Magtrieve™ (CrO2) and MnO2 mediated oxidation of aldoximes: studying the reaction course

doi:10.1016/j.tet.2010.10.029

Tetrahedron

Volume 66, Issue 50, 11 December 2010, Pages 9582-9588

DB dedicates this work to Dr. Santosh Maji on the occasion of his 60th birthday

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Abstract

Magtrieve™ (CrO₂) and MnO₂ mediated oxidation of aldoximes to nitrile oxides were studied in details. In presence of external radical source, TEMPO, these reagents did not furnish nitrile oxides, instead favoured deoximation to aldehydes. A common trend of deoximation was established from electronically tuned aldoximes, which is: aliphatic>aromatic>aldoximes with strong electron-withdrawing group, though the extent of deoximation was less in case of CrO₂. Above effects were not observed with chloramine-T and diacetoxyiodobenzene, reagents known to produce nitrile oxides via hydroximoyl halide or equivalent ionic intermediates. A putative reaction mechanism is proposed for MO₂ (M=Cr, Mn) mediated oxidation of aldoximes through formation of a nitroso-oxime tautomeric pair. Formation of nitrile oxide is possibly occurred from the oxime tautomer via a σ-type iminoxy radical intermediate. The deoximation process, dominating in presence of external radical environment, is explained following decomposition of the nitroso tautomer.

Graphical abstract

Introduction

Nitrile oxides are versatile intermediates in organic synthesis. The most useful synthetic application of nitrile oxides is perhaps the synthesis of isoxazoline and isoxazole heterocycles, via 1,3-dipolar cycloaddition (1,3-DC) reactions.1, 1(a), 1(b), 1(c), 1(d), 1(e), 1(f), 1(g), 1(h) Recently we have communicated^2a an efficient methodology for direct oxidation of aldoximes to nitrile oxides using Magtrieve™ (CrO₂),3, 3(a), 3(b), 3(c), 3(d), 3(e) and subsequent 1,3-DC reactions with dipolarophiles to furnish the desired five-member heterocycles in one-step. Additionally for this type of synthesis, the use of CrO₂ has been shown to have specific advantages over some other previously known reagents,^2b and particularly over direct oxidizing agents, such as MnO₂,4, 4(a), 4(b) Pb(OAc)₄⁵ and other transition metal complexes.6, 6(a), 6(b), 6(c)

While developing the CrO₂-methodology, two interesting observations motivated us to develop further understanding of these reactions. Firstly, a mixture of (E)- and (Z)-aldoximes 1 was also oxidized to nitrile oxides 2 in high yields upon treatment with CrO₂ (Scheme 1; Eq. 1). Such results hinted us that CrO₂ might follow a mechanism similar to MnO₂, which also has been reported^4a to produce nitrile oxide with equivalent yield from both the stereoisomers of aldoximes (Scheme 1, Eq. 2). It is important to mention here that Pb(OAc)₄ has been known⁵ to produce nitrile oxides only from (E)-aldoximes via a rigid six-member transition state 3 (Scheme 1, Eq. 3). Secondly, we noticed that CrO₂ led to significantly less deoximation compared to what was reported by Kiegiel et al.^4a for MnO₂ especially in case of aromatic and aliphatic aldoximes. For MnO₂, involvement of iminoxy radical intermediates 4 has been invoked by the original investigators^4a as common intermediates towards the formation of nitrile oxides 2 as well as the corresponding aldehydes 5 as deoximation product (Scheme 1, Eq. 2). However, no further mechanistic insights have been reported in the literature. Therefore, subsequent to our original communication, a series of experiments were designed and carried out in our laboratory to study the reaction course of Magtrieve™ and MnO₂ mediated oxidation of aldoximes. Herein, we report our observations, and at the end propose a putative reaction mechanism to account various facts.

Section snippets

Reactions of CrO₂ with (E)- and (Z)-aldoximes

In our previous study with CrO₂, it was observed that 60:40 regioisomeric mixture of aliphatic aldoximes furnished 1,3-DC products in 63–75% isolated yields.^2a Subsequently for the present investigation, pure (E)- and (Z)-benzaldoximes, (E)-1a and (Z)-1a were independently reacted with CrO₂ following the reported procedure^2a in presence of various dipolarophiles. As anticipated, the corresponding 1,3-DC products 7a–11a were obtained in 75–86% isolated yields irrespective of the geometry of

Conclusions

Based on several experimental results (Scheme 2; Table 1, Table 2) a putative mechanism is derived for CrO₂ and MnO₂ mediated direct oxidation of aldoximes to nitrile oxides. To illustrate the mechanism, initial interaction of either (E)- or (Z)-aldoximes with MO₂ (M=Cr, Mn) lead to formation of a nitroso-oxime tautomeric pair II and III as non-radical common intermediates (Scheme 3). The formation of nitrile oxide is proposed to occur from the oxime tautomer III via a σ-type iminoxy radical

Synthesis of 6a, 7a, 8a, 9a, 10a, 11a

Following the procedure described in our previous communication, the above compounds were synthesized.^2a Both (E) and (Z)-benzaldoximes provided us identical results. Analytical data of 6a,14, 14(a), 14(b), 14(c) 7a,15, 15(a), 15(b) 8a,16, 16(a), 16(b), 16(c), 16(d) 9a¹⁷ and 11a^8b were matched with the literature values.

Acknowledgements

Authors are grateful to Drs. Rashmi Barbhaiya and Kasim Mookhtiar for their support and encouragement. SB is thankful to Andhra University for registering in Ph.D. program, and Professors Y.L.M. Murthy and U.V. Prasad for their guidance. Authors also acknowledge the reviewers for their important suggestions.

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Tetrahedron

Magtrieve™ (CrO2) and MnO2 mediated oxidation of aldoximes: studying the reaction course

Abstract

Graphical abstract

Introduction

Section snippets

Reactions of CrO2 with (E)- and (Z)-aldoximes

Conclusions

Synthesis of 6a, 7a, 8a, 9a, 10a, 11a

Acknowledgements

Tetrahedron Lett.

Org. Lett.

J. Org. Chem.

Chem. Rev.

J. Org. Chem.

J. Am. Chem. Soc.

J. Org. Chem.

Chem. Pharm. Bull.

J. Org. Chem.

J. Org. Chem.

Org. Lett.

Tetrahedron Lett.

Synthesis

Acc. Chem. Res.

Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis

Chem. Rev.

Heterocycles

Chem. Rev.

Tetrahedron Lett.

Synlett

Tetrahedron Lett.

Tetrahedron Lett.

Bioorg. Med. Chem. Lett.

Monatsh. Chem.

Tetrahedron Lett.

Tetrahedron: Asymmetry

Magtrieve™ (CrO₂) and MnO₂ mediated oxidation of aldoximes: studying the reaction course

Reactions of CrO₂ with (E)- and (Z)-aldoximes