Chemistry of Cyclobutadiene-iron Tricarbonyl - ACS Publications

34 Chemistry of Cyclobutadiene-iron Tricarbonyl LEWIS WATTS and R O W L A N D P E T T I T

Downloaded by CORNELL UNIV on June 14, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch034

Department of Chemistry, University of Texas, Austin, Tex.

Cyclobutadiene-iron tricarbonyl is prepared through reac tion of 3,4-dichlorocyclobutene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl-; 1,2,3,4-tetramethyl-; and benzocyclobutadiene-iron tri carbonyl complexes. Cyclobutadiene-iron tricarbonyl is "aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cyclo butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes.

Following repeated failures during several decades to isolate cyclo butadiene, organic chemists came to recognize that this hydrocarbon, if i t is thermodynamically stable at all, is at best an extremely reactive system (1). Hiickel molecular orbital calculations supported these con clusions; such calculations predict that the system will possess no resonance stabilization energy and this, coupled with the fact that the double bonds are part of a highly strained ring system, allows one to predict a high degree of reactivity for the molecule. It was, therefore, a most intriguing suggestion, proposed b y LonguettHiggins and Orgel in 1956 (5) that, despite its instability, cyclobutadiene nonetheless might be able to form stable complexes when coordinated to metal atoms of the transition series. The basis for this prediction was the fact that the molecular orbitals of cyclobutadiene are of the correct symmetry to interact with the atomic orbitals of the metal atom; these interactions allow bonding of the "forward coordination-back donation" type which is commonly necessary in metal complexes involving the metal atom i n a low oxidation state. Shortly thereafter, this prediction was essentially proved to be correct; Hubel and co-workers (4) i n 1959 reported the isolation of tetraphenyl549 Kauffman; Werner Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

550

WERNER

CENTENNIAL

cyclobutadiene-iron tricarbonyl(I) following reaction of diphenylacetylene and F e ( C O ) and, in the same year, Criegee and co-workers (2) prepared the dimer of tetramethylcyclobutadiene-nickel dichloride (II) through reaction of tetramethyldichlorocyclobutene and N i ( C O ) 4 . X - r a y de5

CH

3

CH

8

Fe(CO), Downloaded by CORNELL UNIV on June 14, 2017 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch034

I terminations have subsequently confirmed the presence of the tetrasubstituted cyclobutadiene rings in these two complexes. Since these two initial reports, several other complexes possessing tetra-substituted, cyclobutadiene ligands have been reported, notably by Maitlis and co-workers (6), who have demonstrated that one can transfer a cyclobutadiene ligand from one metal to another to produce new cyclobutadiene-metal complexes. In 1964 we prepared cyclobutadiene-iron tricarbonyl (III), a com plex possessing an unsubstituted cyclobutadiene ligand (3). The reac tion employed in this preparation involves the interaction of m-3,4dichlorocyclobutene with F e ( C O ) 9 . It has subsequently been found that 2

the dehalogenation of dihalocyclobutenes with diiron enneacarbonyl appears to be a general route for the preparation of cyclobutadiene-iron tricarbonyl complexes. Thus, in a similar manner, dibromobenzocyclobutene reacts with F e ( C O ) to produce benzcyclobutadiene-iron tricarbonyl (IV). Likewise, 3,4-dichlorotetramethylcyclobutene reacts to give tetramethylcyclobutadiene-iron tricarbonyl (V) (7) and 3,4dibromo-l,2-diphenylcyclobutene affords 1,2-diphenylcyclobutadiene-iron tricarbonyl (VI) (7). However, perfluorocyclobutene does not react to give tetrafluorocyclobutadiene-iron tricarbonyl. The mechanism of the formation of these cyclobutadiene complexes remains obscure; it is interesting to note that either the cis or trans isomer of 3,4-dichlorocyclobutene reacts with F e ( C O ) to give Complex I I I and that Complex I V can be produced from either the cis or trans isomer of diiodobenzocyclobutene (7). 2

9

2

9

Kauffman; Werner Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

34.

551

Tricarbonyl Complexes

WATTS AND PETTIT

Br Fe (CO)» 2

Br

Fe(CO)

8

IV


CH

3

Br

\

Br

Fe(CO)

