimerization partners. Synthesis of the dibromo compounds such as 9 from 1,6-diynes has been demonstrated by the [2+2+2] cycloaddition reaction with the diol 7a in the presence of Wilkinson’s catalyst followed by bromination with PBr3 (Scheme 1).18 This two step strategy could not be extended to sensitive substrates (e.g., peptides, ether linkage containing compounds) and the starting materials/products decompose during the bromination sequence. Hence, there is a pressing need to develop a new strategy where the halo derivatives such as 9 are assembled directly via a [2+2+2] cycloaddition reaction as a key step without the involvement of the bromination step (Scheme 1). For this purpose, one of the alkyne partners must be a propargyl halide, however, the products formed will be the benzyl halides, which are good substrates in cross-coupling reactions. Moreover, realization of [2+2+2] sequence without the involvement of the diol intermediate amounts to the ‘step economy’.19 In this context, Kudinov and co-workers attempted a [2+2+2] co-trimerization of 1,6diynes with various alkyne partners in the presence of naphthalene cyclopentadienyl ruthenium complex and they were unsuccessful

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Scheme 2. [2+2+2] co-trimerization with propargyl bromide.

Figure 1. General representation of a [2+2+2] cycloaddition reaction.

when the alkyne partner was propargyl bromide 7c.20a A perusal of the literature indicated that Fujihara and co-workers have reported the synthesis and the catalytic activity of dinuclear niobium(III) complexes for the regioselective cyclotrimerization of alkynes. Here, they reported the synthesis of hexakis(chloromethyl)benzene by the [2+2+2] cyclotrimerization of 1,4-dichloro-2-butyne 7f in the presence of a catalytic amount of dinuclear niobium(III) complex. This system tolerates the benzylic halides as the end products.17a However, niobium(V) chloride is air as well as moisture sensitive and hence its handling is not a trivial exercise. Sugihara and coworkers observed that a [2+2+2] co-trimerization is feasible by using Mo(CO)6 and Cr(CO)6 complexes than that of the corresponding isonitrile complexes.20b Mori and co-workers have also showed that the alkyne having an o-hydroxyphenyl group gave the trimerization product, rather than alkyne metathetic product under Mortreux catalytic conditions.20c To expand the utility of [2+2+2] cycloaddition strategy in organic synthesis, we examined the catalytic activity of Mo complexes in [2+2+2] cycloaddition reaction21 with propargyl halides. In this context we chose dipropargylated 1,3-indane dione 11 as a model substrate. The commercially available 1,3-indane dione 10 was treated with propargyl bromide in the presence of K2CO3 as a base under PTC conditions in MeCN at rt to generate the dipropargylated 1,3-indane dione 11.12c,f Later, the diyne 11 was subjected to a [2+2+2] cycloaddition sequence with propargyl bromide 7c in the presence of catalytic amount of Mo(CO)6 under THF refluxing conditions for 10 h. The desired [2+2+2] cycloaddition product 12c was obtained (34%) along with self dimerized product 14 (5%). We also observed the formation of a, b-unsaturated aldehyde 13 (10%) which was confirmed by 1H, 13 C, DEPT-135 NMR, and mass spectrometric analysis (Scheme 2). Since Mo(CO)6 is not a typical catalyst for [2+2+2] cycloaddition reactions as compared to other metal complexes, the mechanism might involve molybdenapentadiene, which would react with the alkyne partner to produce cyclotrimerized product.20c,21d When the propargyl alcohol or propargyl tosylate was subjected to a [2+2+2] co-trimerization with the diyne 11 in the presence of

Scheme 1. General strategy for the synthesis of dibromo derivative via [2+2+2] cycloaddition.

