Das Auftreten von freiem Cyclobutadien bei der Oxidation von Tricarbonyl(cyclobutadien)eisen‐Komplexen

Abstract

Bei der oxidativen Zerlegung von Tricarbonyl( 1,2-dimethylcyclobutadien)eisen (2 b) wurden wie bei dem unsubstituierten Stammkomplex ( 2 4 in Gegenwart von Dimethylmaleat, Dimethylfumarat und Maleinsiiureanbydrid Bicyclo[2.2.O]hexcnderivate gebildet, die formal die Diels-Alder-Addukte von 1,2-Dimethylcyclobutadien (1 b) an die entsprechenden Dienopbile darstellen. Die Bildung von racemischen Addukten bei der Reaktion des chiralen (1-~thyl-2-methylcyclobutadien)tricarbonyleisens [( -)-2c] erlaubt den SchluB, daB intermediar freies Cyclobutadien 1 c entsteht. Gcstutzt wird dicser Befund durch das Vcrhalten von Dicarbonyl(l.2dimethylcyclo-butadienMdimethylma1eat)eisen (11 b) bei der Oxidation. Die hohe Stereo-und Regiospevfitat der Adduktbildung kennzeichnen das 1,2-Diakylcyclobutadien 1 b als ein stark diskriminierendes Dien, dessen Selektivitatdie Addition an Dimethylfumarat verlauft z 175mal schncllcr als an Dimethylmaleatdie des Cyclopentadiens noch ubertrifft. The Intermediacy of Free Cyclobatadiew During the Oxidatioa of Tricarbonyl(cyclobntadiene)iroo Complexes Oxidat. degradation of tricarbonyl(l,2dimethylcyclobutadiene)iron (2 b) in the presence of dimethyl maleate, dimethyl fumarate, and maleic anhydride leadsas in the case of the unsubstituted parent complex (21)to bicyclo[2.2.0]hexene derivatives which are formally the Diels-Alderadducts of 1,2-dimethylcyclobutadiene (1 b) with the corresponding dienophiles. The formation of raamic adducts on reaction of chiral tricarbonyl( l-ethyl-2-methylcyc1obutadiene)iron [( -)-2c]indicates the intermediacy of free cyclobutadiene lc. This finding is corroborated by the behavior of dicarbonyl(l,2dimethylcyclobutadiene~dimethyl maleate)iron (11 b) on oxidation. The high stereo-and regiospecificity of the formation of adducts characterises the 1.2dialkylcyclobutadiene l b as a strongly discriminating diene whose selectivityaddition to dimethyl fumarate being = 175 times faster than to dimethyl maleateexceeds that of cyclopentadiene.Die grok Reaktivitat des Cyclobutadiens (la) hat bisher eine Isolierung und die direkte Bestimmung seiner physikalischen und chemischen Eigenschaften verhindert la). Hochsubstituierte Derivate von la, deren Reaktivitatzumindest im Bezug auf die Diskussion und Literaturhinweise findet man bei: G.

Metal-Assisted Cycloaddition Reactions in Organotransition Metal Chemistry

“…Since the (diene)Fe(CO) 3 group is also known for its ability to effectively stabilize positive charge, Goldschmidt et al 126 138 and others [139][140][141] that the cycloaddition reactions take place with the free cyclobutadiene after oxidative destruction of the complexes. These are therefore not metal-assisted reactions; the Fe(CO) 3 fragment acts only as temporary storage device, and the reactions are not further discussed here.…”

“…Since the (diene)Fe(CO) 3 group is also known for its ability to effectively stabilize positive charge, Goldschmidt et al 126 (Cyclobutadiene)Fe(CO) 3 complexes in conjunction with an oxidant such as ammonium cerium(IV) nitrate (see e.g., refs [129][130][131][132][133][134][135] or lead(IV) acetate 136,137 have found extensive use in [2+2], [3+2], and [4+2] cycloaddition reactions. However, it has unequivocally been shown by Pettit 138 and others [139][140][141] that the cycloaddition reactions take place with the free cyclobutadiene after oxidative destruction of the complexes. These are therefore not metal-assisted reactions; the Fe(CO) 3 fragment acts only as temporary storage device, and the reactions are not further discussed here.…”

Chapter 10. Aromatic compounds

“…The conflicting reports on the photoaddition of furan to benzene have been largely r e c ~n c i l e d , ~~ and the proportions of adducts are dependent on the light source and the reactant concentrations. Compound (19) and its Coperearranged isomer are the major products with high concentrations of benzene if a…”

The mechanism of the oxidative decomposition of cyclobutadienyliron tricarbonyl complexes: intramolecular trapping

“…This suggests that in the earlier3•4 intermolecular trapping experiments, optically active products would have resulted if the cyclobutadiene ligand was not free from the metal at the time of reaction. = 1.5 Hz, (H)(8), 4.47 (multiplet, 2 H) (4), 4.12 (doublet, J = 9 Hz, 1 H) (5), 3.72 (doublet, J= 9 Hz, 1 H) (5), 3.47 (doublet, JB 9 = 1.5 Hz, 1 ), 1.70 (triplet, J410 = 1.5 Hz, 3 H (10); 13C: 191.7(1), 171.1 (3), 145.3(7), 136.0 (8), 131.6 (2), 69.0 (4), 64.6 (5), 62.5 (6), 56.4 (9), 9.45 (10); MS m/e 162.…”

“…NMR, (Me4Si) 5.93 (multiplet, 1 H) (8), 4.47 (multiplet, 2 H) (4), 4.12 (doublet, J = 9 Hz, 1 H) (5), 3.72 (doublet, J = 9…”