Dieter Cremer's contribution to the field of theoretical chemistry

This legacy article reviews the contributions of Dieter Cremer (1944-2017) to the field of theoretical chemistry, highlighting his major accomplishments in method development and applied quantum chemistry, which has led to many advances in the field. His work reflects an extraordinary breadth and deep understanding of theory, which is needed to solve complex chemical problems. His passion for science has inspired many colleagues in the past and will do so in the future.     

4-Oxo-2,3,5,6-tetrafluorocyclohexa-2,5-dienylidene--a highly electrophilic triplet carbene

The 4-oxocyclohexa-2,5-dienylidenes are an interesting class of highly electrophilic carbenes. We investigated the reactivity of the 2,3,5,6-tetrafluorinated and -tetrachlorinated derivatives 1b and 1c with small molecules under conditions of matrix isolation. The reactions with molecular oxygen and with carbon monoxide produce the expected carbonyl O-oxides and ketenes, respectively. As a result of the extreme electrophilicity of 1b and c both carbenes insert with no or very small activation barriers into H2 or the CH bonds of hydrocarbons. Obviously, spin restrictions for these formally spin-forbidden reactions do not result in substantial thermal activation barriers.     

Diabatic ordering of vibrational normal modes in reaction valley studies

Diabatic ordering of the normal model of a reaction complex along the reaction path has several advantages with regard to adiabatic ordering. The method is based on rotations of the vibrational normal modes at one point, s, of the reaction path to maximize overlap with the vibrational modes at a neighboring point. Global rotations precede the rotations of degenerate modes so that changes in the direction of the reaction path and changes in the force constant matrix, which represent the two major effects for changes in mode ordering, can be separated. Overlap criteria identify resolved and unresolved avoided crossings of normal modes of the same symmetry. Diabatic mode ordering (DMO) can be used to resolve the latter by reducing the step size, thus guaranteeing correct ordering of normal modes in dependence of s. DMO is generally applicable to properties of the reaction complex that depend on s such as normal mode frequencies, orbital energies, the energy of excited states, etc. Additional applications are possible using a generalized reaction path vector, which may describe the change in atom masses, geometrical parameters, and/or the force constant matrix. In this way, the vibrational spectra of isotopomers can be investigated or the vibrational frequencies of different molecules correlated. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1282–1294, 1997     

p-Benzyne

A noble gas matrix at low temperature was used to investigate the photochemical behavior of diacetyl terephthaloyl diperoxide and dipropionyl terephthaloyl diperoxide as well as 1,4-diiodobenzene. All three photoreactions formed small quantities of a compound with IR absorption bands at 725 and 980 cm⁻¹, which disappeared when the matrix was annealed. These bands correspond to the most intense of the calculated bands (B3LYP) for 1,4-didehydrobenzene (p-benzyne) ( 1 ). That decomposition of the peroxides in fact leads to 1 is confirmed by vapor-phase pyrolysis experiments in which (Z)- 2 was obtained in high yield.     

Dimesitylketone O-oxide: spectroscopic characterization, conformation, and reaction modes: OH formation and OH capture.

Dimesitylketone O-oxide 1b was synthesized by photolysis of dimesityldiazomethane dissolved in an oxygen saturated CCl3F solution at 140 K. Conformation and geometry of 1b were determined by comparing measured NMR chemical shifts with the corresponding chemical shifts calculated at the DFT-IGLO level of theory where it had to be considered that the molecule exists in two enantiomeric forms. Measured and calculated 1H chemical shifts agree within 0.1 ppm while the calculated 13C shift of the COO carbon (210.6 ppm) differs by only 0.4 ppm from the measured shift of 211.0 ppm. The two mesityl rings are perpendicular to each other and enclose angles of 40 and 57 degrees with the COO plane. The preferred rearrangement process of 1b is an H migration from one of the ortho-methyl groups to the terminal O atom of the COO unit. The calculated activation enthalpy of this process is 12.7 kcal/mol (B3LYP/cc-pVTZ). In contrast, the activation enthalpy for isomerization to dioxirane is 5 kcal/mol higher. In CCl3F, the activation barrier for the thermal decay was determined to be 13.8 +/- 0.2 kcal/mol and in acetonitrile 13.1 +/- 0.4 kcal/mol. H migration initiates cleavage of the OO bond and the production of an OH and a benzyl radical. Recombination of the latter in the solvent cage leads to the formation of 2-methylhydroxy-pentamethylbenzophenone, while escape of the OH radical from the solvent cage yields a ketone. These results confirm the possibility of OH production from carbonyl oxides in the solution phase.     

Synthesis of 2-Carboxy-Substituted Sphingosine Derivatives

ABSTRACT

From the sphingosine biosynthesis pathway it is deduced that 2-carboxy-substituted sphinganine derivatives should be suitable inhibitors of sphingosine biosynthesis. For their synthesis enantioselective acylation and α-hydroxyalkylation of serine was performed via its optically pure 2-tert-butyl-oxazolidine derivatives 4A, B and ent-4A, known to undergo partial chirality transfer from serine to the 2- position of 4 and then to the 4-position. Thus, after acid hydrolysis compounds R-7Aa, -7Ab, -7Ac, -7Bb, S -7Aa, -7Ab, and 11 are provided highly stereoselectively.