Mo(CO)6, they were unreactive and the starting material was recovered. However, when we used propargyl halides as co-trimerization partners, the yield was low. To improve the yields of cotrimerization products and to minimize the side products, the reaction was carried out under different conditions (Fig. 2, Table 1). Based on Sugihara and co-workers report,20b when we used Cr (CO)6 catalyst for [2+2+2] cyclotrimerization, the reaction did not happen and the starting material was recovered. In view of the literature report,20c later we used the Mortreux catalytic system by adding phenol or p-Cl-phenol as an additive, however, no improvement in the yield of the products. We thought changing the ligands may improve the yields, hence, we prepared Mo(PPh3)2CO4 and Mo (Py)3(CO)3 as reported in the literature22 and carried out the [2+2 +2] co-trimerization with propargyl bromide. When the catalyst was Mo(Py)3(CO)3, the reaction did not proceed at all and the starting material was recovered. When the catalyst was Mo(PPh3)2CO4, the reaction was incomplete even after 10 h of reflux conditions. At this point, we thought changing the solvent system may improve the yields. Hence, we changed the solvent from THF to toluene and later to dioxane, but the results were not encouraging (Table 1). Recently, Roglans and co-workers reported the Rh catalyzed fully intramolecular [2+2+2] cyclotrimerization of N-tosyl-, carbon-, and oxygen-tethered cyanodiynes to afford highly functionalized pyridines and bipyridines. They claimed that the solvent and the mode of thermal activation play a dramatic role on the outcome of the reaction. Surprisingly, the self dimerized product was observed under conventional heating conditions; whereas, microwave (MW) irradiation conditions gave the co-trimerized product.23a Hence, we performed the co-trimerization of the diyne 11 with propargyl bromide 7c under MW conditions in dry THF at 75 °C, but we didn’t observe much difference in the yield. However, the reaction was completed within 2.5 h instead of 10 h. Furthermore, when we changed the solvent from THF to 1,4-dioxane under MW conditions at 110 °C, the reaction was completed within 15 min and the yield was also improved to 45%. We felt this may be the suitable conditions, hence the 1,4-dibromo-2-butyne 7b was reacted with the diyne 11 under similar reaction conditions and found that the dibromo derivative 12b was formed in 11% yield. As chlorides are less reactive than bromides and we anticipated improved yields. Hence, we performed the [2+2+2] co-trimerization reaction of diyne 11 with propargyl chloride 7e and 1,4dichloro-2-butyne 7f in 1,4-dioxane under MW conditions and the isolated yields of co-trimerized products were found to be 42% (12e) and 13% (12f), respectively (Scheme 3).

Figure 2. List of alkyne partners studied in [2+2+2] cycloaddition reaction.

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S. Kotha, G. Sreevani / Tetrahedron Letters xxx (2015) xxx–xxx Table 1 Reaction conditions under conventional heating

Entry b

1 2 3 4 5 6 7c 8c 9 10 11d 12 13 a b c d e

Alkyne

Catalyst (mol %)

Solvent

Time [h]

% yielda

7c 7c 7b 7d 7g 7c 7c 7c 7c 7c 7c 7c 7c

Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Mo(CO)6 Cr(CO)6 Mo(CO)6/PhOH Mo(CO)6/p-ClPhOH Mo(PPh3)2(CO)6 Mo(Py)3(CO)3 Mo(CO)6 Mo(CO)6 Mo(CO)6

THF THF THF THF THF THF THF THF THF THF Toluene Toluene Dioxane

10 12 10 15 15 10 12 12 10 10 20 13 06

34:10:5 30:12:6 21:10:8 N.R. N.R. N.R. 31:10:5 31:9:6 Incompletee N.R. 28:10:5 31:10:6 31:11:5

Yields calculated after column chromatography. 10 mol % catalyst was used; Propargyl bromide:PhOH/p-ClPhOH = 1:6. Reaction was performed at 80 °C. The reaction did not proceeded 100%; N.R. = no reaction.

Scheme 3. [2+2+2] co-trimerization with propargyl halides under MW conditions.

