Introduction Signpost Open Access Journal of Organic ...

Bubun Banerjee (2015) Signpost Open Access J. Org. Biomol. Chem., 3, 80 - 86. Volume 03, Article ID 010321, 07 pages. ISSN: 2321- 4163 http://signpostejournals.com

Signpost Open Access Journal of Organic and Biomolecular Chemistry

Zn[(L)-proline]2 Compiled by Bubun Banerjee

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Bubun Banerjee was born in Mamudpur, Burdwan (West Bengal), India in 1987. He received B.Sc. (Honours) degree in Chemistry from St. Xavier’s College, Kolkata (University of Calcutta) in 2008 and M.Sc. (Organic specialization) from the University of Delhi in 2010. After qualifying CSIR-UGC NET (2010), he joined the Ph.D. programme (Synthetic Organic Chemistry) as Junior Research Fellow under the supervision of Prof. (Dr.) Goutam Brahmachari at the Chemistry Department, (This feature focuses on any Visva-Bharati (a Central University), Santiniketan (West important chemistry and/or Bengal), India, and presently he is working as Senior Research chemical biology related Fellow. His research interest focuses on the development of topic of current research novel synthetic methodologies for heterocyclic compounds interests, compiled with special emphasis on green chemistry.

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exclusively by a postgraduate research scholar)

C/O Prof. (Dr.) Goutam Brahmachari, Laboratory of Natural products & Organic Synthesis, Department of Chemistry, VisvaBharati (a Central University), Santiniketan-731-235, West Bengal, India. E-mail: [email protected] In memory of Bandyopadhyay

my

heavenly

grandfather,

Amritapada

Received: June 5, 2015

Abstract The present feature outlines the preparation and synthetic applications of Zn[(L)-proline]2 as an eco-friendly, reusable, acidic catalyst.

Keywords: Zn[(L)-proline]2; Preparation; Synthetic utility elements needed for human metabolism. A large number of specific enzymes require metallic zinc for their catalytic function. On the other hand, proline is an important amino acid. It can coordinate with Zn2+ to make Zn[(L)-proline]2 through its secondary amino group and carboxylate functionality simultaneously in low coordination number (Figure 1). Zn[(L)-proline]2

Introduction The design and synthesis of organometallic complexes is an important field of research due to its various advantages [1]. Among the transition metals used for the preparation of organometallic complexes, zinc is of choice because it is the most ubiquitous of all trace 80


can catalyze various organic reactions based on its Lewis acidic properties. Zn[(L)Proline]2 is an efficient, stable, environmentally benign, recyclable, watertolerable Lewis acidic catalyst which is soluble in water but least soluble in organic solvents that helps

the easy and quantitative recovery of the catalyst. This spotlight highlights the preparation and the use of Zn[(L)proline]2 in organic reactions as a reusable Lewis acid catalyst.

Figure 1: Catalyst Zn[(L)-proline]2

appeared which was then collected by filtration and dried at 70 C in vacuum for 6 h (23–95% yield). Zn[(L)Proline]2 is stable, inexpensive and water-soluble Lewis acid catalyst which does not dissociate under reaction conditions. It is insoluble in most of the organic solvents, which helps simple and quantitative recovery of the catalyst from the reactions carried out in organic solvents.

Preparation of Zn[(L)-proline]2 Zn[(L)-proline]2 complex was first reported by Darbre et al. in 2003 [2]. It was prepared by adding Et3N (0.6 ml) to L-proline (4.34 mmol) in methanol (10 ml), followed by addition of zinc acetate (2.17 mmol) after 10 min. The contents were vigorously stirred at room temperature. After 45 min stirring, a white precipitate

1. Darbre et al. [2] first synthesized Zn[(L)-proline]2 and demonstrated its catalytic applicability to the asymmetric aldol addition of acetone (1) and pnitrobenzaldehyde (2) to form the corresponding aldol-product (3) in high yield with enantiomeric excesses up to 56% in aqueous medium at room temperature (Scheme 1).

Synthetic applications Zn[(L)-proline]2 is a reusable, nontoxic water-soluble Lewis acid catalyst with enhanced binding ability towards guest molecules relative to native L-proline. Synthetic uses of such catalyst are documented below:

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Scheme 1: Direct aldol reaction in aqueous media [2] 2. A mild and efficient protocol was reported by Manvar et al. [3] for the synthesis of 3-methyl-1substituted phenyl-1H-chromeno[4,3-c]pyrazol-4ones (6) by the reaction of 3-acetyl-4-

hydroxycoumarin (4) and substituted phenylhydrazines (5) using Zn[(L)-proline]2 as a reusable catalyst under the influence of microwave irradiation (Scheme 2).

