Direct Observation of Ylide and Enol Intermediates Formed in Competition with Wolff Rearrangement of Photoexcited Ethyl Diazoacetoacetate

Abstract

The photoexcitation of α-diazocarbonyl compounds produces ketenes by both concerted and stepwise Wolff rearrangements. The stepwise mechanism proceeds through singlet carbene intermediates which can also participate in bimolecular reactions such as ylide formation with nucleophiles. Here, ultrafast transient infra-red absorption spectroscopy is used to show competitive production of singlet carbene and ketene intermediates from the photoexcitation of ethyl diazoacetoacetate. We provide direct spectroscopic evidence for ylide formation by singlet -carbonyl carbene capture in aprotic nucleophilic solvents (with ylide bands at 1625 cm -1 in acetonitrile, and 1586 cm -1 and 1635 cm -1 in tetrahydrofuran) and report an enol mediated pathway for singlet -carbonyl carbene reaction with alcohols (ethanol or tbutanol) identified by an absorption band at 1694 cm -1 , but find no evidence for a previously proposed ylide pathway. The -carbonyl carbene is monitored using a band with solvent-dependent wavenumber in the range 1627 -1645 cm -1 . A computed two-dimensional cut of the potential energy surface for the reaction of the singlet -carbonyl carbene with methanol shows that the enol forms without a barrier, and that this reaction is promoted by an intermolecular hydrogen bond from methanol to the carbonyl oxygen atom. The corresponding ylide structure lies higher in energy, with a barrierless downhill path to isomerization to the enol.

Sc(OTf)3-Catalyzed [3+2]-Cycloaddition of Diazoacetoacetate Enones and N-Aryl Nitrones: Diastereoselective Synthesis of Functionalized Isoxazolidines with Three Contiguous Stereogenic Centers

“…1 H NMR (400 MHz, CDCl 3 ):  = 8.38 (d, J = 8.5 Hz, 1 H), 8.21 (d, J = 7.1 Hz, 1 H), 8.00 (d, J = 8.1 Hz, 1 H), 7.92 (d, J = 8.2 Hz, 1 H), 7.71 (t, J = 8.0 Hz, 3 H), 7.67-7.57 (m, 2 H), 7.53-7.43 (m, 3 H), 7.38 (t, J = 7.8 Hz, 2 H), 7.27 (d, J = 5.9 Hz, 2 H), 7.07 (t, J = 7.3 Hz, 1 H), 6.19 (d, J = 7.0 Hz, 1 H), 5.69 (d, J = 8.7 Hz, 1 H), 5.34 (t, J = 7.9 Hz, 1 H), 3.21 (s, 3 H). 13…”

“…1 H NMR (400 MHz, CDCl 3 ):  = 7.52 (d, J = 7.6 Hz, 2 H), 7.37 (t, J = 7.5 Hz, 2 H), 7.33-7.21 (m, 4 H), 7.06-7.01 (m, 4 H), 6.94 (t, J = 7.3 Hz, 1 H), 6.87 (dd, J = 8.2, 2.4 Hz, 1 H), 5.33 (d, J = 8.9 Hz, 1 H), 5.17 (d, J = 7.5 Hz, 1 H), 5.05 (t, J = 8.3 Hz, 1 H), 3.79 (s, 3 H), 3.45 (s, 3 H). 13 C NMR (100 MHz, CDCl 3 ):  = 188.7, 160.3, 159.9, 151.9, 140.9, 137. 4…”

“….43 (d, J = 8.9 Hz, 1 H), 5.06 (t, J = 8.0 Hz, 1 H), 3.08 (s, 3 H), 2.87 (sept, J = 7.0 Hz, 1 H), 1.21 (d, J = 6.9 Hz, 6 H). 13 C NMR (100 MHz, CDCl 3 ):  = 188.9, 159.9, 152.0, 149.…”

“…8 Accordingly, they have been applied with great success in the construction of the core skeletons of natural products, 9 mostly due to their propensity to participate in cyclopropanation, 10,11 C-H functionalization, 12 and ylide transformations. 13 Conventional access to structurally complex diazoacetoacetates has been fulfilled by the installation of the diazo group at a late stage during the synthetic planning. Nevertheless, the prerequisites of synthesizing an appropriate precursor and optimization of the conditions for the subsequent diazo transfer have undermined the overall efficiency of the synthetic process.…”

Spirocyclic Amide Acetal Synthesis by [CpRu]‐Catalyzed Condensations of α‐Diazo‐β‐Ketoesters with γ‐Lactams

“…Decomposition of diazo derivatives in the presence of Lewis bases is a recognized strategy to generate ylides efficiently [1–9] . With aldehydes and ketones, but also esters and amides, carbonyl ylides are formed, usually under light irradiation or metal‐catalyzed conditions ( Scheme 1, A ) [8,10–25] .…”