Pentacoordinate and tetracoordinate carbon and boron compounds (27, 38, 50-52, 56-61) bearing an anthracene skeleton with two oxygen or nitrogen atoms at the 1,8-positions were synthesized by the use of four newly synthesized tridentate ligand precursors. Several carbon and boron compounds were characterized by X-ray crystallographic analysis, showing that compounds 27, 56-59 bearing an oxygen-donating anthracene skeleton had a trigonal bipyramidal (TBP) pentacoordinate structure with relatively long apical distances (ca. 2.38-2.46 A). Despite the relatively long apical distances, DFT calculation of carbon species 27 and boron species 56 and experimental accurate X-ray electron density distribution analysis of 56 supported the existence of the apical hypervalent bond even though the nature of the hypervalent interaction between the central carbon (or boron) and the donating oxygen atom was relatively weak and ionic. On the other hand, X-ray analysis of compounds 50-52 bearing a nitrogen-donating anthracene skeleton showed unsymmetrical tetracoordinate carbon or boron atom with coordination by only one of the two nitrogen-donating groups. It is interesting to note that, with an oxygen-donating skeleton, the compound 61 having two chlorine atoms on the central boron atom showed a tetracoordinate structure, although the corresponding compound 60 with two fluorine atoms showed a pentacoordinate structure. The B-O distances (av 2.29 A) in 60 were relatively short in comparison with those (av 2.44 A) in 59 having two methoxy groups on the central boron atom, indicating that the B-O interaction became stronger due to the electron-withdrawing nature of the fluorine atoms.
The bimolecular nucleophilic substitution (S N 2) reaction at saturated carbon such as hydrolysis of methyl halides is one of the most important and the most popular reactions in organic chemistry. 1,2 The mechanism of the reaction invoking inversion of configuration of the central carbon is one of the fundamental ideas of organic reactions and is described commonly in textbooks for undergraduate students. 3 The structure of the transition state (TS) of S N 2 should be trigonal bipyramid (TBP) around the central carbon. 4 Hence the bonding about the carbon involves, at least formally, expansion of the valence shell and is called hypervalent. 5 Due to the fundamental importance of S N 2, there have been a variety of efforts to stabilize the TS and even to prepare model compounds of TS; typical examples are by Hojo 6 and Martin. 7-10 They claimed that they observed symmetrical TBP structure for the model compounds in solution. However, the X-ray structure of dimethyl-1-fluorenylcarbenium hexachloroantimonate bearing two methylthio groups at the 9-position 6 revealed that the two S-C + distances were different and the compound should be regarded as a sulfonium structure, 6 and X-ray analysis of the 9-anthracenylmethyl dication bearing phenylthio groups at 1 and 8 positions has not been reported. 7-10 Theoretical calculations on S N 2 are also numerous and they conclude that hypervalent 10-C-5 species should be TBP and energy maximum. 11 On the other hand, recently reported exotic highly coordinate carbon species such as CH 5 + 12,13 and CLi 5 , 14 CLi 6 , 15 and (Ph 3 PAu) 5 C + 16 are electron deficient carbocations and/or are stabilized by metalmetal cage interactions. Therefore, these compounds cannot be regarded as hypervalent 10-C-5 species for models of the S N 2 transition state. Here we report the synthesis and crystal structure of 1,8-dimethoxy-9-dimethoxymethylanthracene monocation (1) as the first fully characterized hypervalent 10-C-5 compound.Preparation of 1 is illustrated in Scheme 1. Commercially available 1,8-dihydroxyanthraquinone (2) was converted to 5 via methylation 17 followed by reduction 18 and trifluoromethanesulfonation. Carbon monoxide insertion 19 to 5 in methanol mediated by Pd(PPh 3 ) 4 gave 6 in 52% yield. 6 was treated with trimethyloxonium tetrafluoroborate (Me 3 O + BF 4 -) under CH 2 Cl 2 reflux for 20 h, and after filtration of excess Me 3 O + BF 4 -and removal of the solvent, 1 was obtained as a yellow-green solid. 1 is thermally stable but is sensitive to atmospheric moisture. 13 C NMR chemical shift of the central carbon is found at δ 192.58 ppm, which is confirmed by independent preparation of 1 using carbon monoxide ( 13 C 99%) in the process to convert 5 to 6.Crystals of 1 suitable for X-ray analysis were obtained by careful recrystallization from dry CDCl 3 , and the X-ray structure is shown in Figure 1. 20 The counteranion is B 2 F 7 -unexpectedly but it is well separated from the cationic part. The structure clearly shows the symmetrical nature of the compound. The sum ...
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