Chemistry - A European Journal
10.1002/chem.201602801
COMMUNICATION
Sarkar, R. Fröhlich, S. Grimme, A. Studer, Chem. Sci. 2013, 4, 2177; g)
Y. Hayashi, S. Santoro, Y. Azuma, F. Himo, T. Ohshima, K. Mashima, J.
Am. Chem. Soc. 2013, 135, 6192. h) K. Sugahara, N. Satake, K.
Kamata, T. Nakajima, N. Mizuno, N. Angew. Chem. 2014, 126, 13464;
Angew. Chem. Int. Ed. 2014, 53, 13248; i) D. Nakatake, Y. Yokote, Y.
Matsushima, R. Yazaki, T. Ohshima, Green Chem. 2016, 18, 1524.
Utility of tertiary butyl ester : P. G. M. Wuts, Greene’s Protective groups
in organic synthesis, 5th ed.; John Wiley & Sons, Inc. 2014.
Next, to demonstrate the usefulness of the present iron
catalysis for tertiary butyl ester formation from the corresponding
methyl ester, we performed the reaction with various N-Boc-
protected -amino methyl esters. N-Boc glycine was a good
substrate and the corresponding tertiary butyl ester 10oa was
obtained in high yield. Notably, chiral -amino tertiary butyl
esters were obtained without any racemization (10pa–10ua).
Even when using more easily epimerized chiral phenylglycine
methyl ester, the reaction proceeded with only slightly
decreased enatioselectivity (10va). Synthetically useful tertiary
ester 10gb[18] was isolated in high yield. Linalool (9c) was also
applicable; no rearrangement reaction of the desired product
10gc was observed and linalool (9c) was recovered unchanged.
In conclusion, we developed a chemoselective O-acylation
with activated esters in the presence of amines catalyzed by -
oxo-dinuclear iron(III) salen complex 6a. The present catalysis
was applicable not only to functionalized amino alcohols, but
also to less reactive aminophenols, which is unprecedented.
Furthermore, 6a catalyzed the transesterification of various
tertiary alcohols, including tert-BuOH, for the first time. Further
studies on elucidation of precise reaction mechanism and
enantioselective reaction are in progress.[19]
[5]
[6]
1-Adamantanol was used for catalytic transesterification; see ref 4e and
4i. Other tertiary alcohols have never been used under catalytic
conditions.
[7]
[8]
In the absence of catalyst, only trace amount of 5aa was detected.
Recent example of iron catalyst transesterification: S.-S. Weng, C.-S.
Ke, F.-K. Chen, Y.-F. Lyu, G.-Y. Lin, Tetrahedron 2011, 67, 1640.
-Oxo-dinuclear iron(III) salen complex; see a) S. K. Edulji, S. T.
Nguyen, Organometallics 2003, 22, 3374; b) T. Glaser, R. H. Pawelke,
M. Heidemeier, Z. Anorg. Allg. Chem. 2003, 629, 2274; c) K. J.
Gallagher, R. L. Webster, Chem. Commun. 2014, 50, 12109.
[9]
[10] Commercially available iron(II)-salen complex 6b from TCI afforded
slightly inferior result, presumably because commercially available
iron(II)-salen complex 6b would contain -oxo-dinuclear iron(III) salen
complex 6a. The solution of iron(II)-salen complex 6b in untreated
chloroform gave the same UV- spectrum as -oxo-dinuclear iron(III)
salen complex 6a. In contrast, the solution of iron(II)-salen complex 6b
in degassed chloroform gave similar but distinct UV-spectrum,
suggesting that complex 6b was easily converted into complex 6a with
the assistance of tiny amount of oxygen. We also confirmed the almost
identical UV- spectrums after the reaction conditions. See Supporting
Information for detail. Also see Ref. 8c
Acknowledgements
[11] Previously developed Zn4(OCOCF3)6O (11) was less effective. See
Supporting Information for detail.
This work was financially supported by Grant-in-Aid for Scientific
Research (B) (#24390004), Scientific Research on Innovative
Area 2707 Middle molecular strategy and Platform for Drug
Discovery, Informatics, and Structural Life Science from MEXT.
[12] NHC catalyst reported in ref. 4f was found to be less effective for the
chemoselective transesterification of aminophenols. See Supporting
Information.
[13] S. Uesugi, Z. Li, R. Yazaki, T. Ohshima, Angew. Chem. 2014, 126,
1637; Angew. Chem. Int. Ed. 2014, 53, 1611
Keywords: iron • chemoselective • transesterification •
aminophenol • tert-butyl ester
[14] O. Robles, D. Romo, D. Nat. Prod. Rep. 2014, 31, 318.
[15] Selected examples of catalytic esterification of tert-alcohols using highly
reactive acid anhydrides, see; a) K. Ishihara, M. Kubota, H. Kurihara, H.
Yamamoto, J. Am. Chem. Soc. 1995, 4413; b) E. Vedejs, O. Daugulis,
J Org. Chem. 1996, 61, 5702; c) P. A. Procopiou, S. P. D. Baugh, S. S.
Flack, G. G. A. Inglis, J. Org. Chem. 1998, 63, 2342; d) A. Orita, C.
Tanahashi, A. Kakuda, J. Otera, Angew. Chem. 2000, 112, 2999;
Angew. Chem. Int. Ed. 2000, 39, 2877.
[1]
a) R. C. Larock, Comprehensive Organic Transformations, 2nd ed.;
Wiley-VCH: New York, 1999; b) J. Mulzer, Comprehensive Organic
Synthesis; B. M. Trost, I. Fleming, Eds.; Pergamon Press: New York,
1992; Vol 6; c) J. Otera, Esterification; Wiley-VCH: Weinheim, 2003.
M. Nahmany, A. Melman, Org. Biomol. Chem. 2004, 2, 1563.
[2]
[3]
[16] In the absence of catalyst, no reaction was observed.
[17] We speculated that amine would also function as base to activate
alcohol under heated conditions in Table 1.
For reviews on chemoselective reactions: a) B. M. Trost, Science 1983,
219, 245; b) R. A. Shenvi, D. P. O’Malley, P. S. Baran, Acc. Chem. Res.
2009, 42, 530; c) N. A. Afagh, A. K. Yudin, Angew. Chem. 2010, 122,
270; Angew. Chem. Int. Ed. 2010, 49, 262; d) J. Mahatthananchai, A. M.
Dumas, J. W. Bode, Angew. Chem. 2012, 124, 11114; Angew. Chem.
Int. Ed. 2012, 51, 10954.
[18] 1,1-dimethylallyl ester could chemoselectively be deprotected in the
presence of Fmoc group and stepwise preparation from corresponding
carboxylic acid was required; M. Sedighi, M. A. Lipton, Org. Lett. 2005,
7, 1473.
[4]
Catalytic chemoselective transesterification; a) M M.-H. Lin, T. V.
RajanBabu, Org. Lett. 2000, 2, 997; b) T. Ohshima, T. Iwasaki, Y.
Maegawa, A. Yoshiyama, K. Mashima, J. Am. Chem. Soc. 2008, 130,
2944; c) S. De Sarkar, S. Grimme, A. Studer, J. Am. Chem. Soc. 2010,
132, 1190; d) M. Hatanao, Y. Furuya, T. Shimmura, K. Moriyama, S.
Kamiya, T. Maki, K. Ishihara, Org. Lett. 2011, 13, 426; e) M. Hatanao, K.
Ishihara, Chem. Commun. 2013, 49, 1983; f) R. C. Samanta, S. De
[19] Proposed catalytic cycle was described in Supporting Information.