In-Silico Guided Synthesis and Characterization of Novel Electrophilic Carbenes
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The synthesis and electronic characterization of two new electrophilic carbenes is described. The synthesized carbenes were selected based on computational studies investigating the HOMO and LUMO energies of various new carbenes.
The cyclic(amido)arylcarbene (CAmArY) could not be isolated due to rapid dimerization when the carbene was generated. The carbene dimer was characterized through NMR, mass spectrometry, and X-ray crystallography. A sulfur trapping experiment was performed to confirm the generation of the carbene. The π-accepting ability of CAmArY was studied via selenium NMR. As of this writing, the CAmArY selenium adduct is the most π-accepting carbene with a selenium NMR shift of 1240 ppm. Tolman Electronic Parameter (TEP) studies could not be completed due to decomposition of the iridium and rhodium metal complexes which is discussed in further detail below.
A series of diiminocarbenes (DICs) were synthesized in good yields. DICs were isolated successfully and characterized through NMR and a single crystal X-ray structure of one free DIC was obtained. The π-accepting ability of the DICs were also studied via 77Se NMR. The DIC selenium adducts displayed 77Se NMR shifts ranging from 640 - 699 ppm, which suggests a π-accepting ability greater than CAAC but less than the DACs. TEP studies were performed by synthesizing the iridium (I) dicarbonyl chloride complex of the parent DIC and examining the stretching frequencies of the metal carbonyls via IR spectroscopy. The TEP value of the DIC was determined to be 2046 cm-1 which places its overall donating ability very similar to CAAC. To determine the effect of para-substitution of the arylimino substituents, DFT calculations were performed and a Hammett analysis was performed which showed that the level of the LUMO energy was more effected than the HOMO energy level. The selenium adducts of some of these DIC derivatives were prepared to conduct a Hammett analysis by plotting the 77Se NMR chemical shift versus the respective LUMO level. This Hammett analysis revealed a direct correlation of the LUMO energy level to the 77Se NMR shift.
The synthesis of two other carbenes, an abnormal diamidocarbene (aDAC) and an indole derived carbene (IC) has been started. The synthetic routes currently being employed to synthesize the carbene precursors is described herein along with future efforts to prepare and characterize these carbenes.