The highly enantioselective addition of acetone to 2-nitrostyrene, using N–diphenylphosphinyl-trans-1,2-diphenylethane-1,2-diamine (PODPEN) as catalyst, is described

A. Simon Partridge

Bernardi

Burns

Charles V. Manville

Chaun

Connon

Dahlin

Daugidas T. Racys

David J. Morris

Denmark

Eder

Fletcher

García-García

Gary Woodward

Gordon Docherty

Hayashi

Hine

Huang

Kohmura

Laars

Lattanzi

Martin Wills

Massi

Melchiorre

Pansare

Rishel

Seayad

Sulzer-Mosse

Suzuki

Tsandi

Tsogeova

Vijaikumar

Wang

Watson

Zhang

English

Crossref

Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN)

Morris, David J.

Partridge, A. Simon

Manville, Charles

Racys, Daurgidas Tomas

Woodward, G.

Docherty, Gordon

Wills, Martin

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 University of Warwick institutional repository: http://go.warwick.ac.uk/wrap  This paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): David J. Morris, A. Simon Partridge, Charles V. Manville, Daugidas T. Racys, Gary Woodward, Gordon Docherty, Martin Wills Article Title: Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN) Year of publication: 2009 Link to published article: http://dx.doi.org/10.1016/j.tetlet.2009.10.131 Publisher statement: Morris, D. J. et al. (2009). Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN), Tetrahedron Letters, Vol. 51(1), pp. 209-212   Graphical Abstract To create your abstract, type over the instructions in the template box below. Fonts or abstract dimensions should not be changed or altered.Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN) David J. Morris, A. Simon Partridge, Charles V. Manville, Daugidas T. Racys, Gary Woodward, Gordon Docherty and Martin Wills.             NO2 NO2O10-15% ligand 10SacetoneX X3 X=H; up to 97% ee16 X=OMe; 93% ee17 X=Cl; 96% eePhPhNH NH2POPhPh(R,R)-10  Leave this area blank for abstract info.  Tetrahedron Letters  1 Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN) David J. Morris,a A. Simon Partridge,a Charles V. Manville,a Daugidas T. Racys,a Gary Woodward,b Gordon Dochertyb and Martin Wills. a* a Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, UK. b New Products and Phosphine Development, Rhodia Consumer Specialities Ltd, PO Box 80, Trinity Street, Oldbury, W. Midlands, B69 4LN, UK  Abstract— The highly enantioselective addition of acetone to 2-nitrostyrene, using N–diphenylphosphinyl-trans-1,2-diphenylethane-1,2-diamine (PODPEN) as catalyst, is described.  © 2010 Elsevier Science. All rights reserved ——— * Corresponding author. Tel.: +44-24-7652-3260; fax: +44-24-7652 4112; e-mail: m.wills@warwick.ac.uk.  Pergamon TETRAHEDRON LETTERS  Tetrahedron Letters  2 The use of enantiomerically pure amines in organocatalysis has enjoyed a period of rapid recent development.1-3 Several of these reactions involve the formation of an active species (iminium or enamide) by a reversible condensation reaction between the amine catalyst and the carbonyl group in one of the reagents. In a large proportion of examples of amine-catalysed reactions, a second functional group in the catalyst serves to activate and direct the reaction. In the case of proline, the carboxylic acid may act in this capacity.2 The asymmetric addition of acetone to 2-nitrostyrene (Figure 1) may be catalysed by a number of amine derivatives including thiourea derivatives of S,S-1,2-diphenyl-1,2-ethylenediamine (DPEN) and R,R-1,2-diaminocyclohexane (DACH).4 Examples include ureas 1 and 2, which give products 3 in up to 91%4a and 99% ee,4b respectively (Scheme 1). PhPhH2N HNHNSMe(S,S,R)1H2N HNHNStBuONMeBn(R,R,S)2 The mechanism of the amine-catalysed addition reaction is believed to proceed via the enamine, whilst the urea engages the nitro group in a hydrogen bond (Figure 1).1d NO2 NO2O10-15% ligand 1,2, 8-15See Table 1Sacetone, AcOH, H2OX X3 X=H; up to 97% ee16 X=OMe; 93% ee17 X=Cl; 96% ee  Scheme 1. Asymmetric addition of acetone to nitrostyrene.  