1 loo1 2d Fig. . Dependence of the differences in the enthalpy of formation AAH: " (AHScF@y~li~) -AHfCF(acyclic)) on the substituents of the pyrazolyl groups of boranes 2a-d. and thermodynamic stability of species 4 relative to the corresponding acyclic structure ( 3 / 3 ) upon further increase in the internal crowding of the molecule. Hitherto, all attempts to introduce a substituent substantially larger than a methyl or ethyl group have failed at the P-diketone, starting material stage.
Experimental Procedure
2a: A solution of l a (1.5g, 8.3 mmol) and (9H-9-BBN), (l.Og, 4.15 mmol) dissolved in 10 mL of toluene was heated to reflux for 18 h, during which time 200 mL of H, gas evolved. The solvent was removed in vacuo and the solid residue recrystallized from hexane by slow cooling to -78 "C. Crystalline 2a: 2.4 g (96%). m.p. 89-90°C. MS: miz 300 (Me, B,, 90%) 285 (43, 271 (43, 257 (6% 243 (100). IlB NMR (64.2 MHz, CDCI,): 6 = 63.1 (h,,, = 350 Hz). 'HNMR(200.1 MHz,CDCI3):6=6.09(s,1H),2.33(br,2H),1.X8(m,12H), 1.26 (s, IXH). "C NMR (50.3 MHz, CDCI,, T = 311 K): 6 = 162.8 (s, 2C),
106.5 (d, lC), 33.6 (t, 4C), 30.5 (4, 6C). 30.0 (s, 2C). 28.5 (br., d, 2C). 22.8 (t, 2C). I3C NMR (75.5 MHz, C,D8, T = 173 K): 6 = 165.1 (s, 1 C), 161.9 (s, 1 C). 108.0 (d, 1 C), 33.9 (t, 4C), 32.8 (s, 1 C), 32.5 (s, 1 C), 31.4 (q, 3C), 29.9 (q. 3C). 28.4 (br.. d, 2C). 23.5 (t. 2C).
2b: (9H-9-BBN), (0.53 g, 2.2 mmol) and 1 b (0.84 g. 4.3 mmol) were allowed to react as above for 24 h to give 1.3 g (94%) of a colorless solid. Purified by sublimation in vacuum; m p. 70-71 "C. MS: m/z = 314 (Me, B,, 80%), 299 (20). 285 (40). 257 (100). "B NMR (64.2MHz. CDCI,). 6 =64.2 (hl;z = 500 HZ). ' H N M R (200.1 MHz,CDCI,): d = 1.95 (s, br., 12H); 1.27 (s, 3H); 1.39 (s, 20H). 13C NMR (50.3 MHz, CD,CI,, T = 311 K): 6 = 157.4 (s, 2C),115.8(s,1C),33.8(t,4C),32.9(s,2C),30.1(br.,d,2C),29.9(q,6C),22.8 (t, 2C), 12.7 (9, 1 C) "C NMR (75.5 MHz, C,D,O, T = 163 K): 6 = 158.5 (~,2C).117.3(s,1C),34.7(t,4C),33.9(s,2C),31.2(d,2C),30.3(q,6C),24.0 (t. 2C), 13.6 (q, 1 C). 2c: (9-H-9-BBN), (0.84 g, 3 4 mmol) and 1 c (1.36 g, 6.5 mmol) were allowed to react as above for 24 h to give 1.9 g (88%) of a colorless solid. Purified by sublimation in vacuum; m.p. 82-83°C. MS: m/z = 328 (Me. B,, loo%), 285 (65). 271 (8% 245 (85), 232 (90). "B NMR (64.2 MHz, CDCI,). 6 = 65.3 (h,,, = 550 Hz). 'H NMR (200.1 MHz, CDCI,): 6 = 2.67 (q, 2H), 1.95 (br., 2H). 1.88(m. 12H). 1.36(s. lXH), 1.14(t,3H). "CNMR(50.3MHz,CDC13, T = 3 1 1 K ) : 6 = 157.9(s,2C),122.3(s,lC),33.8(t,4C),33.2(s,2C),30.8(q, 6'21% 30.0 (br., d, 2C), 22.8 (1, 2C), 17.8 (t. lC), 16.5 (q, 1C).