Alkynes oxidative rearrangement

Oxidative Rearrangements Toste et al. recently developed various oxidative rearrangements of alkynes using sulfoxides as stoichiometric oxidants through carbenoid intermediates. These reactions could provide an entry into products that contain a carbonyl group susceptible to further functionalization [142] (Scheme 8.26). ... 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

Alkynes upon heating with HTI in methanol underwent an oxidative rearrangement to methyl carboxylates which were hydrolysed in situ to afford carboxylic acids a typical procedure is given. 

Background information on the chemistry of oxirenes is presented in Chapter 1.03, and this area has been reviewed by Zeller <2002SOS(9)19>. The potential antiaromatic character of oxirenes, the strain of the unsaturated three-membered ring, and the possible involvement in important reactions (e.g., oxidation of alkynes, Wolff rearrangement)... 

Oxidative rearrangement. (PhIF)OTf induces rearrangement of nonterminal alkynes in an alcohol to give oi-branched esters. 

Oxidative cleavage of acetylenes RC=CH (R = Ph, Bu, hexyl, heptyl, cyclopentyl or PhCH2CH2) with bis[trifluoroacetoxy)iodo]pentafluorobenzene in wet benzene gives carboxylic acids RC02H. Alkynes 263 (R, R = H, Me, Et, Pr, Bu or Ph) undergo an oxidative rearrangement by the action of [hydroxy(tosyloxy)iodo] benzene in methanol to yield the esters 264. ... 

Reaction conditions have been developed for a metallonitrene-initiated alkyne oxidation cascade with intermolecular cascade termination by ylide formation/[2,3] Wittig rearrangement upon reaction of alkyne 245 with enantioenriched allyl ethers 246 to provide heterocyclized AT-sulfonyl imine products 247 efEciendy (13AGE5836). 

An oxidative rearrangement of r-alcohols mediated by m-CPBA generates tetrasubsti-tuted alkenes with a carboxylic acid substituent. The proposed mechanism of the reaction involves the epoxidation of the alkyne forming an oxirene that undergoes a 1,2-aryl shift. " ... 

Gold-catalysed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant has been reported to involve a gold carbene generation via alkyne oxidation. DFT studies suggest that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts-type cyclization (Scheme 12) ... 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

The reaction using 11a as a substrate in the presence of several oxides as additives revealed that addition of tributylphosphine oxide, hexamethylphos-phoric triamide, and dimethyl sulfoxide all accelerate the reaction considerably. Furthermore, when about 10 molar amounts of N-methylmorpholine M-oxide (NMO) is added to the alkyne-cobalt complex 12b in THF,the reaction proceeds even at room temperature and cyclopentenone 13 b is obtained in 37% yield accompanied by another rearranged product, the methylenecyclobutanone 35, obtained in 23% yield as a mixture of ( )-and (Z)-isomers (Scheme 14). These facts indicate that dissociation of the carbonyl ligand of the alkyne-cobalt complex 12 is the rate-determining step in this rearrangement. This is also supported by the fact that under a CO atmosphere in refluxing THF the reaction is completely suppressed. 

---