The enol of isobutyric acid

We present the gas-phase synthesis of 2-methyl-prop-1-ene-1,1-diol, an unreported higher energy tautomer of isobutyric acid. The enol was captured in an argon matrix at 3.5 K, characterized spectroscopically and by DFT computations. The enol rearranges likely photochemically to isobutyric acid and dimethylketene. We also identified propene, likely photochemically formed from dimethylketene.


The Enol of Isobutyric Acid
Akkad Danho, Artur Mardyukov, and Peter R. Schreiner*     Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2024

Experimental Procedures
Matrix Apparatus Design.A Sumitomo cryostat system consisting of an RDK 408D2 closedcycle refrigerator cold head and an F-70 compressor unit was used for matrix isolation experiments.A polished CsI window was mounted in the cold head sample holder.The sample holder, connected with silicon diodes for temperature measurements, was covered by a vacuum shroud, which was equipped with KBr windows to allow for IR measurements.In some experiments BaF2 windows were used due to their higher transparency when measuring UV/vis spectra.The sample and the host gas (Ar, purity of 99.999%) were co-deposited at 3.5 K.All spectral data were collected at 3.5 K.The pyrolysis zone was equipped with a heatable 90 mm long quartz tube (inner diameter 7 mm), controlled by a Ni/CrNi thermocouple.The travel distance of the sample from the pyrolysis zone to the matrix was ∼45 mm.Ar was stored in a 2 L gas balloon, which was evacuated and filled three times before every experiment.The sample was evaporated from a Schlenk tube at 80 °C (water) and reduced pressure (∼3 × 10 - 6 mbar) and co-deposited with a high excess of argon on both sides of the matrix window in the dark (preventing unwanted photochemistry) at a rate of ∼1 mbar min -1 , based on the pressure inside the Ar balloon.Pyrolyses were carried out at 750 °C.IR spectra were recorded between 7000 and 350 cm -1 with a resolution of 0.7 cm -1 with a Bruker Vertex 70 FTIR spectrometer.A spectrum of the cold matrix window before deposition was used as background spectrum for the subsequent IR measurements.UV/vis spectra were recorded between 190 and 800 nm with a resolution of 1 nm with a Jasco V-760 spectrophotometer.A high-pressure-mercury lamp equipped with a monochromator (LOT Quantum Design) or a low-pressure-mercury lamp (Gräntzel) fitted with a Vycor filter were used for irradiation of the matrix during photochemical experiments.
Computations.All DFT computations were performed with the Gaussian 16, 1 Revision C.01 (full citations for electronic structure codes are given at the end of this document) at the B3LYP/def2-TZVP 2-3 level of theory.The keywords Opt and Freq=NoRaman were used for the characterization of minima on the PES.For transition structures the keyword Opt=(ts,tight,calcfc,noeigen) was used.UV/Vis absorptions were computed by using the keyword td(50-50,nstates=10).

Synthesis Diethyl dimethylmalonate-d6
To a solution of 0.550 g (3.43 mmol, 1.00 equiv) diethyl malonate in 7 mL anhydrous N,Ndimethylformamide 1.42 g (10.27 mmol, 3.33 equiv) K2CO3 was added.The reaction was stirred at room temperature for 1 h and 0.440 mL (1.000, 7.03 mmol, 2.05 equiv) iodmethane-d3 was added and the reaction mixture was stirred at room temperature for 4 d.The remaining salts were filtered and the yellow solution was quenched with 6 mL (5% wt) Na₂S₂O₃-solution and extracted with ethyl acetate.The combined organic layers were dried with MgSO4, filtered, and the solvent was removed under reduced pressure, giving 0.454 g (2.32 mmol) of diethyl dimethylmalonate-d6 in 68% yield.The residue contained DMF, yet proved inconsequential for the subsequent step, allowing for the utilization of the crude product without additional purification.
After cooling to room temperature the reaction mixture was acidified with conz.HCl-solution to a pH of 2 and extracted with acetone.The combined organic layers were dried with MgSO4, filtered, and the solvent was removed under reduced pressure, giving 0.180 g (1.30 mmol) of dimethylmalonic acid-d6 in 84 % yield.

Figure S1 :
Figure S1: IR spectra showing the pyrolysis product of 1 with subsequent trapping in an argon matrix at 3.5 K (a) IR difference spectra showing the photochemistry of 4 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to 4 disappear after 20 min irradiation time.(b) IR spectrum of 4 computed at B3LYP/def2-TZVP.(c) IR difference spectra showing the photochemistry of d1-4 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to d1-4 disappear after 20 min irradiation time.(d) IR spectrum of d1-4 computed at B3LYP/def2-TZVP.

Figure S2 :
Figure S2: (a) IR difference spectra showing the photochemistry of 1 after irradiation with λ = 254 nm in argon at 3.5 K. (b) IR difference spectra showing the photochemistry of d2-1 after irradiation with λ = 254 nm in argon at 3.5 K.

Figure S3 :
Figure S3: IR spectra showing the pyrolysis product of 1 with subsequent trapping in an argon matrix at 3.5 K. (a) IR difference spectra showing the photochemistry of 2 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to 2 disappear while upward bands assigned 3 appear after 20 min irradiation time.(b) IR spectrum of 2 computed at B3LYP/def2-TZVP.(c) IR difference spectra showing the photochemistry of d2-2 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to d2-2 disappear

Figure S4 :
Figure S4: IR spectra showing the pyrolysis product of 1 with subsequent trapping in an argon matrix at 3.5 K. (a) IR difference spectra showing the photochemistry of 2 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to propene appear after 20 min irradiation time.(b) IR spectrum of 2 computed at B3LYP/def2-TZVP.(c) IR difference spectra showing the photochemistry of d6-2 after irradiation with λ = 254 nm in argon at 3.5 K. Downward bands assigned to d6-propylene appear after 20 min irradiation time.(d) IR spectrum of d2-2 computed at B3LYP/def2-TZVP.