Time-Resolved IR Detection and Study of an Iminooxirane Intermediate

Abstract

[reaction: see text] Iminooxirane 4 has been generated by the reaction of phenylchlorocarbene (2) with phenyl isocyanate (3) and studied by nanosecond time-resolved infrared spectroscopy. Iminooxirane 4 (1830 cm(-1)) isomerizes to alpha-lactam 5 (1880 cm(-1)) at an observed rate of 3 x 10(4) s(-1). Peak assignments were confirmed by a combination of B3LYP calculations and isotopic labeling. An acyclic transition state was found computationally for the isomerization reaction.

Computational studies on the formation of aza‐oxypentadienyl intermediates from alkylidene oxaziridines and keteneimine oxides and their conversion to 1,5‐dihydropyrrolones

“…In contrast to the vinyl allene oxide case (Scheme B), no transition structures for the one‐step rearrangement of 1a to 3a or 2a could be characterized. This failure also contrasts with the characterization of the reverse concerted rearrangement of iminooxirane VI to aziridinone VII (Scheme A) in the gas phase and points to the role of the vinyl group in altering the reaction pathway by providing extended conjugation to the forming intermediate species. Therefore, independent mechanistic pathways to afford 3a were studied for the constitutional isomers 1a and 1’a (see S.I.…”

“…With a few exceptions involving species stabilized by strong steric and electronic effects of the substituents, the alkylidene-substituted threemembered heterocycles [7] have often been invoked as fleeting intermediates in reactions of cumulene-containing reactants or by rearrangement of structural isomers. [9,10] After formation these species undergo rearrangements due to their highly strained nature that start with C-C or C-X ring-opening reactions, which are more favorable processes for small heterocyclic compounds than for the parent hydrocarbons. In these structures, stereoelectronic effects determine to a large extent the reactivity patterns as a result of the orientation of the electron pairs relative to the atoms of the ring.…”

“…[9] However, only in recent years the formation of a species with structure VI (Scheme 2) by carbene insertion into a phenyl isocyanate precursor and its conversion into aziridinone (α-lactam) VII has been confirmed through the detection of the functional groups using nanosecond time-resolved IR spectroscopy. [10] B3LYP/6-31-G* calculations in the gas phase located a one-step transition structure for this formal transfer of methylene carbon from O to N. [10] Aziridinone VII was computed to be thermodynamically more stable than the vinyloxiran-2-imine VI. Moreover, similarly to the easy interconversion between vinyl allene oxide species Ib and cyclopropanones IIb previously studied,4b,5,6b aziridinones IIc could presumably interconvert with Ic and I'c via the intermediacy of acyclic species…”

“…The energy of activation for the concerted rearrangement of iminooxirane VI to aziridinone VII (Scheme 2) was estimated to be approximately 23 kcal/mol in the gas phase. [10] Preliminary ab initio computations (at the SCHEME 1 (A) Rearrangement of compounds with a parent allylidene-substituted three-membered ring. (B) Rearrangement of vinyl allene oxide Ib via one-step and stepwise pathways, the latter through oxypentadienyl intermediate IVb.…”

“…SCHEME 2 (A) Conversion of iminooxirane VI into aziridinone VII. [10] (B) Examples of (4 + 3)-and (3 + 2)-cycloaddition reactions of azaoxyallylic zwitterions IX with butadiene X and C3-substituted indoles XII, to produce XI and XIII, respectively [13,14] SCHEME 3 Two possible mechanisms, of one-step and stepwise nature, postulated for the rearrangement of (E)-3-(but-3-en-2-ylidene)-2-methyl-1,2-oxaziridine 1a and (R,E)-N,3-dimethyl-3-vinyloxiran-2-imine 1'a species to afford 3-methyl-1,5-dihydro-2H-pyrrol-2-ones 3a and/or 1,3-dimethyl-3-vinylaziridine-2-one 2a, and their N-OMe analogues (1b-4b) All geometry optimizations, IRC analysis and wavefunction calculations have been carried out using the Gaussian 09 [19] suite of programs. These calculations were performed using the hybrid meta-GGA exchange correlation functional M06-2X, developed by Truhlar and co-workers.…”