Stereoselective Oxidation of Titanium(IV) Enolates with Oxygen

Abstract

A novel approach to synthesize enantiomerically pure α-hydroxy carboxylic derivatives is reported. A highly stereoselective oxidation of titanium(IV) enolates from chiral N-acyloxazolidinones is performed with oxygen under simple experimental conditions that do not require any reducing steps. The success of this approach depends on the biradical character of titanium(IV) enolates.

Key words

stereoselective synthesis - hydroxylation - titanium enolates - chiral auxiliaries - oxygen - radicals

• References

• 1a Chen K. Baran PS. Nature 2009; 459: 824
• 1b Newhouse T. Baran PS. Angew. Chem. Int. Ed. 2011; 50: 3362
• 1c Saint-Denis TG. Zhu R.-Y. Chen G. Wu Q.-F. Yu J.-Q. Science 2018; 359: 747
• 2a Chen B.-C. Zhou P. Davis FA. Ciganek E. Org. React. 2003; 62: 1
• 2b Smith AM. R. Hii KK. Chem. Rev. 2011; 111: 1637
• 3 Davis FA. Chen B.-C. Chem. Rev. 1992; 92: 919
• 4a Evans DA. Morrissey MM. Dorow RL. J. Am. Chem. Soc. 1985; 107: 4346
• 4b Zhang L. Zhu L. Yang J. Luo J. Hong R. J. Org. Chem. 2016; 81: 3890
• 5a Punniyamurthy T. Velusamy S. Iqbal J. Chem. Rev. 2005; 105: 2329
• 5b Bryliakov KP. Chem. Rev. 2017; 117: 11406
• 6a Córdova A. Sundén H. Engqvist M. Ibrahem I. Casas J. J. Am. Chem. Soc. 2004; 126: 8914
• 6b Sundén H. Engqvist M. Casas J. Ibrahem I. Córdova A. Angew. Chem. Int. Ed. 2004; 43: 6532
• 6c Córdova A. Sundén H. Bøvegig A. Johansson M. Himo F. Chem. Eur. J. 2004; 10: 3673
• 7 Lubin H. Tessier A. Chaume G. Pytkowicz J. Brigaud T. Org. Lett. 2010; 12: 1496
• 8a Sano D. Nagata K. Itoh T. Org. Lett. 2008; 10: 1593
• 8b Sim S.-BD. Wang M. Zhao Y. ACS Catal. 2015; 5: 3609
• 9 Ma P.-J. Liu H. Xu Y.-J. Aisa HA. Lu C.-D. Org. Lett. 2018; 20: 1236
• 10a Moreira I. deP. R. Bofill JM. Anglada JM. Solsona JG. Nebot J. Romea P. Urpí F. J. Am. Chem. Soc. 2008; 130: 3242
• 10b Heras C. Gómez-Palomino A. Romea P. Urpí F. Bofill JM. Moreira I. deP. R. J. Org. Chem. 2017; 82: 8909

Zakarian has convincingly demonstrated the biradical character of titanium(IV) and zirconium(IV) enolates in highly stereoselective reactions:
• 11a Beaumont S. Ilardi EA. Monroe LR. Zakarian A. J. Am. Chem. Soc. 2010; 132: 1482
• 11b Gu Z. Herrmann AT. Zakarian A. Angew. Chem. Int. Ed. 2011; 50: 7136
• 11c Herrmann AT. Smith LL. Zakarian A. J. Am. Chem. Soc. 2012; 134: 6976
• 11d Ilardi EA. Zakarian A. Chem. Asian J. 2011; 6: 2260
• 12a Gómez-Palomino A. Pellicena M. Romo JM. Solà R. Romea P. Urpí F. Font-Bardia M. Chem. Eur. J. 2014; 20: 10153
• 12b Mabe PJ. Zakarian A. Org. Lett. 2014; 16: 516
• 13 Evans DA. Urpí F. Somers TC. Clark JS. Bilodeau MT. J. Am. Chem. Soc. 1990; 112: 8215

Enolization with mild titanium(IV) Lewis acids is only possible in more acidic α-halo or α-aryl substrates. See, for instance:
• 14a Nebot J. Romea P. Urpí F. J. Org. Chem. 2009; 74: 7518
• 14b Alvarado J. Herrmann AT. Zakarian A. J. Org. Chem. 2014; 79: 6206
• 15 Zirconium(IV) enolates from 1a were prepared according the experimental procedure reported in reference 11b.
• 16 Radical α-hydroxylations of non-chelated chiral N-acyloxazolidinones with oxygen yield the corresponding adducts with low diastereoselectivities, see: Kihara N. Ollivier C. Renaud P. Org. Lett. 1999; 1: 1419
• 17 Adam W. Metz M. Prechtl F. Renz M. Synthesis 1994; 563

Stereoselective oxidation of enolates by peroxide intermediates has been occasionally proposed in α-hydroxylation reactions with molecular oxygen. See, for instance:
• 18a Yang Y. Moinodeen F. Chin W. Ma T. Jiang Z. Tan C.-H. Org. Lett. 2012; 14: 4762
• 18b Wang Y. Yin H. Tang X. Wu Y. Meng Q. Gao Z. J. Org. Chem. 2016; 81: 7042
• 18c Wang Y. Zheng Z. Lian M. Yin H. Zhao J. Meng Q. Gao Z. Green Chem. 2016; 18: 5493
• 18d Ding W. Lu L.-Q. Zhou Q.-Q. Wei Y. Chen J.-R. Xiao W.-J. J. Am. Chem. Soc. 2017; 139: 63

For the role of peroxide intermediates in nonstereoselective hydroxylation of enolates with molecular oxygen, see:
• 19a Liang Y.-F. Jiao N. Angew. Chem. Int. Ed. 2014; 53: 548
• 19b Chaudhari MB. Sutar Y. Malpathak S. Hazra A. Gnanaprakasam B. Org. Lett. 2017; 19: 3628
• 20 For a comprehensive analysis of the copper-catalyzed oxidation of ketones with oxygen, see: Tsang AS.-K. Kapat A. Schoenebeck F. J. Am. Chem. Soc. 2016; 138: 518

For the removal of 5,5-disubstituted oxazolidinones, see:
• 21a Davies SG. Sanganee HJ. Tetrahedron: Asymmetry 1995; 6: 671
• 21b Hintermann T. Seebach D. Helv. Chim. Acta 1998; 81: 2093
• 22 SuperQuat chiral auxiliary (S)-4-benzyl-5,5-dimethyl-1,3-oxazolidin-2-one was prepared according to a procedure previously reported, see: Bull SD. Davies SG. Jones S. Polywka ME. C. Shyam Prasad R. Sanganee HJ. Synlett 1998; 519

Spectroscopical and analytical data of 1a–h are in agreement with those reported in the literature, see:
• 23a Ref. 11.
• 23b Bull SD. Davies SG. Nicholson RL. Sanganee HJ. Smith AD. Org. Biomol. Chem. 2003; 1: 2886