Cyclobutadienes reactions with

The cyclobutadiene analogue diazadiboretidine reacts with hexafluoroacetone to give a nng expansion product [776] (equation 90) 6-(3-Fluoroaryl)decaborane is formed by alkylation of decaboranyl anion and separation of the two isomers (5- and 6-benzyl) formed by reaction with dimethyl sulfide [777] (equation 91). 

Polymercuration of (cyclobutadiene)cyclopentadienylcobalt complexes has also been reported. Upon reaction with Hg(OAc)2, complexes 88a-c afford a mixture of products containing one to five mercury atoms. Higher yields of the pentamercurials 89a-c were obtained when Hg(OCOCF3)2 was employed (Equation (34)). 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-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. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

Although aUcenes react rapidly with singlet oxygen, reaction with triplet oxygen is normally slow. Exceptions are very electron-rich aUcenes such as tetraaminoethylenes , and normal aUcenes with low-lying triplet states such as cyclobutadienes andketenes . 

Acetylenes are discussed separately here because, in reactions with transition metal compounds, they can act in a variety of ways 34). Thus, they can act as monodentate (2-electron donors) or as bridging groups (4-electron donors), or they can undergo chemical transformations to form cyclobutadiene, cyclopentadienone, or other moieties that incorporate the parent acetylene as a part of the cyclic ir-ligand. Some examples of this last type have already been mentioned (Sections IV,C,l and 4). 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

Cyclobutadiene (26) is antiaromatic and its isolation is not possible. Flowever, it can be stabilized by -coordination of Fp+ to one of the double bonds to give 27, and the uncomplexed double bond in 27 undergoes Diels-Alder reaction with cyclopentadiene to give 28 [4]. As described in Section 9.2, cyclobutadiene (26) can be stabilized as a diene by the )/4-coordination of Fe(CO)3. 

The reactivity of the incarcerated cyclobutadiene was probed by warming the NMR sample to 220°C for 5 min. This resulted in the formation of free cyclooctatetraene (6.108), clearly from ejection of the cyclobutadiene from the cavity and its subsequent dimerisation via 6.107. Reaction with 02 (which is able to enter the cavity of 6.101 gave incarcerated malealdehyde (6.109). 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

In another version of a diphenylcarbodiimide cycloaddition reaction, it produces a [2+2+2] cycloadduct 333 in 78 % yield in its reaction with diethyl 2,4-bis(diethylamino)-cyclobutadiene- 1,3-dicarboxylate 332. ... 

Supporting evidence for the above mechanistic patterns in the majority of metal systems was first established in elegant isotopic labeling studies, which showed clearly that no intermediate with the symmetry of a cyclobutadiene was involved.In one system, the reaction of a cobaltacyclopentadiene with MeC>2CC CC02Me, benzene formation does not involve direct complexation of the third alkyne to the metal. It has therefore been suggested that the conventional insertion process has been here replaced by a direct Diels-Alder reaction with the metallacycle, perhaps as a result of electronic factors (Scheme 25). o ... 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

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, in 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 cyclobutadiene-iron tricarbonyl with methylchlorothio-formate followed by hydrolysis gives rise to cyclobutadienecarboxylic acid-iron tricarbonyl (XII). A Curtius rearrangement of the acid azide derived from Complex XII affords aminocyclobutadiene-iron tricarbonyl (XIII). The dimethylaminomethyl derivative (XIV) is readily available through the Mannich reaction with formaldehyde and dimethylamine. The chloromercury cyclobutadiene complex (XV) is produced upon reaction of Complex III with Hg(OAc)2, followed by treatment with hydrochloric acid. In the simplest substitution reaction, treatment of cyclobutadiene-iron tricarbonyl with CF3COOD produces a mixture of deuterated derivatives of Complex III. 

Cyclopropene provides isomeric tricycles upon reaction with cyclobutadiene generated from its iron tricarbonyl complex (equation 54) but this thermal reaction is more likely to... 

Interest in the photochemistry of the phthalimide systems has continued. The phthalimide derivatives (316) are phot ochemically reactive and on irradiation in acetone yields the cyclized products (317). The reaction involves hydrogen abstraction to yield the biradical (318) which subsequently bonds to afford the observed products. A recent study has examined the behaviour of the anion (319) in an attempt to reduce electron transfer processes. In t-butanol irradiation affords the solvent addition product (320) as the principal product presumably by a free radical path. Minor products (321) and (322) are also formed but are probably artefacts of the work-up procedure. Irradiation of (319) in methanol with added cyclohexene follows a different reaction path. In this system the reaction with methanol is minor while the dominant reaction is addition of the alkene to afford the adduct (323) in 20 % yield. The Dewar benzene derivative (324) is photocheraically unstable and irradiation affords t etramet hyl cyclobutadiene. ... 

---