Cyclobutadienes isolation

These compounds have to be regarded as the closest analogs to the parent cyclobutadiene isolated to date and their preparation indicates the powerful stabilizing ability of sterically demanding groups. 

The story of the generation, detection, and determination of the structure of cyclobutadiene isolated in low temperature matrices is one of the most celebrated and long running in the history of matrix isolation, and it illustrates both the strengths and pitfalls of the technique. 

The same thought occurred to early chemists However the complete absence of natu rally occurring compounds based on cyclobutadiene and cyclooctatetraene contrasted starkly with the abundance of compounds containing a benzene unit Attempts to syn thesize cyclobutadiene and cyclooctatetraene met with failure and reinforced the grow mg conviction that these compounds would prove to be quite unlike benzene if m fact they could be isolated at all... 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

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) ... 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

In most cases involving syntheses using cyclobutadiene, it is advantageous to use an excess of the trapping agent, but here excess -benzoquinone hampers isolation of the pure adduct. 

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. ... 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

In some cases, double bonds add to triple bonds to give cyclobutenes, apparently at about the same rate that they add to double bonds. The addition of triple bonds to triple bonds would give cyclobutadienes, and this has not been observed, except where these rearrange before they can be isolated (see 15-63) or in the presence of a suitable coordination compound, so that the cyclobutadiene is produced in the form of a complex (p. 60). 

In at least one case the mechanism is different, going through a cyclobutadiene-nickel complex (p. 60), which has been isolated. " ... 

With the successful chemistry of the cymantrenes and the (cyclobuta-diene)tricarbonyl iron, the quest for tetraethynylated cyclobutadienes based on CpCo-stabilized complexes arose. Why would they be interesting Whereas all derivatives of 63 and 68 exhibit reasonable stability when their alkynyl substituents are protected by either an alkyl or a trimethylsilyl group, the desilylated parents are isolated only with difficulty and are much more sensitive. 

All of the ethynylated cyclobutadienes are completely stable and can be easily manipulated under ambient conditions, as long as the alkyne arms carry substituents other than H. For the deprotected alkynylated cyclobutadiene complexes, obtainable by treatment of the silylated precursors with potassium carbonate in methanol or tetrabutylammonium fluoride in THF, the stability is strongly dependent upon the number of alkyne substitutents on the cyclobutadiene core and the nature of the stabilizing fragment. In the tricarbonyUron series, 27b, 27c, 29 b, and 28b are isolable at ambient temperature and can be purified by sublimation or distillation under reduced pressure. The corresponding tetraethynylated complex 63 e, however, is not stable under ambient conditions as a pure substance but can be stored as a dilute solution in dichloro-methane. It can be isolated at 0°C and kept for short periods of time with only... 

Stable, isolable at ambient temperature, can be purified by subiimation under reduced pressure. The CpCo-ligated cyclobutadienes are more stabie than the tricarbonyiiron ones. 

The CpCo-stabilized ethynylated cyclobutadienes are considerably more robust, and the parent 76 can be isolated as a yellow crystalline material, stable at ambient temperature for several hours. At 0°C 76 decomposes in the course of several days, which is indicated by darkening of the formerly brillant-yellow needles. The stability of 76 made in X-ray analysis feasible and the bond angles/distances obtained are in good agreement with reported values for ethynylated cyclobutadiene complexes already described [35,36]. 

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. 

During a very early study on tetramethylcyclobutadiene16 it was anticipated by one of us that 2,2,4,4-tetramethyl-l,3-dicarbenacyclobutane formed in the pyrolysis of dry di-sodium salt of 2,2,4,4-tetramethyl-l,3-cyclobutanedione-bisto-sylhydrazone might be a reasonable precursor for the cyclobutadiene derivative. But the only product — isolated in a cooling trap — was tetramethylbutatriene. 

During all these studies on cyclobutadienes and tetrahedranes formed via carbenes as transient species we wondered whether matrix isolation IR spectroscopy might be a good tool for the direct observation of cyclopropenylidene (2) and trimethylenemethane (3). This is indeed the case. 

Our interest in ketocarbenes originated from the aim to matrix-isolate oxirene (76) (Scheme 1), the oxygen-containing hetero analog of cyclobutadiene (1)... 

The case of this dimerisation can be related to the closeness in energy of cyclobutadiene HOMO and LUMO. In contrast, the tritertiary butyl cyclobutadiene in which the bulky t-butyl groups hinder dimerisation can be isolated at room temperature. 

Species identified by matrix isolation include cyclobutadiene and benzyne, where products derived from these molecules are formed at higher temperatures. 

Thermal cyclization of alkynes with Fe(CO)5 proceeds predominantly with CO incorporation to afford (cyclopentadienone)Fe(CO)3 complexes, however small amounts of cyclobutadiene complexes can be isolated (see Section VI.B.)15. 1,6-FIeptadiyne and 1,7-octadiyne substrates 107 have been utilized to prepare bicyclo[3.3.0] and bicyclo[4.3.0] complexes 108 in excellent yield (equation 12)115, while 1,8-nonadiynes gave bicyclo [5.3.0] complexes in low yield. 

It has to be pointed out, however, that additional valence isomers (CR)4 such as cyclopropenyl carbene, are found in local energy minima, if other cuts through the 54 dimensional hyperspace are calculated (30) - in agreement with compounds isolated recently e.g. from the reactions of both isomers, tetra (tert. butyl) tetrahedrane and cyclobutadiene, with tetracyanoethylene (32). 

Cyclooctatetraene, CgHg, has a D2g tublike structure with four rather isolated double bonds. Cyclooctatetraenes can be the products of the catalytic cyclotetramerization of alkynes, and cyclobutadienes may be the intermediates. The BN homologues of cyclooctatetraenes have been known since 1962 (67). Like cyclooctatetraene, molecules of [(SCN)BNtBu]4 were shown to have a tublike ring structure of S4 symmetry with alternating bond lengths of 140 and 146 pm, the shorter ones perpendicular to the direction of the S4 axis (68). 

Some years after the successful isolation and characterization of cyclobutadiene, another C4H4 isomer, methylenecyclopropene (3), was prepared by Billups and... 

Unlike pyridine (85MI1), tri-rm-butylazete (101) has, as judged from its reactivity, olefinic character [88AG(E)(27)272]. As in the case of cyclobutadiene, the push-pull substitution [69CC240 88AG(E)(24)1437] promotes the stabilization of azete, as has been demonstrated by the isolation of the first thermodynamically stable substituted azete, tris(dimethylamino)azete... 

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