Hexamethyl Dewar benzene is a derivative of Dewar benzene with application in organometallic chemistry. It consists of the Dewar benzene core, with a methyl group substituent on each of its six carbon positions.
Attempting a similar reaction with potassium tetrachloroplatinate results in the formation of a pentamethylcyclopentadiene complex, [(η4-Cp*H)PtCl2], indicating that the rhodium and iridium metal centres are necessary for the step in which the aromatic anion is formed.[5]
The epoxide products are stable when the oxidation is performed under neutral conditions, such as when using DMDO that has acetone as a byproduct. When Using a peracid (mCPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.[10]
In 1973, the dication of hexamethylbenzene, C 6(CH 3)2+ 6, was produced by Hepke Hogeveen and Peter Kwant.[13] This can be done by dissolving the hexamethyl Dewar benzene monoepoxide in magic acid, which removes the oxygen as an anion.[14] NMR had previously hinted at a pentagonal pyramidal structure in a related cation[15] as had spectral data on the Hogeveen and Kwant dication.[16][17] The pyramidal structure having an apex carbon bonding to six other carbon atoms was confirmed by X-ray crystallographic analysis of the hexafluoroantimonate salt published in 2016.[14]
Left: Structure of C 6(CH 3)2+ 6, as drawn by Steven Bachrach[18] Right: Three-dimensional representation of the dication's rearranged pentagonal-pyramid framework, from the crystal structure[14]
Computational organic chemist Steven Bachrach discussed the dication, noting that the weak bonds forming the upright edges of the pyramid, shown as dashed lines in the structure he drew, have a Wiberg bond order of about 0.54; it follows that the total bond order for the apical carbon is 5×0.54+1=3.7<4, and thus the species is not hypervalent, but it is hypercoordinate.[18] From the perspective of organometallic chemistry, the species can be viewed as having a carbon(IV) centre (C4+ ) bound to an aromatic η5–pentamethylcyclopentadienyl anion (six-electron donor) and a methyl anion (two-electron donor), thereby satisfying the octet rule[19] and being analogous to the gas-phase organozinc monomer [(η5 –C 5(CH 3) 5)Zn(CH 3)], which has the same ligands bound to a zinc(II) centre (Zn2+ ) and satisfies the 18 electron rule on the metal.[20][21] Thus, while unprecedented,[14] and having attracted comment in Chemical & Engineering News,[22]New Scientist,[23]Science News,[24] and ZME Science,[25] the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with Konrad Seppelt,[14] commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."[23]
↑ Hogeveen, Hepke; Kwant, Peter W. (1975). "Pyramidal mono- and dications. Bridge between organic and organometallic chemistry". Acc. Chem. Res.8 (12): 413–420. doi:10.1021/ar50096a004.
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