Diamantane. III. Preparation and Solvolysis of Diamantyl Bromides

The bromination of diamantane (I) may be controlled to give mono-, di-, or polybrominated derivatives. At 25° in neat bromine, 1-bromodiamantane (III) is obtained in high yield. In refluxing bromine, 1,6- and 1,4-dibromodiamantane (V and VI) predominate. 4-Bromodiamantane (IV) is best prepared as a 59:41 equilibrium mixture with III, by reaction of I with tert-butyl bromide-aluminum bromide at 0°. Reaction of I in neat bromine with trace amounts of AlBrg gives 4,9-dibromodiamantane (VII) as the major product together with dibromides V and VI. Addition of larger quantities of Lewis acid produces 1,4,9-tribromodiamantane (VIII) and 1,4,6,9-tetrabromodiamantane (IX). The structure of the various bromides can be determined from their nmr spectra, as a chemical shift additivity relationship holds. The monobromides and dibromides were solvolyzed in 80% aqueous ethanol. The relative rates at 75° follow: III, 1.0; IV, 3.2 × 10^-2; V, 2 × 10^-3; VI, 8 × 10^-3; VII, 7 × 10^-4.1-Bromodiamantane (III) solvolyzes eight times faster than 1 -bromoadamantane (II), and IV three times slower. Although carbocation strain is less favorable for III and IV than for II, III is accelerated by relief of axial leaving group strain and by the greater stability of the 1-cation owing to β-chain branching. No detectable hydroxy bromide intermediates formed during solvolysis of V and VI. The solvolysis rates of dibromides V, VI, and VII were analyzed in terms of two limiting models for the transmission of nonconjugative substituent effects—σ inductive (through bond) and field models. The field effect contribution was evaluated by calculations based on the Tanford modification of the Kirkwood-Westheimer ellipsoidal model. The magnitude of each transmission mode is independent on the geometrical relationship between the two bromines. Through-bond coupling is favored by the parallel arrangements found in V and VII, and contributes factors in the range of [Formula Omited] to the rate depressions observed.