Cyclobutadiene intermediates

In the bisdecarboxylation of the cyclobutenedicarboxylic acid 52, products are obtained whose formation possibly involves a cyclobutadiene intermediate [322], A case of a 1,3-bisdecarboxylation has been reported in the preparation of a bicyclobutane (Table 11, No. 26). An elimination, that involves the cleavage of an carbon-oxygen bond after the decarboxylation, has been observed with the carboxylic acid 53 (Eq. 33) [282]. 

This mechanism is supported by the transformation of preformed metallacyclo-pentadienes with alkynes,73,76-78 and labeling experiments79 that excluded the involvement of cyclobutadiene intermediates. It also accounts for the observation that terminal alkynes yield 1,2,4- (and 1,3,5-) trisubstituted benzene derivatives as the main product but not 1,2,3 derivatives. In contrast with this picture in cyclotrimerization with PdCl2-based catalysts, stepwise linear insertion of alkynes takes place without the involvement of palladacyclopentadiene.80... 

Another reaction which may proceed via a cyclobutadiene intermediate and subsequent proton shifts is the dimerization of (23) to yield cycloocta[b]naphthalin (50)15). 

However, convincing evidence has emerged recently which works against participation of cyclobutadiene intermediates in these reactions. Whitesides and Ehmann (198) have shown that l,2,3-trimethyl-4,5,6-tri(methyl-d3)benzene is not a product in the Co2(CO)b assisted cyclo-trimerization of MeCsCCDa. This evidence establishes that the activity of the catalyst in this system does not depend on the generation of free or metal complexed cyclobutadiene intermediates. 

The transition metal-catalyzed chemistry of acetylenes appears consistent with the S5unmetry principles described in this section. Much of the catalytic chemistry of acetylene very likely proceeds on a single metal center. In this case, smooth transformation to a cyclobutadiene ligand i.e., 26 27) should be a rare event. Cyclobutadiene intermediates are clearly... 

Another mechanism for olefin disproportionation had been proposed earlier by these same authors (29). In this reaction scheme, the olefins first undergo dehydrogenation to acetylenes which subsequently cyclize to cyclobutadiene surface intermediates. The cyclobutadiene intermediate is suggested as the quasi-cyclobutane proposed by Bradshaw et al. (27). There is, however, no evidence supporting a mechanism involving acetylene or cyclobutadiene intermediates. 

Other preparative methods make use of cyclobutadiene intermediates which react with acetylene compounds. The first method was reported by Criegee and his coworker 6) who prepared tetramethylcyclobutadiene from the 1,2-diiodo derivative (4). In the second method, the cyclobutadiene irontricarbonyl complex is oxidized by Ce,v to produce free cyclobutadiene (5) 7). This reaction is widely used for the synthesis of cyclobutadiene. 

Other synthesis using cyclobutadiene intermediates are as follows (25,26) ... 

The application of this reaction to an aza analog leads to an azacyclobutadiene 138). This compound is cleaved to two 1-alkyne and two nitrile molecules which suggests that an equilibrium exists between the two azacyclobutadiene structures (129). This reaction is widely used for the synthesis of cyclobutadiene intermediates. 

Scheme 7.24. The trimerization of l-fluoro-3,3-dimethyl-l-butyne [f-butylfluoroacetylene, F-CsCC(CH3)3] to l,2,3-trifluoro-4,5,6-tri(l,l-dimethylethyl)benzene. The final step is an allowed six-electron ring opening. The cyclobutadiene intermediate is presumably formed in a diradical process (see Viehe, H. G. Merenyi, R. Oth, J. F. M. Valange, P. Angew. Chem. Int. Ed. Engl., 1964, 76,888).

Biphenylenes undergo oxidative addition to various low-valent metals to form the corresponding dibenzometalloles [7]. The reaction with Cp RhfPMcj) involved C-H activation prior to C-C activation, as with the case of C-C activation of cyclopropane [7c]. On the other hand, the reaction with [Rh(Pi-Pr3)2] initially formed the r -arene complex, which led to the dibenzorhodacycle [7g]. Density functional theory (DFT) calculation suggested the C-C activation proceeds via the rhodium Tj -cyclobutadiene intermediate (Scheme 1.4). 

Cyclobutadiene escaped chemical charactenzation for more than 100 years Despite numerous attempts all synthetic efforts met with failure It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable Beginning m the 1950s a variety of novel techniques succeeded m generating cyclobutadiene as a transient reactive intermediate... 

Stabilization of Unstable Intermediates. Transition metals can stabilize normally unstable or transient organic intermediates. Cyclobutadiene has never been isolated as a free molecule, but it has been isolated and fully characterized as an iron tricarbonyl complex (138) ... 

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

Since cyclotrimerization and metathesis of alkynes occur simultaneously, a common intermediate might be involved, which would mean that the metathesis of alkynes does not proceed via a cyclobutadiene complex. 

The compound was generated (as an intermediate that was not isolated) and two isomers were indeed found. The cyclobutadiene molecule is not static, even in the matrices. There are two forms (52a and 52b) that rapidly interconvert. ... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

Transient intermediates like benzynes or cyclobutadiene are extremely reactive dienophile. 

Flash vacuum pyrolysis of deuterium-labeled [l,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 [l,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. 

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. 

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