Croconates

Infrared-absorbing dyes 88 have been prepared from croconic acid and 4-methyl-chalcogenopyrylium salts. Red shifts of 7 , are noted as the size of the heteroatom increases and range from 845 nm for (88 X = O) to 1081 nm for (88 X = Te) . 

By a similar reaction phenanthraquinone yields biphenylene-glycollic acid. (The equation should be written.) The benzilic acid rearrangement also plays a part in many other reactions (croconic acid, purpurogallin). 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

The squarylium (4.6) and croconium (4.7) dyes are closely related structurally to cyanines but are in fact donor-acceptor molecules and consequently the design principles of near-IR absorbers based on these chromophores are different. The synthesis of these chromophores is achieved easily by reacting either squaric acid, or preferably an alkylated derivative, e.g. di-n-butyl squarate, or croconic acid with electron-donor molecnles. The croconium dyes absorb at significantly longer wavelengths than the sqnarylinms as shown in Figure 4.2. 

Deltic acid Squaric acid Croconic acid Rhodizonic acid Tropolone... 

Carbon-13 NMR spectra of the H2C On series, such as deltic, squaric, croconic and rhodizonic acids, obtained in anhydrous solvents [304] display carbonyl shifts similar to those reported for quinones (Table 4.33). Considerable shielding of the carbonyl carbon of deltic acid diethyl ester is not only attributed to the three-membered ring but also to an electron releasing effect of the ethoxy groups. 

As shown above, di-tert-butoxyethyne 1s the only acetylenic diether prepared so far whose stability allows its use as a synthetic Intermediate. It has been used 1n the synthesis of all the members of the series of monocyclic oxocarbons (deltic, squaric, croconic, and rhodizonic acids), as well as in the synthesis of semisquaric acid, the parent compound of the natural mycotoxins, noniliforndna... 

The reaction of croconic acid with 2,6-di-f-butyl-4-methylthiopyrylium salts yields the croconate dye 623 which absorbs in the infrared at 950 nm compared with 845 nm for the pyrylium analogue. This croconate dye is quite soluble in dichloromethane (180 mg ml-1), a feature that renders it attractive for thin film applications <2000JOC2236>. 

Sterically hindered seleno- and telluropyrylium salts 40 react with squaric acid 41 <1993H(35)1149> and also with 4,5-dihydroxy-4-cyclopentene-l,2,3-trione (croconic acid) 42 <2000JOC2236> in the presence of base to afford squarylium and croconate dyes 43 and 44, respectively (Equations 11 and 12). Dyes of this type have important industrial applications (Section 7.11.8.2). 

The rhodizonate dianion C60g (Fig. 20.4.19) is a member of a series of planar monocyclic oxocarbon dianions C 0 (n = 3, deltate n = 4, squarate n = 5, croconate n = 6 rhodizonate) which have been recognized as nonbenzenoid aromatic compounds. However, this six-membered ring species... 

In the host lattice of [( -C3H7)4N+]2C505 3(NH2)2C0-8H20, the croconate anion resides in a rather symmetrical hydrogen-bonding environment (Fig. 20.4.23) and thus its measured dimensions are consistent with its expected charge-delocalized Z>5h structure in the ground state, as shown in... 

I (n (11/ U N 14 s v -3(NH2)2C0-8H20. The slanted vertical (urea dimer—croconate—urea)co chain constitutes a side wall of the [110] channel system. The parallel [urea dimer-tFLOLlx, ARAB... and [croconate-urea-(H20)4]oo DCEDCE... ribbons define the channel system in the c direction. From C.-K. Lam, M.-F. Cheng, C.-L. Li J.-P. Zhang, X.-M. Chen, W.-K. Li and T. C. W. Mak, Chem. Commun., 448-9 (2004). 

Hydrogen-bonding environment of the croconate dianion in the crystal structure of[(C2H5)4N+]2C502-.3(CH3NH)2CO. 

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