1,3-DIPOLAR CYCLOADDITION REACTIONS TO THE C=X-M FRAGMENT .9. SYNTHESIS AND CHARACTERIZATION OF MONONUCLEAR IMINO KETONE COMPLEXES FE(CO)3(T-BU-N=C(H)-C(R)=O) AND THEIR USE IN CHEMOSELECTIVE CYCLOADDITION REACTIONS WITH DIMETHYL ACETYLENEDICARBOXYLATE RESULTING IN THE FORMATION OF FE(CO)3(BUTENOLIDE) COMPLEXES

Reaction of Fe2(CO)9 at room temperature in THF with the alpha-imino ketones t-Bu-N=C(H)-C(R)=O(R = Ph, Me) results in initial formation of mononuclear Fe(CO)3(t-Bu-N=C(H)-C(R)=O (9) in moderate (9b: R = Me) to high (9a: R = Ph) yield. The sigma-O, sigma-N chelate coordination behavior of the heterodiene ligand in complexes 9 was confirmed by the spectroscopic data (IR, H-1 and C-13 NMR). With respect to their spectroscopic properties and chemical behavior the Fe(CO)3(alpha-imino ketone) complexes closely resemble the related Fe(CO)3(R-DAB) complexes. Under the conditions of their formation, complexes 9 undergo subsequent reactions with Fe(CO)n fragments or free ligand, giving rise to the formation of the stable dinuclear complexes Fe2(CO)6(t-Bu-N=C(H)-C(R)=O)(R = Ph, Me) and Fe2(CO)4(L-L) (10) (R = Me). Reaction of the mononuclear Fe(CO)3(t-Bu-N=C(H)-C(R)=O) complexes (9) with dimethyl acetylenedicarboxylate at -78-degrees-C under an atmosphere of CO results in the formation of the Fe(CO)3(butenolide) complexes (13a,b), which have been characterized both spectroscopically (IR, H-1 and C-13 NMR) and by elemental analysis. The solid state structure of complex 13b was determined by single-crystal X-ray diffraction. Crystals of 13b, C17H19NO9Fe, are triclinic, space group P1BAR, with cell constants a = 8.627(1) angstrom, b = 9.597(2) angstrom, c = 11.557(1) angstrom, alpha = 81.60(1)degrees, beta = 88.75(1)degrees, gamma = 83.19(2)degrees, V = 939.8(2) angstrom3, Z = 2, R = 0.0405 (R(w) = 0.0669) for 4165 unique reflections with I > 2.5sigma(I). The coordination geometry around the central iron atom in 13b is distorted trigonal bipyramidal, with the intact sigma-donating imine nitrogen and the eta2-coordinated endocyclic C=C bond of the newly formed butenolide ligand occupying axial and equatorial positions, respectively. The proposed initial step in the formation of 13 is a chemoselective 1,3-dipolar cycloaddition of DMADC to the Fe-O=C fragment, yielding a bicyclo[2.2.1] intermediate 11. As opposed to the similar reactions of related M(CO)3(R-DAB) complexes, no intermediates can be observed in the present case, because the subsequent CO insertion into the Fe-O bond and the product forming reductive elimination of the newly formed Fe-C bonds are too fast even at very low temperatures.