Cyclobutadiene stabilization

Bunz et al. have made an interesting contribution to this field as they developed the area of highly alkynylated cyclobutadienes stabilized by ty -bonding to CoCp moiety. 

A pentafulvalene has been trapped as a diiron complex using Fe(CO)5. Thiophene dioxide has also been stabilized as its tricarbonyliron complex. The irradiation of a-pyrone (45) in the presence of Fe(CO)5 gives cyclobutadiene stabilized by complexation with the Fe(CO)3 moiety (46), together with the (ct-pyrone)Fe(CO)3 complex (47) (eq 29). ... 

Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

Cyclobutadiene itself is not stable at room temperature. Several derivatives with stabilizing groups have been prepared by the acid-catalyzed dimerization of alkjmes (R. Gompper, 1975). Less substituted cyclobutadienes could be obtained by photolytic reactions in solid matrix at low temperatures (G. Maier, 1973, 1974). 

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

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). 

A general type of stabilization for iminoboranes is a cyclodimerization, which yields diazadiboretidines (RBNR )2 that are isoelectronic with cyclobutadienes. 

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) ... 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of... 

Both thermochemical and MO approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy which is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncydic polyene models are called antiaro-maticf ... 

A number of alkyl-substituted cyclobutadienes have been prepared by related methods Increasing alkyl substitution enhances the stability of the compounds. The tetra-/-butyl derivative is stable up to at least 150°C but is very reactive toward oxygen. This reactivity reflects the high energy of the HOMO. The chemical behavior of the cyclobutadienes as a group is in excellent accord with that expected from the theoretical picture of the structure of these compounds. 

It is of interest to consider at this point some of the specific molecules in Scheme 9.2 and compare their chemical properties with the calculated stabilization energies. Benzo-cyclobutadiene has been generated in a number of ways, including dehalogenation of... 

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination lo a metal and, indeed, were predicted theoretically on this basis by H. C. Longuei-Higgins and L, E, Orgel (1956) some 3y before the first examples were synthesized, In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy t /i and there would be an unpaired electron m each of the 2 degenerate oibilals 2, Coordination to a metal provides funhei interactions and avoids this unstable configuration, See also the discussion on ferra-boranes (p. 174). 

The reaction of diazirines 2 with the kinetically stabilized cyclobutadiene 1 gives 577-1,3-di-azepines 4 via the initial cycloadducts 3.159... 

Ever since the special stability of benzene was recognized, chemists have been thinking about homologous molecules and wondering whether this stability is also associated with rings that are similar but of different sizes, such as cyclobutadiene... 

There are some simple cyclobutadienes that are stable at room temperature for varying periods of time. These either have bulky substituents or carry certain other stabilizing substituents such as seen in tri-tert-butylcyelobutadiene (57). Sueh... 

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

The kinetic stabilities and the donor-acceptor properties of cyclic conjugated molecules [68] have been described (Scheme 12) in the theoretical subsection (Sect. 2.2.2) to be controlled by the phase property. There is a parallelism between the thermodynamic and kinetic stabilities. An aromatic molecule, benzene, is kinetically stable, and an antiaromatic molecule, cyclobutadiene, is kinetically unstable (Scheme 13). 

Cyclobutadiene owes its observed instability much to the kinetic property. Cyclobutadiene dimerizes in the argon matrix above 35 K [69] and only exists for 2-10 ms under low pressure [70, 71], However, cyclobutadiene is stabilized by bulky substituents (Scheme 30). The ferf-butyl derivative was quantitatively prepared even at a high temperature (130 °C) [72]. Yellow crystals of the cyclobutadiene fused by two seven-membered rings did not decompose below 240 °C [73]. 

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... 

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]. 

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

In contrast to benzene, the pi-conjugative stabilizations of D4h-symmetric cyclobutadiene, 18, are found to be exactly zero. The absence of conjugative stabilization may also be judged from the 7ta—7rb orbital contours in Fig. 3.44. Indeed, it is... 

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