Scheme 2: Synthesis of chromeno[4,3-c]pyrazol-4-ones [3] 3. Kidwai and his group [4] demonstrated an efficient, ‘on-water’ protocol for the synthesis of pyrazole derivatives (9) with excellent yield by the reaction of various 1,3-dicarbonyl compounds (7) and different substituted hydrazines (5) using Zn[(L)-proline]2 as a

recyclable catalyst at room temperature. To explore the further scope of this catalyst they also performed the reaction of different dicarbonyl compounds with o-phenylenediamines (8) to form benzo[b][1,4]diazepine derivatives (10) (Scheme 3).

Scheme 3: On-water synthesis of pyrazoles [4] 4. Siddiqui and his group developed a novel approach [5] for the ‘on-water’ synthesis of a series of chromonyl chalcones (13) from the reaction of 3formylchromones (11) and different cyclic active

methyl compounds (12) using Zn[(L)-proline]2 as a recyclable Lewis acid catalyst under reflux condition (Scheme 4).

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Scheme 4: Synthesis of chromonyl chalcones [5] 5. Siddiqui et al. [6] demonstrated another application of Zn[(L)-proline]2 as a reusable catalyst for the synthesis of a series of Knoevenagel condensation products (17) from 5-chloro-3-methyl-1phenylpyrazole-4-carboxaldehyde (14) with different cyclic active methylene compounds (15) in water

under reflux condition. They also observed that 4hydroxycoumarin (16a) and 4-hydroxy-6-methyl pyron (16b) with the reaction of aldehydes (19) under the same reaction condition gave the pyrazolo-pyrone derivatives (18) instead of the expected Knoevenagel condensation products (Scheme 5).

Scheme 5: Zn[(L)-proline]2 catalyzed Knoevenagel condensation [6] 6. A simple and efficient route was adopted by Kidwai et al. [7] for the synthesis of xanthenediones (21) by the condensation of aldehydes (19) with dimedone

(20) employing catalytic amount of recyclable Zn[(L)proline]2 in water (Scheme 6).

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Scheme 6: Synthesis of xanthenediones [7] 7. Heravi et al. [8] demonstrated a simple and straightforward protocol for the synthesis of quinoxalines (23) from various 1,2-diketones (22) and

1,2-diamines (8) employing catalytic amount of inexpensive and reusable Zn[(L)-proline]2 in acetic acid medium at room temperature (Scheme 7).

Scheme 7: Synthesis of quinoxalines [8] 8. A facile and efficient protocol was developed by Siddiqui et al. [9] for the synthesis of dicoumarols (24) from the reaction of 4-hydroxycoumarin (16a) and

aromatic/heteroaromatic aldehydes (19) using catalytic amount of Zn[(L)-proline]2 in water under reflux condition (Scheme 8).

Scheme 8: Synthesis of dicoumarols [9]

9. Montazeri et al. [10] reported a facile, efficient, high-yielding, straightforward method for the synthesis of tetrahydrobenzo[b]pyrans (26) via a onepot three-component reaction of dimedone (20), aryl

aldehydes (19) and malononitrile (25) employing Zn[(L)-proline]2 as a non-toxic, reusable Lewis acidic catalyst at room temperature under grinding condition (Scheme 9). 84


Scheme 9: Synthesis of tetrahydrobenzo[b]pyrans [10] 10. Ravi et al. [11] developed a facile and selective method for the ‘on-water’ synthesis of 1,2disubstituted benzimidazoles (27) from various

substituted o-phenylenediamines (8) and aldehydes (19) using Zn[(L)-proline]2 as a recyclable catalyst at room temperature (Scheme 10).

Scheme 10: Synthesis of tetrahydrobenzo[b]pyrans [11]

Acknowledgement BB is grateful to his honorable mentor, Prof. (Dr.) Goutam Brahmachari, for his constant encouragement and supports.

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Montazeri N., Pourshamsian K., Keshavarzi S., and Rezaei B. M. (2012) Asian J. Chem. 24, 4791-4794.

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