Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0885 989

SCHEME 63

The (tetraphenylcyclobutadiene)PdX2 dimer reacts with a variety of metal complexes via transfer of the cyclobutadiene ligand to another metal. These reactions and other ligand transfer reactions have been reviewed by Efraty15.

VII. REACTIONS OF CYCLOBUTADIENE–METAL COMPLEXES

A. Isomerizations

1,2-Disubstituted (cyclobutadiene)Fe(CO)3 complexes in which the two substituents are different may exist as enantiomers. Racemic cyclobutadiene carboxylic acids or cyclobutadiene amine complexes of this type have been separated by classical resolution methodology234. These optically active (cyclobutadiene)Fe(CO)3 complexes are stable with respect to racemization at 120 °C for 24 h. This stability contrasts with acyclic

SCHEME 65

(diene)Fe(CO)3 complexes which have been shown to undergo racemization at this temperature (see Section IV.E.1.d).

In a similar fashion, 1-substituted-2,3-bis(trimethylsilyl) (cyclobutadiene)CoCp complexes in which the substituent contains a chiral center (e.g. 259) exist as a mixture of diastereomers235. These diastereomers may be separated either by column chromatography or HPLC. Diastereoisomerization of either 259a or 259b requires extremely vigorous reaction conditions; either flash vacuum pyrolysis (>520 °C) or solution thermolysis (301 °C). The absence of ligand crossover products indicates that the diastereoisomerization occurs in a unimolecular fashion. Two possible pathways may be considered (Scheme 66). Pathway a involves insertion of cobalt into one side of the cyclobutadiene ligand to generate a cobaltacyclopentadiene intermediate. Notably, this type of intermediate is implicated in the cyclodimerization of alkynes to form (cyclobutadiene)CoCp complexes (see Scheme 60). Alternatively, in pathway b, a retro [2 C 2] cyclization would generate a bis-alkyne cobalt complex, which can undergo ‘propeller’ rotation about the alkyne-to-cobalt bond axis followed by [2 C 2] cyclization. Examination of the hexalabeled complex 260a (two stereocenters, two different silyl groups, two 13C labels) sheds light on these possibilities (Scheme 67). Thus, isomerization of 260a leads only to isomer 261b (but not to diastereomer 261a) while isomerization of the 260b leads only to 261a (but not diastereomer 260a). Since 260a and 260b should interconvert via the cobaltacyclopentadiene intermediate, the above results cannot be adequately explained by the ‘pathway a’ mechanistic possibility. The results are most consistent with a retro [2 C 2] alkyne cyclization (‘pathway b’) in such a fashion that the cyclobutadiene ring bond between the two silyl substituents is not broken.

Flash vacuum pyrolysis of deuterium-labeled [1,2-bis(ethynyl)cyclobutadiene]CoCp 262a affords the rearranged product 262b and recovered starting material (Scheme 68)236. None of the dideuteriated product 262c or any of the potential [1,3-bis(ethynyl)cyclobuta- diene] CoCp isomers were observed. These results are difficult to reconcile with a mechanism involving a bis(diyne)CoCp intermediate (263) and are most consistent with the intermediacy of either cyclooctadiendiyne complex 264 or cyclooctahexaene complex 265.

B. Ligand Substitution

Tetramethylor tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,20-bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,20 -bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand.

Neutral (cyclobutadiene)Co(CO)2X complexes and (cyclobutadiene)Co(CO)3C cations undergo displacement in the presence of arenes to generate ( 4-cyclobutadiene)( 6-arene) CoC cations (Scheme 69)38,211e,228a. Neutral (cyclobutadiene)MoCp(CO)I complexes50c and (cyclobutadiene)RuCp(CO)C cations219 also undergo ligand displacement of a coordinated carbon monoxide.

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238.

C. Decomplexation

The majority of studies concerning decomplexation have been carried out on (cyclobutadiene)Fe(CO)3 [223, MLn D Fe(CO)3] and substituted derivatives. As is the case

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