3

VI

Chemical Properties of Cyclobutadiene-iron

Tricarbonyl

Cyclobutadiene-iron tricarbonyl is a pale, yellow solid, m.p. 26°C, which exhibits a single, sharp N M R absorption at r 6.09. I n common with other diene-iron tricarbonyl complexes, the material displays appreciable thermal stability, as well as a pronounced resistance to further replacement of the C O ligands; several hours treatment with triphenylphosphine in refluxing toluene leaves the complex unaffected. One of the most interesting properties of the complex concerns its reactions with electrophilic reagents. It is found that these reactions lead to substituted cyclobutadiene-iron tricarbonyl complexes and, i n this sense, the complex is classified as aromatic just as ferrocene may be so classified. The substitution reactions which have been performed so far are summarized below. Reaction of Complex I I I with acetyl chloride and aluminum chloride under typical Friedel-Crafts conditions affords acetylcyclobutadiene-iron tricarbonyl (VII) in high yields. The corresponding benzoyl derivative is similarly prepared with benzoyl chloride. Formylation with JV-methylformanilide and P O C l produces cyclobutadienecarboxaldehyde-iron t r i carbonyl (VIII), while chloromethylation with formaldehyde and H C I affords the chloromethyl derivative ( I X ) . 3


552

WERNER

CENTENNIAL

It is possible to perform many of the usual reactions characteristic of the functional groups in these substituted complexes. Thus, reaction of Complex V I I with N a B H produces the secondary alcohol ( X ) , which is also obtained when the aldehyde (VIII) is treated with methylmagnesium iodide. The aldehyde (VIII) can also be reduced with N a B H to generate hydroxymethylcyclobutadiene-iron tricarbonyl ( X I ) ; the same complex is produced upon hydrolysis of the chloromethyl derivative ( I X ) . Reaction of cyclobutadiene-iron tricarbonyl with methylchlorothioformate followed by hydrolysis gives rise to cyclobutadienecarboxylic acid-iron tricarbonyl ( X I I ) . A Curtius rearrangement of the acid azide derived from Complex X I I affords aminocyclobutadiene-iron tricarbonyl ( X I I I ) . The dimethylaminomethyl derivative ( X I V ) is readily available through the Mannich reaction with formaldehyde and dimethylamine. The chloromercury cyclobutadiene complex ( X V ) is produced upon reaction of Complex I I I with H g ( O A c ) , followed by treatment with hydrochloric acid. In the simplest substitution reaction, treatment of cyclobutadiene-iron tricarbonyl with C F C O O D produces a mixture of deuterated derivatives of Complex I I I . The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R , adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO) cationic intermediate ( X V I ) ; loss of a proton from this intermediate generates the substituted cyclobutadiene - F e ( C O ) complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation ( X V I I ) , as well 4


4

2

3

+

3

3

+ Fe (Ao), XVII as several of its simple alkyl derivatives; the fact that such cations can be isolated renders more plausible the intermediacy of the species ( X V I ) in the substitution process. One final comment relating to the aromaticity of the cyclobutadiene ligand concerns the orientation effect of substituents towards introduction of a second substituent. T o date, the only reaction bearing on this question which has been performed is the acetylation of methylcyclobutadiene - F e ( C O ) ( X V I I I ) , which was prepared by reducing the chloromethyl complex ( I X ) . Acetylation of Complex X V I I I produces a mixture of 2and 3-acetyl-l-methylcyclobutadiene-iron tricarbonyl complexes with the latter isomer ( X I X ) predominating (~2:1). This is not the orientation 3


34.

Tricarbonyl Complexes

WATTS AND PETTIT

Fe(CO)

R H

R

1/

+

R+

Fe(CO),

8

553

Fe(CO),

XVI CHOHCH,

COCH3


Fe(CO)

Fe(CO)

3

VII

CHO

Fe(CO),

8

X

VIII

I COOH

Fe(CO)

CH OH 2

Fe(CO)

3

Fe(CO),

3

XII

III

XI

i

I

T I

NH

HgCl

2

Fe(CO),

Fe(CO)

XIII

CHjCl

Fe(CO),

3

XV

IX i

CH N(CH ) 3

Fe(CO) XIV

3

8

CH

2

CH CO 3

Fe(CO) XIX

3

CH,

3

Fe(CO), XVIII

one would expect by analogy with the acetylation of toluene.

Whether

this is a result of electronic factors or is a consequence of enhanced steric hindrance to attack at the "ortho" position i n Complex X V I I I remains to be determined.


554

WERNER CENTENNIAL

Literature Cited (1) Baker, W., McOmie, J. F. W., Special Publication No. 12, The Chemical Society, London, 1958. (2) Criegee, R., Schroder, G . , Ann. 623, 1 (1959). (3) Emerson, G. F . , Watts, L., Pettit, R., J. Am. Chem. Soc. 87, 131 (1965). (4) Hubel, W., Braye, E . H . , J. Inorg. Nucl. Chem. 10, 250 (1959). (5) Longuet-Higgins, H . C., Orgel, L. E., J. Chem. Soc. 1956, 1969. (6) Maitlis, P. M . , "Advances in Organometallic Chemistry," F. G. A. Stone and R . West, eds., Academic Press, in press. (7) Pettit, R., et al., in preparation. 1966.


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Chemistry of Cyclobutadiene-iron Tricarbonyl - ACS Publications

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