reaction of trimethyl(pentamethy1cyclopentadienyl)germane (I )[' I with boron trichloride. Reaction of (2) with dimethylamine affords the bis(dimet hylamino)(pentamethyIcyclopenta-dieny1)borane (3), and with trimethylamine the adduct (4)13].
These three compounds differ in their dynamic behavior, since the rate of migration of the boryl group depends on the Lewis acidity of the boron atom.
Compound (2) has a fluxional structure owing to very rapid degenerate sigmatropic rearrangements of the BC12 group. At 40°C a sharp 'H-NMR signal is observed at 6 = 1.81 ppm (in CHzCIz) for the three types of chemically non-equivalent methyl groups; cooling to -80°C leads to a slight broadening of the signal.
Compound (3) has a static structure at 40°C. The 'H-NMR spectrum (in CH,CI,) shows separate signals for vinylic and allylic methyl groups: 6CH3 vinyl.: 1.83 ppm S(12); 6CH3 allyl.: 1.15 ppm S(3); 6N-CH3: 2.55ppm, broad (12). However, on increasing the temperature rearrangements also take place in the case of ( 3 ) and lead to coalescence of the signals of all methyl groups attached to the cyclopentadiene ring. In addition, the non-equivalence of the N-methyl groups effects a temperature-dependent hindrance to rotation.
Compound ( 4 ) is a static molecule at all temperatures. The 'H-NMR spectrum still shows sharp separated signals for the various methyl groups attached to the cyclopentadiene ring, even at 100°C: 6CH3 vinyl.: 2.07 ppm S(6), 1.85 ppm, S(6); 6CH3 allyl.: 1.22 ppm S(3); 6N-CH3: 2.75 ppm S(9).
Experimental :
Compound (2): A solution of ( I ) (6.6g, 26mmol) in CHzCIz ( 20ml) is added dropwise and slowly at -30°C to a solution of BCI3 (3.2 g, 27 mmol) in CH2Cl2 (20 ml). After one hour's stirring at room temperature (2) is isolated by fractional distillation in vacuo as a colorless liquid, b. p. 55"C/0.5 torr; yield 3.2g (57 %); M' : m/e=216 (0.5 %).
Compound (3): A solution of (2) (3.2 g, 15 mmol) in pentane ( 20ml) is added at -30°C to a solution of HN(CH3), (2.7g, 59mmol) in pentane (25ml). After filtration, (3) is obtained by fractional distillation in oacuo as a colorless viscous liquid, b.p. 74"C/0.05 torr; yield 1.8g (52%); M' : m/e=234 (1.5 %).
Compound ( 4 ) : N(CH3)3 (0.2 g, 3 mmol) is added at -20°C to a solution of (2) (0.4g, 2mmol) in pentane (35ml). The precipitate is filtered off and recrystallized from methylcyclohexane; m. p. 135 "C (dec.), yield 0.4 g (73%); M' : m/e= 275 (0.1 %).