At this stage perusal of literature indicated that the better MW absorbing entity in the reaction mixture will improve the yields of the products. Hence we added chlorobenzene23a as an additive and there was no improvement in the yield. Hence, we attempted various other solvents such as acetonitrile, DMF, and ethanol which are known to be good MW absorbing solvents. When acetonitrile was used as a solvent under MW irradiation conditions at 90 °C the reaction was completed within 10 min and the yield was improved to 75%. The side product a,b-unsaturated aldehyde 13 was not observed, however, we couldn’t avoid the formation of self-dimerized compound 14 (7%). When the solvent was DMF or ethanol the reaction did not proceed at all (Table 2).

Table 2 Reaction conditions under MW heating

a b c d

Entry

Alkyne

Solvent

T [°C]

Time

% yielda (12:13:14)

1 2 3 4 5 6b 7 8 9 10 11 12b 13c

7c 7c 7b 7e 7f 7c 7c 7c 7c 7c 7c 7c 7c

THF Dioxane Dioxane Dioxane Dioxane Dioxane Toluene/chlorobenzene (3:1) Dioxane/chlorobenzene (3:1) MeCN DMF EtOH MeCN MeCN

75 110 110 110 110 110 90 110 90 90 85 90 90

2.5 h 15 min 15 min 15 min 15 min 30 min 1h 15 min 10 min 10 min 10 min 30 min 30 min

33:11:5 45:10:4 11:12:5 42:10:4 21:9:4 N.R. 25.5:11:6 26:10:6 75:0:7 N.R. N.R. N.R. Incompleted

Yields calculated after column chromatography. The catalyst used was Mo(Py)3(CO)3. Mo(PPh3)2(CO)6. The reaction did not proceed 100%; N.R. = no reaction.

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S. Kotha, G. Sreevani / Tetrahedron Letters xxx (2015) xxx–xxx

Table 3 [2+2+2] co-trimerization of different active methylene compounds (AMCs) under MW conditions with different propargyl halides Entry

AMC

Propargyl halide

Product

Yielda (%)

1

72

2

80

3

79

4

71

5

69

6

77

7

72

8

71

9

70

10

75

11

73

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S. Kotha, G. Sreevani / Tetrahedron Letters xxx (2015) xxx–xxx Table 3 (continued) Entry

a

AMC

Propargyl halide

Product

Yielda (%)

12

61

13

70

Yields calculated after column chromatography.

Science and Technology-New Delhi for the award of J. C. Bose fellowship. We also thank Dr. S. Chattopadhyay (BARC) for his valuable suggestions. Supplementary data

Figure 3. Representative self-dimerized products formed in our study corresponding to AMCs 15 and 17.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.09. 029. References and notes

We are pleased to note that the best yields were observed under MW conditions with acetonitrile as a solvent (Table 2, entry 9). There might be two reasons for the improvement in the yields of the products under these conditions: (i) may be due to the in situ formation of air sensitive active catalyst (CH3CN)3Mo (CO)3, when Mo(CO)6 was heated with acetonitrile and which undergoes rapid substitution with acetylenes to afford [2+2+2] cycloaddition,24 (ii) the dielectric properties of acetonitrile,25 which aids for the better absorption of MW radiations for rapid conversion of reaction and reduces the reaction time. Therefore, we performed the [2+2+2] co-trimerization with different propargyl halides, with different active methylene compounds (AMCs) under similar reaction conditions and the corresponding benzyl halide derivatives were isolated in good yields (Table 3), along with the respective self-dimerized products (21, 22) (5–8% yields). In the case of dipropargylated 6-methoxy a-tetralone we found a mixture of two diastereomers which were not separable 21a and 21b (Fig. 3). We have demonstrated a simple and atom economic route for the synthesis of benzyl halide derivatives in a single step. Improved yields of co-trimerization products under MW irradiation conditions was observed in contrast to the conventional heating conditions, and the acetonitrile was found to be a better choice of solvent for improving the yields of co-trimerization products under MW conditions. The scope and expansion of this strategy are still under investigation. Acknowledgments We thank the DST-New Delhi for their financial support for our research programs and G. S. thanks the CSIR-New Delhi for the award of research fellowship. S. K. thanks the Department of

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Please cite this article in press as: Kotha, S.; Sreevani, G. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.09.029