Cubane, expanded

Structure (5g), with X = SBu1 and MY = ZnMe, is adopted by (MeZnSBut)5,207 formed in the reaction Me2Zn + HSBu This cage can be regarded as an expanded cubane (4w) where the atoms marked Mc, Xc correspond to seven vertices of the cube, but in place of the last M triconnected to the (Xc)3 tripod there is an M—X—M unit connected to two pairs of Xc. 

In tackling problems of molecular and electronic structure such as those posed by these sulfur-bridged tetrametallic clusters, two approaches are possible. One is the construction, often heavily reliant upon symmetry arguments, of a general, essentially qualitative model intended to provide a broad description of a series of molecules the other is a detailed quantum mechanical study, at an appropriate level of theory, of individual molecules, followed by a search for generalization or patterns. A combination of both approaches is usually the most valuable for chemical understanding, and much effort along these lines has been expanded on cubane-type clusters and their derivatives. 

The last example is the acidity of the ethynyl-expanded cubane 3. Inspired by the synthesis of the methoxy-substituted buta-l,3-diynediyl expanded cubane 4, Bachrach examined some of the properties of its smaller homolog 3. Since the s-character of the C-H bonds of 3 should be large and its conjugate base 3cb can potentially delocalize the anion into three neighboring alkynyl units, 3 might be quite acidic. 

Predicted to be even more acidic is the hydrocarbon 10, the diethynyl-expanded cubane. The free energy of deprotonation of 10 is 309.4 kcal mol using Eq. (3.2). The DPE of 10 is so small due to the resonance stabilization of the conjugate base. The formal anion can be delocalized into the three adjacent diethynyl... 

Manini, P Amrein, W. Gramlich, V. Diederich, F. Expanded cubane synthesis of a cage compound with a C56 core by acetylenic scaffolding and gas-phase transformations into fullerenes, Angew. Chem. Int. Ed. 2002,4339-4343. 

Bachrach, S. M. Structure, deprotonation energy, and cation affinity of an ethynyl-expanded cubane, 7. Phys. Chem. A 2003,107,4957-4961. 

Bachrach, S. M. Demoin, D. W. Computational studies of ethynyl- and diethynyl-expanded tetrahedranes, prismanes, cubanes, and adamantanes, 7. Org. Chem. 2006, 77, 5105-5116. 

Fe-4S] clusters, all with cysteinyl-S completing tetrahedral Fe coordination (Figure 1) and the function was exclusively electron transport. New types of electron transfer Fe-S clusters involving cubane-type [3Fe S] clusters and double-cubane-type [8Fe-7S] clusters have subsequently been characterized (Figure 1). Moreover, the known functions of Fe-S centers have now expanded to include coupling... 

Another structural motif effective in binding anions is that of two face-to-face 1,3,5-trisubstituted benzenes with bridges containing amines (12). These receptors were observed to bind small anions with some selectivity. In the majority of binding studies, monovalent species such as nitrate and chloride bind more weakly (log Kg2.5) compared to dianions such as sulfate (log Ks 6.0). This structural motif was expanded even further in the elegant azaparacyclophane (13). At pH 4, this cubane is tetraprotonated and binds anionic fluorescent molecules such as l-anilinonaphthalene-8-sulfonate (ANS). ... 

Superb experimental skills led to the construction of the first substituted three-dimensional alkane carbomer, C56 carbo-cubane (38, the experimental structure carries eight methoxy groups for every hydrogen), containing linear dialkyne -C=C-C=C, buta-l,3-diynediyl) fragments as the first member of the expanded... 

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