65213-15-2Relevant articles and documents
The First Reported Halogenation of a tert-Butyl Group with HCl or HBr in CHCl3. Unexpected Differences in the Reactions of HCl, HBr, HI, and HF with sp-9-(o-ferf-Butylphenyl)-9-fluorenol
Meyers, Cal Y.,Hou, Yuqing,Lutfi, Hisham G.,Saft, Howard L.
, p. 9444 - 9449 (2007/10/03)
The reactions of sp-9-(o-tert-butylphenyl)-9-fluorenol (1) with HCl, HBr, HI, and HF, respectively, were found to follow diverse pathways. Most unexpected is the unprecedented monohalogenation of a tert-butyl group, sp-9-[o-(β-chloro-α,α-dimethylethyl)phenyl]fluorene (2) being formed quantitatively from 1 treated with HCl-CHCl3 and sp-9-[o-(β-bromo-α,α-dimethylethyl)phenyl]fluorene (3) (>90%), along with a very small amount of sp-9-(o-tert-butylphenyl)fluorene (4), being formed from 1 treated with HBr-CHCl3. The absolute structure of 2 was ascertained by X-ray crystal analysis. Likewise, the expected 9-chloro- and 9-bromofluorenes from the usual substitution of OH by halogen in reactions of alcohols with SOCl2 and SOBr2 were not obtained from treatment of 1 with these reagents; the only products were 2 and 3, respectively. These products are formed by nucleophilic attack of halide ion on a tert-butyl methyl group of the 9-cation (1a) with concerted intramolecular displacement of hydride to 9-C+, while 4 results from the slower electron transfer from Br to 1a to form free radical 1b, which captures an H atom from HBr. The high redox potential of I- and weak HI bond ensures the rapid conversions 1a → 1b → 4, making 4 the exclusive product from treatment of 1 with HI-CHCl3. In contrast to the other halides, fluoride is a poor nucleophile in displacement reactions and poor electron-transfer agent. Consequently, strongly electrophilic 1a reacts with F- to provide sp-9-(o-tert-butylphenyl)-9-fluorofluorene (5) as the only product from the reaction of 1 with pyridine (HF)x. Dynamic NMR provided strong evidence that the reaction with HI occurs with inversion, the ap rotamer (4a) of 9-(o-tert-butylphenyl)fluorene being formed initially, followed by rotation to the isolated sp rotamer, 4. The largely planar configuration of C-9 of the unsymmetrically hindered cation intermediate 1a, responsible for this inversion and the related inversions, was supported by NMR.
