Cyclobutadienes derivatives

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

Conjugated diene (e.g. butadiene), cyclobutadiene derivatives Dienyl (e.g. cyclopentadienyl derivatives, cycloheptadienyl derivatives)... 

Recently, synthetic routes to longer ladderanes with C-C double bonds in the terminal rungs have been improved the [3]-, [5]-, and [7]ladderane derivatives were synthesized by the repeated cycloaddition of cyclobutadiene derivatives,30 and the [w]ladderane derivatives (w = 3,4,5,6,7,9) were synthesized by the successive and alternate cycloaddition with cyclobutadiene and dimethyl acetylenedicarboxylate.31 X-ray crystallographic analysis of the [5]ladderane derivative31 shows a corrugated backbone without twist, in contrast with saturated and unsubstituted ladderanes.22,23... 

During a very early study on tetramethylcyclobutadiene16 it was anticipated by one of us that 2,2,4,4-tetramethyl-l,3-dicarbenacyclobutane formed in the pyrolysis of dry di-sodium salt of 2,2,4,4-tetramethyl-l,3-cyclobutanedione-bisto-sylhydrazone might be a reasonable precursor for the cyclobutadiene derivative. But the only product — isolated in a cooling trap — was tetramethylbutatriene. 

The formation of cyclobutadiene derivatives from zirconacydopentadienes can be accomplished as follows (Eq. 2.76) [55]. 

Example IV The Thermal and Oxidative Isomerization of Tetraalkyl Substituted Tetrahedrane Clusters to Cyclobutadiene Derivatives. Other remarkable structural changes during redox reactions, i.e. charge redistributions enforced by the respective energy differences, are observed for cluster compounds (1). On... 

Tetrahedrane (CR)4 with bulky tert. butyl substituents R has been synthesized by G. Maier and coworkers in 1978 (28). In their already classic investigations, an interesting valence isomerization has been observed on melting, the C4 cluster rearranges to the thermodynamically more stable cyclobutadiene derivative, which on photochemical energy uptake forms again the tetrahedrane (28) ... 

The mass spectrum of tricarbonyliron cyclobutadiene derivative 282 shows fragments corresponding to the stepwise loss of four CO molecules and finally of C2H2 (78AJC1607). 

While cyclobutadiene itself yields only a dimer ([3]-ladderane) in such process, some derivatives, such as (2b) or (2c), provide higher members of the series. The difference in reactivities stems from a difference in the HOMO-LUMO energy gap of the reactants. The energy gap is readily calculated, providing a way to select the most suitable cyclobutadiene derivative to undergo oligomerization to a ladder. ... 

The cyclobutadiene derivatives [Co(Tj -C4Fc Ph4 )Cp] (n = 1-3) (Cp = 17 -cyclopentadienyl) undergo one-electron oxidation to stable cations (27) which show charge-transfer bands in the near infrared region. An analysis of the spectra led to the conclusion that the cations are Class II mixed-valence compounds and that the charge-transfer transition arises from a weak interaction between Co(I) and Fe(III) rather than between Fe(II) and Fe(III). Each compound showed further oxidations at more positive potentials, including an irreversible process ascribed to electron loss from the cobalt center. 

Ring expansion of a number of (cyclopropenyl)diazoalkanes gives cyclobutadiene derivatives, e.g. reaction of 7, though in some cases, fragmentation occurs leading to alkynes (see Section 10.A.2.2.). ° There is evidence that the cyclobutadienes are formed through singlets, while the alkynes are derived from triplets. 

Other examples of NVET include the triplet energy acceptors 1,3,5,7-cyclooctate-traene,116 which has a puckered structure in the ground state but relaxes to a planar structure in the triplet state, and a cyclobutadiene derivative,117 which changes from a rectangular to a square structure. 

Thermal cycloaddition, to form a cyclobutane ring, is not confined to fluorinated ethylenes. Cyclobutadiene derivatives are formed as intermediates and dimerize to yield a cyclobutane ring 48... 

The PE spectra of four cyclobutadiene derivatives 134 - 137 have been recorded. Due to the large alkyl groups only the first two bands could be discriminated and assigned to the two rr orbitals. 

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