The Cyclobutadiene Molecule

The Hiickel secular equation for the square ring with N = 4 is  

It is also seen that they belong to the same eigenvalue s a  

---

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. ... 

Do not forget that the cyclobutadiene molecule differs from two acetylenes in both JT and a systems. There are a total of eight orbitals to be sketched for each side of this reaction. 

Using the Internet, find a Huckel determinant solver and determine the relative energies of the tt molecular orbitals for (a) the butadiene molecule, which has four tt electrons, and (b) the cyclobutadiene molecule, which also has four tt electrons. Can you explain why cyclobutadiene is labeled with the word antiaromaticl... 

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... 

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). 

Our interest in carbenes is closely related to our work on conjugated hydrocarbons containing four electrons. The parent molecules of three completely different families within this category are 1,3-cyclobutadiene (1), cyclo-propenylidene (2), and trimethylenemethane (3). 

Trimerization of 1-alkynes to substituted cyclobutadienes occurs in reactions of RhCl(l-alaninate)Cp with HC CR (R = Ph, tol), which afford Rh -C4HR2 (C=CR) Cp (310) possibly via intermediate dialkynylrhodium(III) complexes. Reductive coupling to an /j -diyne complex, which coordinates the third molecule of alkyne, is followed by further coupling to the rhodacyclopentadiene and reductive elimination of the cyclobutadiene (Scheme 72). ... 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

Whereas for the Dih structure of cyclobutadiene the 50-5, splitting calculated using a multireference double (MRD) Cl treatment is 46 kcal/ mol, for (62) this value is about 89 kcal/mol [87AG(E)170], which also exceeds the value of the S0-S, splitting for (63) (Fig. 2). Ab initio calculations show (89JA6140) that 1,3,2,4-diazadiboretidine (62) has, unlike cyclobutadiene, a rhombic structure without bond length alternation in the ring, which is characteristic of the antiaromatic molecules. 

Also, there is an esthetic element mixed into this motivation, which probably is derived from the tradition of stripped-down test molecules in physical organic chemistry, in which studies of the parent member of a series tend to be valued more highly than those of its derivatives. One thinks, for example, of the emphasis placed on studies of the unsubstimted molecules methylene, norbomyl cation, cyclobutadiene, tetrahedrane, benzyne, and so on. The higher valuation also is associated in some cases with the formidable difficulties experienced by experimentalists in the synthesis and observation of these species. 

Cyclobutadiene 4 is extremely unstable under normal conditions. However, it was obtained and kept at room temperature for several months inside 5 by Cram and coworkers [5], who called the latter molecule a molecular flask. 

However, attempts along these lines, although they may well have led to the production of cyclobutadiene molecules as short-lived intermediates, gave dimers and oligomers of the desired product. Free cyclobutadiene does appear to be unstable, but a complex can be prepared by the reaction ... 

Fig. 4.17 The n molecular orbitals and n energy levels for a cyclic four-p-orbital system in the simple Hiickel method. The MOs are composed of the basis functions (four p AOs) and the eigenvectors, while the energies of the MOs follow from the eigenvalues (Eq. 4.69). This particular diagram is for the square cyclobutadiene molecule. The paired arrows represent a pair of electrons of opposite spin, in the fully-occupied lowest MO, i/q, and the single arrows represents unpaired electrons of the same spin, one in each of the two nonbonding MOs, ij/2 and 1//3 the highest n MO, 1I/4, is empty in the neutral molecule...

---