The Synthesis and Utility of Metal-Nitrosophenolato Compounds-Highlighting the Baudisch Reaction

Abstract

The syntheses of the title compounds demonstrate a privileged introduction of a nitroso (and a hydroxyl via the Baudisch reaction) group to an aromatic ring. These complexes first appeared in the literature as early as 1939, and a range of applications has subsequently been published. However, optimisations of the preparative sequences were not considered, and as such, the reactions have seldom been utilised in recent years; indeed, there remains confusion in the literature as to how such complexes form. In this review, we aim to demystify the misunderstanding surrounding these remarkable complexes and consider their renewed application in the 21st century.

Keywords: Baudisch reaction; C-nitrosation; Copper complexes; ortho-nitrosophenols.

Figure 1

The molecular structure of the commercially available dye, ‘Pigment green 8’ (aka ‘Pigment Green B’) [6].

Figure 2

The structures of four antibiotics isolated from Stroptomyces bacteria, Viridomycin A [13], Actinoverdin A, Ferroverdin [9] and Viridomycin E. Viridomycin E has been described in only a single publication [12] and could well exist preferentially as the hexadentate sodium salt, equivalent to the other 3 examples.

Scheme 1

Five different reactions (1–5) that Baudisch chose to publish, across two publications, which have subsequently become known as the Baudisch reaction [20,21]. ^aBaudisch states in the literature that ‘freshly prepared yellow cuprous hydroxide’ was used; it is known that this is not a stable entity, so the actual active substance could well be an alternative such as cuprous oxide, Cu₂O.

Scheme 2

A clear distinction between copper-mediated aromatic nitrosation of phenols and the Baudisch conditions is demonstrated by the reporting of a different major product from the same starting material (3) [15,26].

Scheme 2

A clear distinction between copper-mediated aromatic nitrosation of phenols and the Baudisch conditions is demonstrated by the reporting of a different major product from the same starting material (3) [15,26].

Scheme 3

Three literature procedures for the successful synthesis, isolation and characterisation of 3-substituted-2-nitrosophenols [39,40]. Yields refer to pure, isolated compounds.

Figure 3

The possible tautomers of copper(II) bis(4-methyl-2-nitrosophenol), (2b, 2bi).

Figure 4

The unit cell of the obtained crystal structure of copper(II) bis(4-methyl-2-nitrosophenol) with ethanol as a solvate [41,51].

Figure 5

The suggested structure of Cobalt(III) tris(4-methyl-2-nitrosophenolato) (5a) as specified in the publication by Mahmoud et al. and tautomeric forms (5b,c) indicating potential hydrogen bond donor sites [54].

Scheme 4

The proposed sequence of copper complex formation, as suggested by Baudisch [20].

Scheme 5

The suggested common intermediate complex (7a) between the reactions at different pH. At a pH~3, the favoured path leads to the copper 2-nitrosophenol complex (7b), while at a pH > 4 the preferred pathway leads to catechol (7c).

Scheme 6

Unexpected products obtained from the Baudisch reactions of naphthols, methyl-phenols and catechol, obtained by Maruyama et al. [42].

Scheme 7

Reactions from Maruyama et al. [26,42].

Scheme 8

The mechanism under which phenol is formed from benzene [42,43].

Scheme 9

The accepted mechanism for the Baudisch Reaction with hydroxylamine [26,42].

Figure 6

The results published by Maruyama [26], which appear to contradict other results from Konecny (not quantified), who instead implied that ortho-nitrosation dominated for conditions comparable to the ‘NaNO₂, pH 2.5, Cu(II)’ shown [43]. The product ratios were calculated by obtaining the isolated ligand, then reacting with Cu(II) salt, separation via a preparative TLC method and finally, quantification via absorbance measurements.

Scheme 10

The results of reactions of methyl-substituted phenols show that the favoured product has the methyl group para to the nitroso (n.b. given of course the nitroso must also be ortho to the phenol).

Figure 7

Accepted nitrosation mechanism for phenol in the absence of copper [33,36,63], followed by three examples of our results [41] for copper-mediated nitrosation.

Figure 8

Dessouky’s colourimetry determination of phenylephrine, an example of copper-mediated nitrosation [25].

Figure 9

Reactions of Grignard reagents and reductions—Mustafa and Kamel [68].

Figure 10

General epoxidation procedure and alkenes tested, with isolated yields given in the table for all four substrates and four catalysts [30].

Figure 11

An adapted diagram to show the Catalytic reduction of CO₂ to methanol. Overall equation: CO₂ + 3H₂ → CH₃OH + H₂O [72].

Figure 12

Reactions reported by Charalambous et al. [77].

Scheme 11

The general procedure reported for benzoxazole synthesis [75].