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-tert-butylphenyl)-9-fluorenol

The reactions of sp-9-(o-tert-butylphenyl)-9-fluorenol (1) with HCl, HBr, H I, and HF, respectively, were found to follow diverse pathways. Most unexpe cted is the unprecedented monohalogenation of a tert-butyl group, sp-9-[o-( beta-chloro-alpha,alpha-dimethylethyl)phenyl]fluorene (2) being formed quan titatively from 1 treated with HCl-CHCl3 and sp-9-[o-(beta-bromo-alpha,alph a-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 ana lysis. Likewise, the expected 9-chloro- and 9-bromofluorenes from the usual substitution of OH by halogen in reactions of alcohols with SOCl4 and SOBr 2 were not obtained from treatment of 1 with these reagents; the only produ cts were 2 and 3, respectively. These products are formed by nucleophilic a ttack 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 f rom the slower electron transfer from Br- to 1a to form free radical 1b, wh ich 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 halide s, fluoride is a poor nucleophile in displacement reactions and poor electr on-transfer agent. Consequently, strongly electrophilic la reacts with F- t o provide sp-9-(o-tert-butylphenyl)-9-fluorofluorene (5) as the only produc t 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 (4 a) of 9-(o-tert-butylphenyl)fluorene being formed initially, followed by ro tation 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.