Buta-1,3-diene complexes bonding

We have shown [13,79, 80] that the electronic spectra of c -l,3-buta-diene [81], cyclopentadiene, aromatic five-membered heterocycles [48,79], and norbornadiene [80] can be understood on the basis of a model with two interacting double bonds. Cyclopentadiene (CP) is the prime example of a ring-shaped molecule with a conjugated rr-electron system, and its structure can be related to that of short polyenes such as cM-1,3-butadiene (CB) and the simplest heterocycles, such as pyrrole (PY), furan (FU), and thiophene (TP). In the series, ci5-l,3-butadiene, cyclopentadiene, norbornadiene (NB), the latter is the most complex system, with the two ethylenic units coupled through indirect conjugation and 77, a interaction. One more system will be added here to the set of molecules with two interacting double bonds methylenecyclopropene (MCP) [(1) in Fig 3] the simplest cross-conjugated 77-electron system. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

Cycloaddition of buta-1,3-diene to the C2 ligand affords 356, containing complexed cyclohex-l-en-4-yne, this time attached to an Ru3 face.538 With cyclopenta-1,3-diene, formal insertion of one of the C2 carbons into a C=C double bond occurs (possibly via a three-membered ring and ringopening) to give 357.539 Two molecules of the cyclic diene have been incorporated into the organic ligand, which shows C=C double bond disorder. 

Figure 4.9. Frontier orbitals available for metal-carbon bonding in complexes of acyclic unsaturated systems C I I 2- (a) Allyl ligand H2CCHCH2 (b) buta-l,3-diene H2C=CHCH=CH2 (c) trimethylenemethane 6(012)3.

TT-Allyl and related complexes can be prepared by reactions such as 23.81-23.85 the last two reactions illustrate formation of allyl ligands by deprotonation of coordinated propene, and protonation of coordinated buta-1,3-diene respectively. Reactions 23.82 and 23.83 are examples of pathways that go via a-bonded intermediates (e.g. 23.48) which eliminate CO. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

Cyclo-octa-l,5-diyne, prepared in 2% yield by dimerization of buta-l,2,3-triene, has been examined by X-ray diffraction and shown to be almost planar with C—C C bond angles of 159.3°. Irradiation of cyclo-octa-l,5-diyne gave buta-l,2,3-triene, and treatment with ba gave cyclo-octa-tetraene. Cyclo-octa-l,5-diyne underwent Diels-Alder reactions with two molecules of buta-1,3-diene and 2,3-dimethylbuta-1,3-diene, but gave complex mixtures of products when treated with bromine, iodine, dimethyl acetylenedicarboxylate, or tcne. ... 

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