Formyl shifts

Attempts to use acetic-formic anhydride with Friedel-Crafts catalysts resulted only in acetylation. However, using anhydrous HF as a catalyst, a small amount of aldehyde is also formed in accordance with the fact that acetic—formic anhydride gives both acetyl and formyl fluoride with HF. By continuous removal of the low boiling HCOF, the reaction can be shifted to the formation of this compound (118). 

Thiophene, 3-pentadeuterophenyl-chemical shifts, 4, 730 Thiophene, 2-phenyl-oxidation, 4, 800 phototranspositions, 4, 743 rearrangement, 4, 42 reduction, 4, 775 synthesis, 4, 865, 914 UV spectrum, 4, 735 Thiophene, 3-phenyl-photochemical rearrangements, 4, 735 phototranspositions, 4, 743 lsmeier formylation, 4, 759 Thiophene, 2-pivaloyl-Birch reduction, 4, 775 Thiophene, polybromo-reactivity, 4, 829 Thiophene, polylithio-synthesis, 4, 831 Thiophene, (propargylthio)-rearrangement, 4, 746 Thiophene, 2-(3-pyridinyl)-synthesis, 4, 781 Thiophene, 2-(5-pyrimidinyl)-synthesis, 4, 781 Thiophene, 3-pyrrolidinyl-cycloaddition reactions, 4, 68 with dimethyl acetylenedicarboxylate, 4, 788-789... 

Cieplak apphed this effect to the explanation of the shift of selectivity in the reactions of the pentamethylcyclopentadienes 21,19, and 20 having substiments of formyl, vinyl, and (hydroxyimino)methyl moiety at the 5-positions [14]. He pointed out that the observed result is consistent with the notion that in the case of formyl moiety the hyperconjugative effect is enhanced by lone pair back-donation due to ... 

The Stille coupling reaction is very versatile with respect to the functionality that can be carried in both the halide and the tin reagent. Groups such as ester, nitrile, nitro, cyano, and formyl can be present, which permits applications involving masked functionality. For example, when the coupling reaction is applied to l-alkoxy-2-butenylstannanes, the double-bond shift leads to a vinyl ether that can be hydrolyzed to an aldehyde. 

Although the Vilsmeier reaction is known best in aromatic systems, aliphatic olefins also undergo formylation. Synthesis of forwocortal (257) involves such a step. Formation of the monoketal of 255 involves the 3-ketone function with the familiar concomitant shift of the double bond to C-5,6. 

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

The extreme downfield shift of the intermediate signal is characteristic either of a metal formyl, or a hydridocarbene complex, suggesting one of the following structures (or, perhaps, a rapid equilibrium between the two) ... 

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... 

It is considered that [1,5] alkyl shifts usually require temperatures above 330 °C and proceed with free energies of activation greater than 180 kJmol-1. The 1,5-migratory aptitude of the formyl group is comparable with that of a trimethylsilyl group. However, 5-acetyl- and 5-ethoxycarbonyl-PMCPDs under the same conditions show a temperature-invariant XH NMR spectra, i.e. the migratory aptitude decreases in order CHO COMe COOMe114. 

It is reasonable to assume that the initial step in the cycloaddition reaction is an electrophilic attack by the carbene on the nitrogen atom to form the A -ylid. Where proton shift is possible, cyclization does not occur and the A-ylid produces the N-formyl compound (Scheme 7.30) [36]. 

The primary products are methyl and formyl radicals [36, 37] because potential energy surface crossing leads to a H shift at combustion temperatures [35], It is rather interesting that the decomposition of cyclic ethylene oxide proceeds through a route in which it isomerizes to acetaldehyde and readily dissociates into CH3 and HCO. Thus two primary addition reactions that can be written are... 

Substituting a formyl, carboxy, or carbalkoxy group into a thieno-thiophene or selenophenothiophene molecule has no substantial effect on Ae coupling constants but considerably affects the chemical shifts (see Litvinov and Fraenkel and Michael and Gronowitz ). The chemical shifts of the protons in the selenophene ring are characteristic of different t rpes of condensation of the thiophene and selenophene rings and increase in the order 15 > 14 > 16 irrespective of the solvent used (acetone, CCIJ. A similar sequence is observed in the carbonyl derivatives of the isomeric selenophenothiophenes. 

The H and nuclear magnetic resonance (NMR) chemical shift of all the parent structures are fully reported in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, the complete proton and carbon chemical shift assignments have been made for 2- and 3-formyl, acetyl, or methyl phenoxathiin <1996PJC36>. 

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