Symmetric Transfer Hydrogenation of Quinolines Using Ru (II) Catalysts
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symmetric Transfer Hydrogenation of Quinolines Using Ru (II) Catalysts. Vimal Parekh*, Ian Lennon, 2 James Ramsden 2 and Martin Wills 1 1. Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, UK

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symmetric Transfer Hydrogenation of Quinolines Using Ru (II) Catalysts

Vimal Parekh*, Ian Lennon,2 James Ramsden2 and Martin Wills1

1. Department of Chemistry, The University of Warwick, Coventry, CV4 7AL, UK

2. CPS Chirotech, Dr Reddy’s Laboratories, Unit 162 Cambridge Science Park, Cambridge, CB4 OGH, UK

Introduction

Synthesis of the catalyst

The studies carried out showed that the 4C ‘tethered’ link is required for rapid conversion and functionality on the aromatic ring might be essential for a high ee, which was why the catalyst shown in Scheme 3 was synthesized.

Asymmetric reduction of C=N bonds represents a powerful method for the asymmetric formation of chiral amines.1 Whilst many methods exist for the asymmetric reduction of isolated C=N groups, their reduction when they are a part of an aromatic ring represents a more challenging objective. Tetrahydroquinoline derivatives have attracted considerable attention owing to their importance as:

-Synthetic intermediates for drugs,

-Agrochemicals, and

-Dyes2

A number of reports on the pressure hydrogenation of quinolines have been published3 mostly using Ir based catalysts. Reductions using Ru based catalysts have also been reported using pressure hydrogenation, but Ir based catalysts are by far more active for quinoline reductions. My poster presentation will describe the use of asymmetric transfer hydrogenation (ATH) of quinolines using tethered and untethered Ru (II) catalysts.

Scheme 1. General scheme for the asymmetric transfer hydrogenation

of quinolines.

Results and Discussion

Catalyst (9) was tested out with the conditions used previously (Table 3), and the reaction proved to be unsuccessful as only 29% conversion was obtained after 24hrs, with an ee of 42%.

Scheme 3. Shows the reaction scheme for the formation of the 4C ‘tethered’ catalyst (9) with added functionality on the aromatic ring about the Ru.

In this project our aim was to carry out ATH of quinolines using ‘tethered’ and untethered

Ru (II) catalysts. The results obtained from preliminary studies showed that the 4C ‘tethered’

catalyst (7) was far more active than the untethered catalyst (3) for the reduction of quinolines, which was why optimization of conditions to give high e.e. and conversion was carried out using catalyst (7) as a starting point along with the least bulkier quinoline, 2-methylquinoline (1). (Scheme 1)

Reductions using different solvents

ATH of quinolines was carried out using different solvents, and from the results obtained it was quite clear that overall methanol gave the best conversion and ee.

Reductions using different Ru (II) catalysts

The graph clearly shows that the 4C ‘tethered’ catalyst (7) proves to be the best Ru (II) catalyst for ATH of quinolines (Figure 2).

Figure 2. Shows the conversion vs time at 60°C for different catalysts.

Reductions carried out on various quinoline substrates using catalyst (7) and (10)

The reduction of a series of further quinolines, 12-19 (Figure 3) was examined using tethered catalyst 7 (Table 4). As this proved to be the most successful catalyst for conversion of quinolines to tetrahydroquinolines. Of these, the phenyl-substituted substrate 12 was reduced in the highest ee of the series, whilst the tBu derivative 13 was successfully reduced but only in racemic form. Other isoquinolines were reduced in high conversion but only moderate-good ee.

Table 1. Shows the reduction of 2-methylquinoline using different solvents.

Reductions carried out at different temperatures

Having determined the best solvent, the next task was to see whether changing the temperature had any effect on the conversion and ee.

Figure 3. Shows the different quinoline substrates used for ATH.

Table 2. Shows the reduction of 2-methylquinoline at different temperatures.

The results clearly show that the fastest conversion was obtained at 60°C with only a drop of 3% ee. The next task was to carry out ATH reductions at 60°C using both untethered and ‘tethered’ Ru (II) catalysts.

Reductions using different Ru (II) catalysts

ATH of quinolines was carried out using both untethered and ‘tethered’ catalysts (Figure 1), and results obtained clearly showed that the 4C ‘tethered’ link (7) is important for the rapid conversion of quinoline to tetrahydroquinolines, and functionality on the aromatic ring above the Ru on the untethered catalyst (3) is vital for obtaining a high ee.

Table 4. Shows the reduction of substrates (1, 12-19) using Ru (II) catalyst (7).

Better results, in terms of enantioselectivity, in several cases exceeding 90% ee, were achieved using the rhodium tethered catalyst 10, which has previous been used for ketone and imine reduction (Table 5). Using 0.5 mol% of 10, the reactions did not go to full conversion after 48 hours, although the use of a higher loading (2 mol%)** of catalyst increased the conversions in most cases. Further work is required to optimize the reductions by these catalysts.

Figure 1. Shows the different Ru (II) and Rh (III) catalysts used.

Table 5. Shows the reduction ofsubstrates (1, 12-19) using Rh (III) catalyst (10).

Conclusion

In conclusion, we have demonstated that tethered Ru(II) and Rh(III) complexes are effective catalysts for the ATH of substituted isoquinolines, which are generally regarded as challenging substrates for this application. To the best of our knowledge, this is the first report5 of the use of such catalysts in a solution of formic acid/triethylamine/methanol. As has been observed in ketone reduction, the increased reactivity of tethered complexes over the untethered ones appears to be key to their capacity to work as effective catalysts in this application.

Table 3. Shows the reduction of 2-methylquinoline at 60°C using different catalysts.

References

1. (a) Noyori, R. Adv. Synth. Catal., 2003, 345, 15-32. (b) Noyori, R.; Sandoval, C. A.; Muniz, K.; Ohkuma, T. Phil. Trans. R. Soc. A2005, 363, 901-912. (c) Noyori, R.; Kitamura, M.; Ohkuma, T. Proc. Nat. Acad. Sci. 2004, 101, 5356-5362. (d) Noyori, R.; Ohkuma, T. Angew. Chem. Int. Ed. 2006, 40, 40-73.

2. Katritzky, A. R.; Rachwal, S.; Rachwal, B. Tetrahedron1996, 52, 15031, and references cited therein.

3. (a) Xu, L.; Lam, K. H.; Ji, J.; Wu, J.; Fan, Q.-H.; Lo, W.-H. Chem. Commun. 2005, 1390-1392. (b) Zhou, H.; Li, Z.; Wang, Z.; Wang, T.; Xu, L.; He, Y.; Fan, Q.-H.; Pan, J.; Gu, L.; Chan, A. S. C. Angew. Chem. Int. Edn. 2008, 47, 8464-8467.

4. Hayes, A. M.; Morris, D. J.; Clarkson, G. J.; Wills, M. J. Am. Chem. Soc.2005, 127, 7318.

5. Parekh, V.; Ramsden, J. A.; Wills, M.Tetrahedron: Asymmetry, 2010, 21, 1549-1556.

Acknowledgements

I would like to thank Martin Wills and the Wills group for their support and encouragement during this project. I would also like to thank my industrial supervisors James Ramsden and Ian Lennon and the EPSRC and Dr Reddy’s for the financial support.


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