The I-dimethoxymethyl-5,6-dimethylidene-7-oxabicyclo[2.2. I jhept-2-ene (9) has been prepared. On treatment with Fe,(CO),, the endocyclic double bond C(2)=C(3) was coordinated first giving the Corresponding exo-Fe(C0)4 complex 10. The latter reacted with Fe,(CO), and afforded cis-heptacarbonyl-p -[( lRs,2sR,3Rs,4sR,sRs,6sR)-2,3-q : C,5,6,C-q-( l-(dimethoxymethyl)-5,6-dimethylidene-7-oxabicyclo[2.2.l]hept-2-ene)]diiron (11) as a major product. On heating, 11 underwent deoxygenation of the 7-oxabicyclo[2.2. llheptene moiety yielding tricarbonyl[C,5,6,C-q-(l-(dimethoxymethyl)-5,6-dimethylidenecyclohexa-1,3-diene)]iron (13). In MeOH, a concurrent, regioselective methoxycarbonylation was observed giving tricarbonyl[C,3,4,C-q-(methyl 5-(dimethoxymethy1)-3,4-dimethylidenecyclohexa-l,5-diene-1-carboxy1ate)jiron (14). Oxidative removal of the Fe(CO)T moiety in 13 and 14 did not afford the expected ortho-quinodimethane derivatives but led to CO insertions giving 2,3-dihydro-2-0x0-1 H-indene-4-carbaldehyde (20) and methyl 7-formyI-2,3-dihydro-2-0~0-1 H-indene-5-carboxylate (21), respectively.
Introduction. -The complexation of 5,6-dimethylidene-7-oxabicyclo[2.2.l]hept-2-ene
(1) by transition metals has been described recently [ 11 121. For various metalcarbonyls (Fe, Ru, Os, Cr, W) it was found that the endocyclic double bond between C(2) and C(3) in 1 is always coordinated faster than the exocyclic diene moiety, leading to the isolation of y~ 2-complexes 2 in good yield, provided the complexation reaction was carried out in a non-coordinating solvent. Prolonged exposure of 2 to metalcarbonyls gave the corresponding doubly complexed systems 3 and 4. Depending on the nature of the metal, of the solvent, and the reaction temperature, condensation products were also formed which arose from a formal [4+2]-cyclodimerization of 2 [3] [4]. On heating the iron double complexes 314 in toluene, the ortho -quinodimethane complex 5 was obtained in 60 % yield. Thermolysis of 3/4 in MeOH afforded instead the carboxylated derivative 6.
Recently, we reported on the syntheses of the acetal derivatives 7 [5] and 8 [6] and on their Diels-Alder stereo-and regioselectivity. We have now prepared the corresponding triene-acetal 9 and report here on its reaction with Fe,(CO),. We shall show that the corresponding complexes 10 and 11/12 can be prepared readily. The double complexes 11/12 can be transformed to the corresponding (ortho -quinodimethane)iron complexes 13 and 14 from which 18 and 19, respectively, were obtained. Contrary to an earlier report [7], we have found that Ce(1V) can oxidize the metallic complexe 5, leading to the ') *)