PhNHNPhNO2HOOPhNHHPh SNHHNPhNHPh SNHRR2 Figure 1. Secondary directing effect of urea. Other derivatives of C2-symmetric diamines have been applied to the nitrostyrene addition reaction. These include the mono N-trifluoromethylsulfonate (Tf) derivative of DACH5 and a series of sulfamides, which promoted the addition of aldehydes in up to 99% ee.6 In addition, closely related pyrrolidine derivatives have been employed,7-16 including the triflate 4, which catalyses additions of aldehydes and ketones in up to 99% ee.7 Good results were also obtained using the 3,5-bis(trifluoromethane)phenylsulfonyl derivative.8 Closely related 2-aminomethylpyrrolidines have also been studied in the nitrostyrene addition reaction (up to 92% ee).9 Proline supported on hydrotalcite clays has been used as a heterogeneous catalyst.10 Diamines derived from cinchona alkaloids have been applied to the addition of 1,3-diketones to nitrostyrenes.11    NHNHTf4NHHNPOPhPhONHPOPhPhNHPOPhPh5 6 7  In a recent report, published during the course of our studies, phosphine oxide 5 was described as an excellent organocatalyst for the asymmetric addition of cyclic ketones to nitrostyrenes, furnishing products in >99% ee and high diastereoselectivity.12 Oxide 5, as well as the related compounds 6,13 and 7,14 have been applied to asymmetric aldol reactions. In previous reports on the use of pyrrolidine-based catalysts, cyclic ketones gave the best results. In contrast, the addition of acetone is less enantioselective.  Given a long standing interest in phosphinamide catalysts,17 we wished to establish whether other functional groups, and particularly those based on P=O directing groups, could be applied to catalytic reactions. Phosphinamides have been used as catalysts in other reactions which may be described as organocatalytic (no metals are present in the catalyst).17,18 These include the catalysis of the asymmetric reduction of ketones using borane, and the addition of trichloroallylsilanes and silyl enol ethers to aldehydes.  Phosphinamide 10 has been reported as a chiral directing group in a Rh-catalysed asymmetric Michael addition reaction.19 Phosphinylated compounds closely related in structure to 10 have been formed in nucleophilic addition reactions to N-diphenylphosphinyl ketimines, but were not used as asymmetric catalysts.20 The attempted use of a phosphinamide in an indium catalysed addition to a hydrazone was reported, but this gave a product with low ee.21  A series of homochiral DPEN derivatives were prepared and screened as organocatalysts in nitrostyrene addition (Scheme 1, Table 1). DPEN alone proved to be capable of high enantioinduction (78% ee), as previously reported,4a but did not furnish a product in high yield. Of its derivatives, N-tosylated amine TsDPEN 8 gave a product of high ee but the reaction was slow. TsDPEN 8 has been successfully used in a closely related addition to nitrostyrene.22 N-Benzoyl amide 9 gave a good result in terms of ee (82% conversion after 96 h, 89% ee) however, the best catalyst, of those tested, proved to be the phosphinamide 1023 (100% conversion after 7 h, 96% ee). The bis-phosphorylated DPEN derivative 1124 was, however, ineffective as a catalyst, thus confirming the expected requirement for a basic amine. Secondary amine derivatives 12-1518 also proved to be essentially inactive, possibly for reasons of steric hindrance. All the DPEN derivatives tested gave the same product enantiomer relative to the diamine, i.e. (R,R)-DPEN derivatives gave the S–configured product. The ee did not change with conversion (samples were taken at regular 1-2 h intervals) in the reaction catalysed by 10 and did not deteriorate when  Tetrahedron Letters  3the reaction was allowed to stand after completion (up to 4 days), suggesting that the addition is essentially irreversible. These results indicate that the phosphinamide group plays an activating role in the reaction, and contributes to the enantiocontrol, possibly in an analogous mechanism to that shown in Figure 1.   