Decarbonylation ring opening

The thermal intramolecular 2 + 2-cycloaddition of 4,4,-disubstituted-2,2/-bis(phenyl-ethynyl)biphenyls (1) yielded the intermediate l,2-diphenylcyclobuta[l]phenanthrenes (2), which could be trapped with 2,3,4,5-tetraphenylcyclopenta-2,4-dione (3) to produce the Diels-Alder adduct (4). Thermal decarbonylative ring opening of (4) gave 9,10,11,12,13,14-hexaphenylcycloocta[l]phenanthrenes (5) as the final product in 12-23% yield (Scheme l).1... 

Uses. Furfural is primarily a chemical feedstock for a number of monomeric compounds and resins. One route produces furan by decarbonylation. Tetrahydrofuran is derived from furan by hydrogenation. Polytetramethylene ether glycol [25190-06-1] is manufactured from tetrahydrofuran by a ring opening polymeri2ation reaction. Another route (hydrogenation) produces furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-methylfuran, and 2-methyltetrahydrofuran. A variety of proprietary synthetic resins are manufactured from furfural and/or furfuryl alcohol. Other... 

The authors also considered two additional reaction pathways for the o-QM decarbonylation to fulvene, both via ring opening (Scheme 2.19).23... 

Insertion of the alkyne into the chromium carbene bond in intermediate B affords vinyl carbene complex D, in which the C=C double bond may be either (Z) or (E). A putative chromacydobutene intermediate resulting from a [2+2] cydoaddition of the alkyne across the metal-carbene bond on the way to chromium vinylcarbene D, as was sometimes suggested in early mechanistic discussions, has been characterized as a high energy spedes on the basis of theoretical calculations [9c]. Its formation and ring-opening cannot compete with the direct insertion path of the alkyne into the chromium-carbene bond. An example of an (E)-D alkyne insertion product has been isolated as the decarbonylation product of a tetracarbonyl chromahexatriene (4, Scheme 4) [14], and has been characterized by NMR spectroscopy and X-ray analysis. 

At temperatures low the decarbonylation barrier, stable dimers are observed. Thus, thermolysis of diphenylcyclopropenone at 150°C or in refluxing toluene leads to the dimer 48 . The formation of 48 can be rationalized as a dipolar addition of the ring-opened species 49 to the C = O group of a second molecule (equation 40). The codimerization of... 

Decarbonylations of furfuraldehyde to furan continue to be of commercial interest and various new catalysts have been recommended.253 Decarboxylations are still occasionally useful,254 and the selective decarboxylation of furan-3,4-dicarboxylic acid to furan-3-carboxylic acid is said to be much improved by omitting any solvent.255 The easy decarboxylation of furan acetic acid derivatives is formulated in structure 139, although the acidic conditions need not preclude ring opening.2553... 

Although 1-methylcyclopropene reacted with bis(trifluoromethyI)ketene in nonpolar solvents by an ene-type reaction, in polar solvents, such as acetonitrile, a fluorinated diene 2 was produced, apparently by electrophilic ring opening followed by decarbonylation of an intermediate cyclopropanone. ... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

---