Formyl cations

Protonation of formic acid similarly leads, after the formation at low temperature of the parent carboxonium ion, to the formyl cation. The persistent formyl cation was observed by high-pressure NMR only recently (Horvath and Gladysz). An equilibrium with diprotonated carbon monoxide causing rapid exchange can be involved, which also explains the observed high reactivity of carbon monoxide in supera-cidic media. Not only aromatic but also saturated hydrocarbons (such as isoalkanes and adamantanes) can be readily formylated. 

The formyl cation, HCO, is also likely to be an intermediate in the modification of the Koch reaction whereby formic acid reacts with olefins to give carboxyhc acids (20) ... 

Vilsmeier type reagent 160 was employed in the direct and efficient synthesis of dimethylformamidrazones from hydrazines . 2-Benzotriazolyl-13-dioxolane (161) has been utilized as a novel formyl cation equivalent . 1-... 

The experimental 13C NMR chemical shift of the simplest member of the acylium ions, the formyl cation [HCO]+ was reported as S = 139.5 (measured under CO pressure of 85 atm) which compares well with the GIAO-MP2 calculated shift of S — 136ppm.116,117 The analogous fluoroformyl cation [FCO]+ and protonated fluoroformic acid [FC(OH)2]+ were characterized by 13C NMR spectroscopy, experimentally118 ((5=117.5 and 157.8 ppm, respectively) as well as computationally (S = 118.6 and 170.6ppm, respectively).119... 

When the pyrrolo[l,2-c]oxazole 269 was treated with trimethyl orthoformate in the presence of BF3 Et20, in dichloromethane at — 78 °C, a mixture of compounds was obtained from which the expected 5-dimethoxymethyl derivative, 270, was isolated in poor yield (12%) with another dimethoxylated compound 271 (23%). The formation of 271 could be explained by the addition of the formyl cation equivalent at C-7, followed by the protonation at C-6 of the resulting enamide 272 leading to the electrophilic iV-acyliminium ion 273 (Scheme 40). The regioselectivity of this electrophilic addition of trimethylorthoformate to the silyloxypyrrole 269 at C-7, in a non-vinylogous manner, is unusual <1999TL2525>. 

The simplest system which offers the possibility of protonation on oxygen or carbon is carbon monoxide. The carbon protonated cation, the formyl cation [211], is known to be unstable from... 

However, the details of the formation of the formyl cation seem to be less assured. 

See S. Raugei, M. L. Klein, Journal of Physical Chemistry B, 2001, 105. 8213 for pertinent references to experiment, and their computational study of the formyl cation. 

Other Formylations. Formyl fluoride, the only known stable formic acid derivative, can be used to perform Friedel-Crafts-type acylation to form aromatic aldehydes. The method was developed by Olah and Kuhn.105 Although a number of Lewis acids may be used, BF3 is the best catalyst. It is dissolved in the aromatic compound to be formylated then formyl fluoride is introduced at low temperature and the reaction mixture is allowed to warm up to room temperature. The aldehydes of benzene, methylbenzenes, and naphthalene were isolated in 56-78% yields. Selectivities are similar to those in the Gattermann synthesis ( toiuene benzene = 34.6, 53.2% para isomer). The reacting electrophile was suggested to be the activated HCOF BF3 complex and not the free formyl cation. Clearly there is close relationship with the discussed CO—HF—BF3 system. 

Chloro-l,3-dithiane (797) has been employed as a formyl cation equivalent . The morpholine enamines of a variety of aldehydes and ketones were shown to react with this dithiane to produce the a-(l,3-dithan-2-yl) aldehydes and ketones (798) in good yield (Scheme 186) (77TL2077). In direct analogy with this work, the reaction of enol silyl ethers with 2-ethoxy-l,3-dithiolane in the presence of zinc chloride has been reported to afford half-protected 1,3-dicarbonyl compounds (81TL3243). 

The formyl cation, H—C=0, has been observed and characterized spectroscopically for the first tune it is stable in HF-SbF5 at room temperature if 200 atm pressure of CO is maintained over it.50 A commentary on this work appeared in the same journal issue.51... 

A calculational study of the formyl cation to isoformyl cation (H—0=C) rearrangement reveals that the latter is much the less stable of the two.52... 

According to a recent ab initio molecular dynamic simulation study639 formation of formyl cation 328 is optimally favored in the 1 1 HF-SbF5 solution. No evidence, however, was found for the formation of the isoformyl cation 329 and diprotonated CO. The fast proton exchange observed earlier was suggested to occur between HCO+ and HF(HF) or SbF6. ... 

While studying the reaction of adamantane with carbon monoxide under super-acidic catalysis, formylation (formation of 1-adamantanecarboxaldehyde) was found by Olah and co-workers447 to effectively compete with Koch—Haaf carboxylation (formation of 1-adamantanecarboxylie acid, major product formed in 60-75% yield). On the basis of results acquired by the reaction of 1,3,5,7-tetradeuteroadamantane, formylation was interpreted by insertion of the formyl cation into the tertiary C—H cr-bond [Eq. (5.163)]. 

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