Carbonyl ylides, aryl cyclizations

Laser flash photolysis of 18 (Ar = -N02C6H4) led to the formation of the aryl chlorocarbene detected at 320 nm. In the presence of acetone, a new species was observed at X = 590 nm that was assigned the structure of carbonyl ylide 19. The ylide, formed by attack of acetone on the carbene, was shown to be irreversible, where the lifetime of the ylide (1.35 ps, fecyciization = 7.40 x 10 s ) was controlled by cyclization to the aryl epoxide 22. The rate constant for the cycloaddition of substituted benzaldehydes to produce dehydrodioxolane (21) was determined experimentally (e.g., p-ClPhCHO=6.16 x 10 M s ). 

Dittami et al. (170,171) was able to generate a carbonyl ylide via photocycliza-tion of an aryl vinyl ether (Scheme 4.85). The photocyclization proceeded through a six-electron rearrangement providing initially the carbonyl ylide, then subsequent to the cyclization, a dipolar cycloaddition takes place with a pendant olefinic tether. 

There are very few examples of photolysis being used for preparation of a carbonyl ylide. The Dittami protocol follows work completed from his lab with aryl vinyl sulfides. Photolysis, followed by cycloaddition, led to the cycloadduct 305 in excellent yield and stereoselectivity. If the aryl vinyl ether 304 was subjected to irradiation in a mixed solution of toluene-methanol at 366 nm rather than a single solvent of toluene, cyclized product was obtained, but no cycloadduct was formed. If a simple phenyl aryl ether was subjected to the same tandem conditions, the cyclized product was generated, but no cycloadduct was detected. 

Multicomponent reactions, which have been known for over 150 years, are those chemical transformations in which more than two reactants combine in a sequential manner to give highly selective products that retain majority of the atoms of the starting materials [36]. Because of the inherent molecular diversity, efficiency, and atom economy, they are highly sought after by chemists and have been widely used in the synthesis of some heterocyclic compounds. Hu and coworkers reported one unique example wherein the electron-rich aryl aldehydes selectively formed a carbonyl ylide with phenyldiazoacetate, which successively cyclized with electron-deficient aryl aldehydes to give the dioxolane (Scheme 2.22) [37]. 

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