Carbonyl ylides and carbenes from thermolysis of oxadiazolines. Substituent effects on intramolecular and intermolecular reactions of carbonyl ylides

Abstract

Thermolysis of a 2-methoxy-Δ3-1,3,4-oxadiazoline involves loss of N2 with formation of a carbonyl ylide. The fate of the carbonyl ylide depends on its environment and on the other substituents present. Thus, the ylides from 2-methoxy-2,5,5-trimethyl-Δ3-1,3,4-oxadiazoline (1) and from 2-methoxy-2-(4-methoxyphenyl)-5,5-dimethyl-Δ3-1,3,4-oxadiazoline (2) are trapped very efficiently by methanol. However, the ylide from 1 is trapped much less efficiently than that from 2 by dimethylacetylene dicarboxylate, cis-1,2-dichloroethylene, or norbornadiene. A major competitive process in the case of 1 is fragmentation of the ylide to carbonyl compounds and carbenes, the latter being trapped by alkenes to form cyclopropanes. An intramolecular 1,4-H transfer is also competitive under some conditions. The ylide from 2 does not appear to fragment, nor does it undergo the 1,4-H transfer, but it cyclizes efficiently to the oxirane in the absence of trapping agents.Preliminary estimates of rate constants for cyclization of the ylide from 2 to form the oxirane [Formula: see text] and for its additions to norbornadiene and to dimethylacetylene dicarboxylate (1 × 105 M−1 s−1 and 1 × 109 M−1 s−1, respectively) are reported. If it is assumed that the ylide from 1 would add to dimethylacetylene dicarboxylate with a similar rate constant, then the yield for that process can be used to place a lower limit of 1010 s−1 on the rate constant for fragmentation of that ylide at 31 °C.

Cascade Processes of Metallo Carbenoids

“…Although a variety of methods exist for dihydro- and tetrahydrofuran synthesis, , few of these are based on an annulation strategy, and, of those that are, single-step procedures are uncommon. , Conceptually, the 1,3-dipolar cycloaddition of carbonyl ylides with π-bonds represents an attractive strategy for tetrahydrofuran formation . Common methods for carbonyl ylide generation involve the thermolysis or photolysis of epoxides possessing electron-withdrawing substituents, − the thermal extrusion of nitrogen from 1,3,4-oxadiazolines, − and the loss of carbon dioxide from 1,3-dioxolan-4-ones . One of the simplest methods for generating carbonyl ylides involves the addition of a metallo carbenoid onto the oxygen atom of a carbonyl group.…”

Intramolecular Cycloadditions of Photogenerated Azaxylylenes with Oxadiazoles Provide Direct Access to Versatile Polyheterocyclic Ketopiperazines Containing a Spiro-oxirane Moiety

“…Extrusion of molecular nitrogen from less strained monocyclic oxadiazolines yields oxiranes, Scheme . This mode of action is not known for bicyclic oxadiazolines, plausibly because cycloaddition reactions of oxadiazoles producing such bicyclic intermediates were explored primarily in the hetero-Diels–Alder, i.e., [4 + 2] context, thus restricting the topology of oxadiazoline intermediates to a particularly strained oxadiazabicyclo[2.2.1] core, where the formation of 5-oxabicyclo[2.1.0]pentanes is not feasible.…”

Reaction of carbonyl ylides with chloroform

“…The hydrazones were recrystallized from absolute ethanoVpetroleum ether. Alk~xy-2,5,5-trialkyl-~~-1.3,4-oxadiazolines, Z Compounds 1 were prepared by oxidative cyclization of acyl hydrazones of ketones with lead tetraacetate in alcohol R~O H or in a solution of that alcohol in methylene chloride (9,10). They were purified, first by methanolysis (MeO-, MeOH) of the 2-acetoxy oxadiazoline that is formed as a by-product, and then by bulb-to-bulb distillation at ca.…”

“…cycloadditions to dipolarophiles ( 3 4 ) , 1,4-sigmatropic hydrogen shifts to form en01 ethers (7)(8)(9), fragmentations to carbonyl compounds and carbenes (9-1 l), and reactions with alcohols to form ketals (10, 12, 1 3 ) .~ The latter reactions involve (formally) the protonation of one ylide carbon and alkoxylation of the other. To the best of our knowledge analogous reactions of carbonyl ylides with acids weaker than alcohols or water have not been reported nor are there any examples of such reactions of ylides with carbon acids of any pKa.…”