Tetramethylheptalenes and their tricarbonylchromium complexes: Synthesis, structures, and thermal rearrangements

The thermal 4:1 equilibrium mixture of 1,3.5:6- and 1,3,5,10-tetramethylhep talene (13a and 13b, resp.) has been prepared. starting from the thermal eq uilibrium mixture of dimethyl 6,8,10-trimethylheptatene-1,2- and -4.5-dicar boxylate (6a and 6b, resp.; cf Scheme 5). These heptalenes undergo double-b ond shifts (DBS) even at ambient temperature. Treatment of the mixture 13a/ 13b 4:1 with [Cr(CO)(3)(NH3)(3)] in boiling 1,2-dimethoxyethane resulted in the formation of all four possible mononuclear Cr(CO)(3) complexes 19a-19d of 13a and 13b, as well as two binuclear Cr(CO)(3) complexes 20a and 20b, respectively, in a total yield of 87% (cf: Scheme 7). The mixture of comple xes was separated by column chromatography, followed by preparative HPLC (c f: Fig. 2). The structures of all complexes were established by X-ray cryst al-structure analyses (complex 19b and 20b; cf. Figs. 6-8) and extensive H- 1-NMR measurements (cf: Table 3). In 20b, the two Cr(CO)(3) groups are link ed in a 'syn'-mode to the highly twisted heptalene pi-skeleton. The corresp ondence of the H-1-NMR data of 20a with that of 20b indicates that the two Cr(CO)(3) groups in 20a also have a 'syn'-arrangement. The thermal behavior of the mononuclear complexes 19a-19d has been studied at 85 degrees in hex afluorobenzene (HFB). At this temperature, all four complexes undergo rearr angement to the same thermal equilibrium mixture (cf: Table 8). The rates f or the thermal equilibration of each complex have been determined by H-1-NM R measurements (cf: Figs.9-12) and analyzed by seven different kinetic sche mes (Chapt. 2). The equilibration rates are in agreement with two different haptotropic rearrangements that take place, namely intra- and inter-ring s hifts of the Cr(CO)(3) group, whereby both rearrangements are accompanied b y DBS of the heptalene pi-skeleton (cf. Scheme 9). All individual kinetic s teps possess similar Delta G* values in the range of 29-31 kcal mol-L (cf: Table 8). The occurrence of inter-ring haptotropic migrations of Cr(CO)(3) groups has already been established for anellated aromatic systems (cf: Sch eme 10); however, it is the first time that these rearrangements have been unequivocally demonstrated for Cr(CO)(3) complexes of non-planar bicyclic [ 4n]annulenes. such as heptalenes. The mechanism of migration may be similar to that proposed for aromatic systems (cf. Schemes 10 and 11).