Oxocarbon

Although this classic picture evolved from "soft, mononuclear transition metal complexes suffices to explain a great deal of carbon monoxide chemistry, it is not clear that it is complete or accurate for understanding processes whereby CO is reduced, deoxygenated, and/or polymerized to form methane, long-chain hydrocarbons, alcohols, and other oxocarbons, especially in cases where heterogeneous catalysts or "hard" metals are involved (6, 7, ,9,J 0). This deficiency of information has led to the search for new modes of carbon monoxide reactivity and to attempts to understand carbon monoxide chemistry in nontraditional environments ... 

A recent theoretical investigation of oxocarbon dianions suggested that the dianion of the cyclic carbon monoxide trimer, (CO)32-, is aromatic.79... 

All of these oxocarbon anions are aromatic according to simple molecular orbital calculations. The aromatic stabilization of the anion is apparently responsible for the fact that squaric acid (HjC O,) is about as strong as sulfuric acid.104 There is a considerable and growing body of knowledge of the chemistry of these systems, but most of it is probably more appropriate to a discussion of organic chemistry.105... 

Fig. 16.35 Cyclic oxocarbon anions (a) squonttc (b) croconutc (c) rhodizonale.

Note that oxalic acid containing carbon in a comparable oxidation stale but not aromatic has a Ki approximately equal to A for squsne and sulfuric acids, and Kj for oxalic odd is three orders of masraiude smaller. For a difference of opinion coneeraing the aromaticity of the oxocarbon anions, see Aihura. J. J. Am. Chem. Soc. 1981.I0S. 1633-1635... 

This chapter is organized according to the nature of X in the grouping X—COf (Table 1) and also includes oxocarbon systems as exemplified by squaric acid, C4H204. 

Table 8 Commonly Occurring Cyclic Oxocarbon Acids...

Protonation of homologous oxocarbon compounds (compounds in which all carbon atoms are bonded to carbonyl or enolic oxygens or their hydrated or deprotonated... 

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... 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

As an illustrative example taken from the current literature, consider the variation of C-C and C-O bond lengths in the deltate species CsO2- held within a dinuclear organometallic uranium(IV) complex. In a remarkable synthesis, this cyclic aromatic oxocarbon dianion is generated by the metal-mediated reductive cyclotrimerization of carbon monoxide, as indicated in the reaction scheme... 

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... 

The self-assembly methodology can be extended to linkers containing oxygen. Oxalate 62 and oxocarbon dianions 63 and 64 are rigid molecules that react immediately with tecton 31 in water/acetone to form neutral metallomacro-cycles 65, 66, and 67, respectively, in 90-98% yields <2005IC7130>. 

Squaric acid [42], which belongs to a family of oxocarbons (Serratosa, 1983 Seitz and Imming, 1992), consists of two s-trans forms of 3-hydroxyenone [43] structures perpendicular to each other, and it shows extremely strong acidity (pATa, = 0.54) as an organic acid (West, 1980). Crystalline squaric acid is the only organic substance whose dielectric property has been shown to be... 

The most dramatic synthetic achievement with cyclobutenediones is the synthesis of deltic acid derivatives. As illustrated below, photolysis43 of diethyl squarate (16 d) yielded, among other products, diethyl debate (21). The bis-trimethylsilyl ester (16 e) of squaric acid similarly furnished a deltate ester which was successfully hydrolysed to deltic acid itself (22), the lowest member of the series of oxocarbon acids48). 

---