Synthesis of Carbonyl Colorants

The synthesis of carbonyl colorants uses a wide diversity of chemical methods, in which each individual product essentially has its own characteristic route. This is in complete contrast to the synthesis of azo dyes and pigments (Chapter 3) where a common reaction sequence is universally used. The subject is too vast to attempt to be comprehensive in a text of this type. The following section, therefore, presents an overview of some of the fundamental synthetic strategies which may be used to prepare some of the more important types of carbonyl colorants. 

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The mono- and dinuclear carbonyl metalates of Cr, Mo, and W, first prepared by us using liquid NHS, have also been of great importance for the synthesis of numerous neutral complexes not accessible by other routes. Special attention is drawn to the deeply colored and very unstable mono- and dinuclear paramagnetic complexes Cr(CO)sI, Cr (CO),oI, Cr(CO)5CN, Cr(CO)sNCS, Cr COl.oNCS, and Cr(CO)sSH. 

Coupling carbonyl compounds with aromatic amines bearing amino, hydroxy, or thiol groups in o -positions is known to be a general method for the synthesis of 1,3-benzazoles, with the intermediate formation of the respective 2,3-dihydro derivatives. No heterocyclization occurs, however, in the reaction of o-phenylene-diamines or o-aminophenols with /i-quinones. In these cases, the reaction stops at the stage of formation of the deeply colored quinoneimine 105 which is similar to that obtained by the previously studied reaction of a variety of / -substituted anilines with derivatives of p -bcn/oquinonc.6 Compounds 10 do not undergo cyclization under heating or irradiation of its solution (Scheme 4). 

An interesting synthesis of methyl 2,4-diphenyloxazole-5-carboxylate has been reported by Arnold and Chang.180 When l-carbomethoxy-4-phenyl-2,2,3,3-tetramethyl-5-oxabicyclo[2.1. 0 Jpentane (115) was heated to 120° in the presence of benzonitrile, a good yield of the oxazole was obtained, together with tetramethylethylene. The colored carbonyl ylide (116), formed upon heating, adds a molecule of benzonitrile, presumably to form the adduct 117 which via a retro-Diels-Alder reaction gives the products. 

The heavily halogenated napthalenones were synthesized well over 100 years ago, but the structural details were sorted out less than 20 years ago. The compound, 2,3,4,4-tetrachloro-l-(4fl)-naphthalenone (I), was discussed in the introduction to this sequence as an example of a substance that gains its photochromic activity by bond homolysis. In the synthesis of I two tetrachloro isomers were originally isolated. Compound I has photochromic activity while the second isomer (II) is simply a yellow-colored material. The structure of II has been determined to be 2,2,3,4-tetrachloro-l-(2TZ)-naphthalenone. A key piece of physical evidence that allowed the assignment of the structures was the infrared frequencies of the carbonyl groups. These frequencies were found to be 1701 and 1675 cm . Which wavenumber value belongs with which structure ... 

While the chemistry of metal carbonyl complexes has enjoyed a rather long and colorful history, being extensively studied and widely reviewed (7-3), the synthesis and reactivity of the group 4B (Ti, Zr, Hf) metal carbonyls have developed relatively slowly. Although the first well-characterized group 4B metal carbonyl complex, bis(i7-cyclopentadienyl)-dicarbonyltitanium (1), was reported by Murray of Monsanto Co. in... 

We do not claim that the Co(III) method is superior to modern methods of chemical synthesis. However it does provide an alternative. The Co(III)-active ester, once made, can be stored for long periods of time, it provides both N-terminal protection and carbonyl-O activation in the one system, it is orange in color (e480 —100 M l cm"1), generally quite water soluble (it is a salt ), and the Co(III) metal plus ancillary ligands can easily be removed by chemical or electrochemical (—1.0 V vs SCE) reduction methods. 

Reaction of ketyls. The carbonyl group reacts with Smlj to generate ketyl species which may be reduced further. Capture of the ketyls with suitable reactants expands the utility of the samarium chemistry. Important reactions include butyrolactone synthesis that is amenable to asymmetric induction when chiral acrylic esters are employed. The reagent system (catalytic in Smij) generated in situ from Smij and Zn-Hg is more economical. The system also contains Lil and MCjSiOTf, and in practice MejSiOTf is added to the mixture of the other components at just the rate to maintain a light blue color (indicating the presence of Smij). When Nilj is added as a catalyst, p-propiolactone instead of acrylic esters can be used. Sometimes steric factors preclude cyclization, and 7-hydroxy esters result. Ketyl addition to acrylamides opens a route to 1,4-amino alcohols. ... 

Zollinger, H., "Ch. 8. Carbonyl Dyes and Pigments," in H. Zollinger, Color Chemistry Synthesis, Properties, and Applications of Organic Dyes and Pigments, 3d ed. Zurich VHCA/Wiley-VCH, 2003. 

Other remarkable stepwise transformations make use of the combination of several biocatalysts to obtain valuable compounds. In particular, a sequential process employing a recombinant enoate reductase (ER) and a carbonyl reductase has been used for the stereoselective redudion of both enantiomers of carvone to dihydrocarveol (Scheme 4.21) [72]. In the first step, hydrogenation of the endocydic C=C double bond of (R)- or (S)-carvone was achieved by ER from Lactobacillus casd (LacER). In the second step, wild-type or mutant reductases from Candida magnolia (CMCR) and Sporobolomyces sahnoni-color (SSCR) were employed, respectively. First, experiments were performed in a sequential manner and later the one-pot synthesis was tested with a substrate concentration of 0.1 M, obtaining the final compound with excellent conversions and de. 

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