PhPhH2N NH2PhPhTsHN NH2(R,R)-DPEN (R,R)-8PhPhNH NH2(S,S)-9OPhPhPhNH NH2POPhPh(S,S)-10PhPhNH HNPOPhPh(S,S)-11POPhPhPhPhTsHN NHRPhPhBnHN NHBn12 R=Bn13 R=cHex14 R=(CH2)2Ph 15  Further studies revealed that added acetic acid and water are important to the reaction. The conversion dropped to just 15% in 96 h when both AcOH and water were omitted from the reaction, and to 75% in 18 h when only AcOH, but not water, were added.  Benzoic acid may be used in place of acetic acid, but gives lower reaction rates. The importance of water in such reactions has been investigated in detail,25 whilst the acid presumably catalyses the formation and decomposition of the intermediate enamine and iminium species. Excess water (>1 eq/ relative to nitrostyrene) is not beneficial; a series of tests on pure acetone revealed that the level of water in the technical grade is close to optimal (see Supporting information). In hexane, the catalyst loading can be reduced to 10%, which gives full reaction in 24 h at rt or 5 h at 40 oC (with a drop in the ee to 93%) but at <5 mol% catalyst loading the reaction requires unacceptable (>72 h) times for completion (Figure 2). Of a series of solvents tested, the nonpolar solvents hexane (94% ee) and toluene (96% ee) gave the fastest rates, whilst reactions in CH2Cl2, THF, EtOAc and acetone were much slower.   Although catalyst 10 worked well in the specific addition of acetone to nitrostyrenes, poor results were obtained in attempts to use both cyclohexanone and ethanal, with <5% formation of products in each case. In view of the known value of pyrrolidine derivatives in the nitrostyrene addition reaction, but mindful of their only moderate reported enantioselectivities for the acetone addition reaction,7-16 proline derivative 18 represented an attractive candidate for evaluation.7-15 An attempt to prepare 18 by direct phosphinylation of homochiral diamine 19 resulted in formation of 20, which was not an active catalyst (Table 2). Table 1. Use of DPEN derivatives in nitrostyrene additions.a Catalyst Product Load-ing (%) Added water Time(h) Conv (%) (ee) h (R/S) (R,R)-DPEN 3 15 b 96 46 78 (S) (R,R)-8 3 15 b 96 68 91 (S) (S,S)-9 3 15 b 96 82 89 (R) (S,S)-10 3 15 b 7 100 96 (R) (S,S)-11 3 15 b 96 0 - (S,S)-12 3 15 b 96 0 - (S,S)-13 3 15 b 96 0 - (S,S)-14 3 15 b 96 0 - (S,S)-15 3 15 b 96 0 - (S,S)-10 3g 15 C (0) 96 16 - (R,R)-10 3 10 b  5 65 95 (S) (R,R)-10 d 3 10 b 5 100 93 (S) (R,R)-10 e 3 10 b 3 62 88 (S) (R,R)-10 f 3 10 b 5 41 94 (S) (R,R)-10 3 10 C (0.75)  18 96 94 (S) (R,R)-10 3 10 C (0.50)  18 89 92 (S) (R,R)-10 3 10 C (0.25)  18 96 94 (S) (R,R)-10 3 10 C (0.10)  18 84 97 (S) (R,R)-10 3 10 C (0)  18 75 95 (S) (R,R)-10 16 10 b 24 100 93 (S) (R,R)-10 17 10 b 24 100 96 (S) a See Scheme 1, all reactions carried out at rt in toluene unless otherwise stated. b Technical grade acetone used; ca 6.7% H2O by volume and 2% H2O by mass. c Reagent grade acetone used, level of water relative to catalyst in parenthesis. d At 40 oC, e At 60 oC, f At 0 oC. g No AcOH added. h ee was determined by chiral HPLC, configuration by optical rotation.    