Bicyclo pentanes, structure

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

Another class of compounds that has attracted much interest are the sesquioxides and sesquichalcogenides of the type (RE)2nY3 [(M2Y3) ] (E = Si, Ge, Sn Y = O, S, Se), due to their unique structural properties. These molecules have polyhedral cage-like structures analogous to those of bicyclo[l.l.l]pentane (n = 1) (1), adamantane (n = 2)... 

FIGURE 7. Structures of 2,4,5-trichalcogena-l,3-disila- and digerma-bicyclo[l.l.l]pentanes... 

Similarly, two stable diastereomeric (synjanti) tricyclic ozonides were obtained from bicyclo[9.4.0]pentadec-l(ll)-en-12-one and ozone at — 75 °C in pentane followed by treatment of the ozonolysis product with H2NOMe. Their structures were confirmed by X-ray crystallography (Scheme 41) <2006EJ01978>. 

Reactions that involve cis-protected Mo2(DAniF)2 units have led to a variety of square structures with dicarboxylate anions. The linkers that have been used are oxalate (121), fumarate (122), ferrocenedicarboxylate (123) and 4,4 -biphenyldicarboxylate (124). The midpoints of the M02 units constitute a square, and their structures have been confirmed by H NMR spectroscopy and X-ray studies. Similar molecular squares containing Rh2 + units have been prepared with linkers such as oxalates (120), bicyclo[l.l.l]pentane-1,3-dicarboxylate (125), tetrafluoroterephthalate (126), 1,4-cubanedicarbboxylate (127), terephthalate (128), fumarate (129) and trans 1,4-cyclohexanedicarboxylate (130). The structures of these molecules have been established by spectroscopic and single-crystal diffraction studies. Detailed electrochemical investigations of these complexes have also been undertaken. ... 

A [l.l.ljpropellane moiety (100) is believed to be involved as an important contributor in the valence bond resonance structure with the bridgehead bicyclo[l.l.l]pentan-l-yl radical 99, which appears to have one of the largest y-proton ESR hyperfine splitting constants for alkyl radicals known to man . 

Notably, the two geminal substituents on the cyclopropane ring stabilize both the norcaradiene and the bicyclo[2.1.0]pentane ( housane ) structure with respect to valence isomerization. The 3,3-dimethyl-3i/-pyrazoles mentioned above are prepared readily by 1,3-dipolar cycloaddition of 2-diazopropane to cyanoacetylenes. A more convenient synthesis of 4,5-dicyano-3,3-dimethyl-3//-pyrazole is the [3-1-2] cycloaddition of 2-diazopropane to fumarodinitrile, followed by oxidation of the 5,5-dimethyl-2-pyrazoline thus obtained with manganese dioxide. 

Electrophilic addition reactions of carbenes to cyclopropane structures are rare and not synthetically important. Reaction of carbenes with bicyclo[2.1.0]pentane generally gave products arising from C —H insertion at one of the methylene groups of the four-membered ring. ° An exception was difiuorocarbene which gave 1,1-difluorohexa-l,5-diene (2) in very low yield by cleavage of two C —C bonds. - ... 

Bicyclo[1.1.0]butane, bicyclo[l.l.l]pentane, [l.l.ljpropellane, and spiro[2.2] pentane are other molecules where strain and rehybridization affect molecular properties. The molecules show enhanced reactivity that can be attributed to characteristics of the rehybridized orbitals. The structures are shown in Scheme 1.6, which also shows calculated AIM charge distributions and strain energy. 

In an attempt to provide the first experimental insight into the solution-phase structure of 4, we, together with the groups of Ryzhkov and Hadad, recently reported a TRIR spectroscopic study [86]. We were unable to cleanly detect IR bands due to 4, but we did analyze the kinetics of bicyclo[2.1.0]pentane (5) formation despite the fact that certain IR bands for 5 in the C-H stretching region overlap with corresponding bands in biradical 4 (Scheme 2.7). This analysis was supported by computational investigations. 

Fused Five-membered Rings.—Structures based on the linear fusion of three five-membered rings are easily synthesized by intramolecular photocyclizations of dicyclopent-l-enylmethanes [viz. (75)], followed by in situ addition of methanol to the presumed intermediate bicyclo[2,l,0]pentanes (76) the general method has been applied in a synthesis of the hirsutane carbon skeleton (77) found in hirsutic acid and related natural sesquiterpenoids. 

Figure 28 presents the PE spectra " of bicyclo[l.l.l]pentane 10 and of di-bicyclo-[l.l.l]pentyl 11, the latter in its conformation. These molecules are again examples of the fact that vicinal interaction terms must be included in an EBO model, for even a qualitative discussion of their electronic structure, and thus of their PE spectra. This... 

The gas-phase reactions of HeT with bicyclo[2.1.0]pentane, bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane (41) have been investigated and the formation of tritiated hydrocarbons retaining the bicyclic structure of the starting substrate has been observed. This finding indicates that the gaseous bicycloalkylium ions are formed via exothermic trition transfer from the HeT to the bicyclic substrate (equation 107). Due to the... 

Padwa, A., Shelter, E., Alexander, E. (1968). The correlation of the crystal and molecular structure with the nuclear magnetic resonance spectrum of a bicyclo[l.l.l]pentane derivative. J. Am. Chem. Soc., 90(14), 3717-3721. 

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