Singlet carbenes incorporating a divalent carbon atom (R–C–R’) have grown from laboratory curiosities and theoreticians’ pet peeves into reagents in the growing field of stable carbene chemistry. Still, the experimental characterization of many simple yet fundamentally important carbenes (e.g., hydroxycarbenes, alkyl carbenes, etc.) is hampered by their high reactivity or lack of precursors.  Hydroxycarbenes have been an unknown class of compounds until 2008, when our group reported the synthesis and characteriza­tion of hydroxymethylene (H–C–OH, 1), whose preparation has been chal­lenging organic chemists for more than 80 years.  The reaction of hydroxycarbene with formaldehyde would be a source of simple sugars (the so-called “formose reaction” in the origin of life theory). Considerable efforts are ongoing to understand the formation and distribution of simple organics in extraterrestrial environments, and the examination of the structures and reactivities of prototypes such as hydroxycarbene may also provide glimpses of the prebiotic earth.

Current work:

Oxathiirane. Peter R. Schreiner, Hans Peter Reisenauer, Jaroslaw Romanski, and Grzegorz Mloston J. Am. Chem. Soc. 2010, 132, 7240–7241. Download

We describe the first preparation of the long sought after parent oxathiirane from sulfine through photochemical rearrangement with light at 313 ± 10 nm in an Ar-matrix at 11 K. Oxathiirane was characterized by the extraordinarily good agreement  of experimentally measured and CCSD(T)/cc-pVTZ  (unscaled) computed vibrational frequencies both for the perhydrogenated and perdeuterated species.  The title molecule is about 10 kcal mol–1 less stable than sulfine, in marked contrast to the isomer energy difference of dioxirane vs. carbonyl oxide (ca. –25 kcal mol–1).  This is due to the strong positive polarization (blue potential) vs. the highly electronegative oxygen atom (red).  The stability ordering and the relative energy differences of carbonyl vs. thiocarbonyl groups underlines the likely role oxathiiranes play in sulfur transfer reactions.

Thermal Reactions of Regioisomeric 1,2,4-Trithiolane S-Oxides. Grzegorz Mloston, Jaroslaw Romanski, Michael L. McKee, Hans-Peter Reisenauer,  and Peter R. Schreiner Eur. J. Org. Chem. 2010, 2132–2137. Download

The products of the gas phase pyrolysis of two regioisomeric 1,2,4-trithiolane S-oxides were collected in an argon matrix at 10 K and studied by means of spectroscopic as well as computational methods. Whereas the main products of the pyrolysis of the ‘symmetrical’ S-oxide were identified as thioformaldehyde S-oxide and thioformaldehyde S-sulfide, the ‘non-symmetrical’ S-oxide gave predominantly dithioformic acid, which exists as a mixture of s-cis and s-trans conformers. We present a rationalization of the reaction pathways including density functional theory computations.

A formal carbon-sulfur triple bond: HCSOH. Peter R. Schreiner, Hans Peter Reisenauer, Jaroslaw Romanski, and Grzegorz Mloston Angew. Chem. Int. Ed. 2009, 48, 8133–8136. Download; Highlight: Neil Withers Nature Chem. 2009. Link; Robert Berger Nachr. Chem. 2010, 58, 7. Blog

Extremely rare: a CS triple bond can be assigned to HCSOH, a new molecule prepared through a photochemical [1,3]H-shift of sulfine H2C=S=O. But does this formal description agree with analyses on the basis of IR vibrations, bond lengths, bond orders, molecular orbitals, and compliance constants? Such types of molecules challenge and refine our current understanding of chemical bonding.

Prototypical Triplet Alkyl Phosphonatocarbenes. Adelina Nemirowski, Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston, and Peter R. Schreiner, J. Phys. Chem. A, 2008, 50, 13244. Download

Abstract.  The current case study focuses on the generation, identification, and characterization of two representative mono- and disubstituted alkyl phosphonatocarbenes by means of matrix isolation techniques in conjunction with density functional theory [B3LYP/6-311++G(d,p)] and coupled cluster [CCSD(T)/cc-pVXZ, X = D, T] computations. The EPR measurements identify both carbenes as triplet ground-state species with D values of 0.660 and 0.623 cm–1 respectively, exhibiting persistency toward intramolecular reactions (the EPR signal observable in perfluoromethylcyclohexane up to around 70 K for the disubstituted molecule). While the reaction of the carbene center of the conformationally rich tetramethyl bisphosphonatocarbene with the CH bonds of the methyl groups leads to phosphaoxetane at room temperature, its fragmentation via a Wittig-type reaction during high vacuum flash pyrolysis (HVFP) results in dimethyl vinylphosphonate and methyl metaphosphate. The latter has been observed for the first time as an isolated entity.

Electronic Stabilization of Ground State Triplet Carbenes. Adelina Nemirowski and Peter R. Schreiner, J. Org. Chem. 2007, 72, 9533. Download

    Abstract.  Based on systematic ab initio (CCSD(T)/cc-pVDZ) studies of substituent effects, we present a concept for the construction of electronically stabilized triplet ground state carbenes with singlet−triplet energy separations (∆E_ST) exceeding that of methylene. Sterically demanding and conjugating substituents were excluded from the selection of model compounds under investigation, as these either destabilize both the singlet and the triplet states or delocalize unpaired spins away from the carbene carbon. Negative partial charges on the carbene center allow for the prediction of the electronic stabilization of substituted carbenes. To decrease carbene reactivity, we chose b-substituents with strong polar bonds. Among them, highly electronegative elements such as fluorine and oxygen enlarge the ∆E_ST value with respect to hydrogen, while chlorine does not due to p-orbital participation.

Dimethoxycarbene: Conformational Analysis of a Reactive Intermediate. Hans Peter Reisenauer, Jaroslaw Romanski, Grzegorz Mloston, and Peter R. Schreiner, Eur. J. Org. Chem. 2006, 4813. Download

    Abstract.  Dimethoxycarbene was prepared from an oxadiazoline precursor under high-vacuum flash pyrolysis (HVFP) conditions and was trapped at low temperatures by matrix isolation techniques (Ar, 10 K). The excellent agreement between the computed [CCSD(T)/cc-pVDZ] IR spectrum of the mixture of conformers of dimethoxycarbene and the experimentally measured IR absorptions allows a detailed analysis ofthe conformational preference of dimethoxycarbene. ItsUV spectrum is in agreement with earlier studies and our TD-B3LYP/6-311+G(d,p) computations. The computed [CCSD(T)/cc-pVDZ] rotational profile is rather steep and separates the s-trans,s-trans and s-cis,s-trans conformers by a 16 kcal mol^-1 barrier, whilst the s-cis,s-cis conformer is too high-lying to be observable (+17 kcal mol^-1). In marked contrast with the gauche,gauche minimum of dimethoxymethane, the s-trans,s-trans conformer of dimethoxycarbene is slightly preferred (0.5 kcal mol^-1). The s-cis,s-trans conformer equilibrates at the high temperatures required during HFVP generation and both conformers can be identified in the IR spectrum of the argon matrix at 10 K. The conformational preference is partly due to the minimization of the overall dipole moment in the s-trans,s-trans conformer.

H–C–SiH₃: Direct Generation and Spectroscopic Identification of Ethylidene’s Cousin. Peter R. Schreiner, Hans Peter Reisenauer, Kurt W. Sattelmeyer, and Wesley D. Allen, J. Am. Chem. Soc. 2005, 127, 12156. Download