1,3-DIPOLAR CYCLOADDITION TO THE FE-N=C FRAGMENT .8. FORMATION OF 1,4,3A,6A-TETRAHYDROPYRROLO[3,2-B]PYRROLES VIA DOUBLE 1,3-DIPOLAR CYCLOADDITION OF ALKYNES TO FE(CO)2(CNR) (ALPHA-DIIMINE) - COMPETITION BETWEEN ISOCYANIDE AND CO INSERTION - DETERMINATION OF CONE ANGLES OF ISOCYANIDES

Fe(CO)2(CNR) (R'-alpha-diimine) (I R' = isopropyl, R = tert-butyl (a), sec-butyl (b), benzyl (c), cyclohexyl (d), o-tolyl (e), and 2,6-xylyl (f); 1g R' = cyclohexyl, R = tert-butyl) reacts with dimethyl acetylenedicarboxylate, via a 1,3-dipolar cycloaddition to the Fe-N=C fragment, to give three products. The first two products, ferra [2.2.2] bicyclic structures (7, 8), are the result of a competition between CO and isocyanide insertion. The trend for preferred insertion proved to be aromatic CNR > CO > aliphatic CNR. The third product, a 1,4,3a,6a-tetrahydropyrrolo-[3,2-b]pyrrole complex (4), is formed via two successive 1,3-dipolar cycloadditions to both Fe-N=C fragments of 1. The distribution of products is governed by the electronic properties of the isocyanides. The pyrrolopyrrole group can be decomplexed by means of substitution by CO. Heating of the uncoordinated heterocycle results in ring cleavage, yielding 1-isopropyl-2,3-dicarbomethoxy-5-(1,2-dicarbomethoxy-2-(isopropylamino)ethenyl)pyrrole (10). The molecular structure of 10 (C20H28N2O8, monoclinic, space group P2(1)/n, a = 8.442 (6) angstrom, b = 29.240 (7) angstrom, c = 9.567 (9) angstrom, beta = 109.797 (8)-degrees, Z = 4, R = 0.059, R(w) = 0.083) shows that the pi systems of the pyrrole and enamine units are not in conjugation. To evaluate the steric properties of isocyanide ligands, the cone angles have been determined. The approach of the nearby and remote cone angle is introduced and discussed as a means to account for the influence of the isocyano carbon atom on the steric properties.