Figure 2. Effect of catalyst 10 loading on reaction rate. Conversion vs catalyst (10) loading (in hexane)01020304050607080901000 5 10 15 20Time, hConversion, %10%5%3%1% Tetrahedron Letters  4  The successful synthesis of 18 was achieved starting from a protected proline derivative 21a, following a related method reported in the literature.26 We favoured the use of 21a because we feared that the acidic conditions required for tBoc removal would also result in loss of the phosphinamide. In the event, 21a was successfully converted into 18 through the sequence shown in Scheme 2, however, the last step gave a very poor yield of product. A search of the literature yielded an example of a method for the removal of a tBoc group in the presence of a phosphinamide.21 The conversion of 21b to 25b followed the previous sequence,26 and the subsequent conversion into 18 was successful, although the work up required the use of triethylamine21 to avoid a highly basic solution from being formed. A better yield was obtained using a combination of isopropylsilane and TFA, but this remained low due to concommitant cleavage of the phosphinamide group.27 NHHNPOPhPh18NHNH219NHNPOPhPh20P PhPhO Compound 18 proved to be an efficient, but not highly enantioselective, catalyst for the nitrostyrene addition reaction (Table 2). The 16% ee of the product was similar to that of the unprotected diamine 19, suggesting that its mechanism of action may be related. This low ee with pyrrolidine derivatives mirrors those obtained in the same reaction using related pyrrolidine-based organocatalysts. 7-16 The use of 18 in other applications is currently being studied.   NOHNHNPOPhPhOX21a (X=CO2CH2Ph, Z)21b (X=tBoc)NOHXBH3.DMS, THFX=Z, 85%X=tBoc, 89%NOTsXTsCl,X=Z, 55%X=tBoc, 78%NaN3, DMSOX=Z, 98%X=tBoc, 77%NN3XNNH2XPPh3, THF,X=Z, 40%X=tBoc, 84%Ph2P(O)Cl,X=Z, 43%X=tBoc, 61%NHHNPOPhPhXX=Z, H2, Pd/CX=tBoc, (iPr)3SiH, TFAX=Z, 9%X=tBoc, 21%H2O22a22b23a23b24a24b25a25b26a26b18 Scheme 2. Synthesis of 18. Table 2. Use of pyrrolidine derivatives in nitrostyrene additions.a Catalyst Product Load-ing (%) Time (h) Conv (%) (ee) b (R/S) (S)-18 3 10 96 100 16 (R) (S)-19 3 15 3/23 50/100 17 (R) (S)-20 3 15 96 0 -  a See Scheme 1, all reactions carried out at rt in toluene unless otherwise stated, with technical grade acetone; ca 6.7% H2O by volume and 2% H2O by mass. b ee was determined by chiral HPLC, configuration by optical rotation.   In conclusion, we have described the promising results of a study directed at establishing the ability of the phosphinamide group to direct asymmetric organocatalytic additions to nitrostyrene. To the best of the our knowledge the phosphinamide group has not been studied in this capacity before and this therefore represents a novel subject for investigation. We are currently investigating further applications of this system. Acknowledgements We thank the EPSRC for funding (postdoctoral grant EP/D031168/1 to DJM) and Rhodia Consumer Specialities Ltd for supporting CVM through a Collaborative Training Account studentship. The EPSRC National MS service (Swansea) is thanked for HRMS analyses as is the EPSRC Chemical Database Service.28   References 1.  (a) Xu, L. W.; Lu, Y. Org. Biomol. Chem. 2008, 2047-2053. (b) Melchiorre, P.; Marigo, M.; Carlone, A.; Bartoli, G. Angew. Chem. Int. Ed. 2008, 47, 6138-6171. (c) Dondoni, A. Massi, A. Angew. Chem. Int. Ed. 2008, 47, 4638-4660. (d) Connon, S. J. Chem. Commun. 2008, 2499-2410. (e) Seayad, J.; List, B. Org. Biomol. Chem. 2005, 3, 719-724. 2.  (a) Eder, U.; Sauer, G.; Weichert, R. Angew. Chem. Int. Ed. 1971, 10, 496-497. (b) Lattanzi, A. Chem. Commun. 2009, 1452-1463. 3.  Sulzer-Mosse, S.; Alexakis, A. Chem. Commun. 2007, 3123-3135. (b) Xu, L.-W.; Luo, J.; Lu, Y. Chem. Commun. 2009, 1807-1821. (c) Xu, L.-W.; Lu, Y. Org. Biomol. Chem. 2008, 6, 2047-2053. 4.  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36, 746. Supplementary Material General experimental details, graphs of experimental results, and 1H and 13C NMR of all new compounds.

In a typical preparation, 10 was made by direct reaction of DPEN with Ph2P(O)Cl, 62% yield on a 0.5g scale, ca. 17% 11 was also formed.

Tetrahedron: Asymmetry

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Asymmetric organocatalysis of the addition of acetone to 2-nitrostyrene using N-diphenylphosphinyl-1,2-diphenylethane-1,2-diamine (PODPEN)

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