名称:
1,3-Dipolar Cycloaddition to the Fe-N:C Fragment. 14. Aromatic Isothiocyanates as Dipolarophiles. Reversible Formation of Novel [3.2.2] Bicyclic Double Isocyanide Insertion Products. Thermodynamics of Isocyanide Deinsertion Reactions
摘要:
The 1,3-dipolar cycloaddition reaction of Fe((2,6-xylyl)NC)3(i-PrDAB) (7) with para-substituted phenyl isothiocyanates (R''C6H4NCS; R'' = H (a), Me (b), OMe (c), NO2 (d)) is followed by one or two isonitrile insertions. Depending on the solvent and type of isothiocyanate used, [2.2.2] and [3.2.2] bicyclic products (10 and 11) are isolated. Crystals of 10a, C51H57N7SFe, are monoclinic, space group I2/a, with cell constants a = 24.041(2) angstrom, b = 12.095(2) angstrom, c = 36.161(6) angstrom, V = 10502(3) angstrom3, Z = 8, and R = 0.109 for 2918 observed reflections with I > 2.5sigma(I). Compounds 11 undergo a unique reversible isocyanide deinsertion reaction when warmed in toluene. The equilibria 11a-d reversible 10a-d + 2,6-xylyl-NC have been studied by H-1 NMR. The equilibrium constants at various temperatures (303 K less-than-or-equal-to T less-than-or-equal-to 368 K) have been determined from which for the first time DELTAG-degrees (a, 16.2 +/- 2.7; b, 22.7 +/- 1.6; c, 23.2 +/- 1.9; d, 24.7 +/- 1.1 kJ/mol), DELTAH-degrees (a, 61.6 +/- 6.3; b, 74.5 +/- 3.3; c, 65.5 +/- 2.0; d, 86.9 +/- 2.5 kJ/mol), and DELTAS-degrees (a, 152 +/- 29; b, 174 +/- 14; c, 142 +/- 12; d, 208 +/- 11 J/(mol.K)) values could be calculated for isocyanide deinsertion. These values provide an explanation for the observed product selectivities in different solvents. The reactions 7 reversible 10 are also reversible; however equilibrium constants could not be determined because at the required temperatures compound 7 is itself too labile.