Bridged bicyclobutanes

The structure of 203 has been redetermined312 by refinements of the XD data in the centrosymmetric orthorhombic space group Pnca. The molecule has C2 symmetry in the crystal. The side bonds in the bicyclobutane unit are longer, Cl—C2 1.540 (6) A, the bridge bond is shorter, Cl —C6 1.442 (5) A, and the fold angle of 110.7 ° (calculated from the coordinates) is narrower than in free 186. 

Parameters of the octabisvalene derivative313 204 also fit the correlations for bicyclobutanes. The bridge bonds are 1.505 (3) and 1.501 (3) A, the dihedral angles 118.0 (1) and 

FIGURE 23. Difference electron-density maps of 201. (a) Section in the plane through the midpoint of and perpendicular to the bridging Cl—C3 bond of the bicyclobutane part, (b) Section in the plane of the three-membered ring C1, C2, C3. Contour lines are at 0.05 e A-3 intervals. Full lines indicate positive, dashed lines negative regions. Reproduced by permission of the International Union of Crystallography from Reference 309 

9 (2)°. The side bonds in the C3 rings which are opposite to the electron-withdrawing PhS02 substituents are shorter (on the average), 1.495 A, than the adjacent bonds, 1.512 

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The photoreactions of electron acceptors with several bridged bicyclobutane systems, e.g. tricyclo[4.1.0.02,7]heptane (75), give rise to CIDNP effects supporting negative hfcs for Ht and FI7 and positive hfcs for H2 and H6 as well as H3 3. and H5<5,. These results would place electron spin density at C, and C7, in agreement with the involvement of the A, HOMO (121) [249, 284],... 

Bicyclobutanes are also cleaved, but small changes in conditions cause different bonds to break, as shown for Moore s bridged bicyclobutane 39 and its derivative 44 in Scheme 11. Thus water, alcohols, and HCN in chloroform or THF add stereo-specifically across the bridging bicyclobutane bond to form 40 [82], implying that... 

Various bridged bicyclobutane analogs have been synthesized and the acidity of their bridgehead hydrogens exemplified by kinetic or synthetic means. A partial listing would include the following systems tricyclo[4.1.0.0 ] heptane (4) ... 

Although the ultimate bridged bicyclobutane system, tetrahedrane (8), has resisted synthetic efforts to date, its tetralithio derivative (9) is claimed" to be a product of low temperature ( —45°C) photolysis of dilithioacetylene in liquid ammonia. Calculations suggest that the most stable structure of tetralithiotetrahedrane should have a lithium cation centered at each of the four faces of the tetraanion rather than at the corners as indicated by structure (9). 

Molecule 5 (Fig. 13.13). A possible synthesis of this, suggested in Chapter 2, is recapitulated here. The carbene-bridged bicyclobutane 25, possibly accessible from... 

Pyramidane, 5 (Chapter 2) is a very promising synthetic goal because of its likely fairly high kinetic stabihty. A plausible route to it is insertion into a C-H bond by a carbene-bridged bicyclobutane. 

Further evidence for the pathways described above comes from labeling studies in which deuterium replaced both hydrogens at the bicyclobutane bridgehead positions, leading to the cyclic dienes with deuteriums at carbons 2 and 3 " This study also included an unsaturated derivative of the tricyclo heptane above as well as measurements of activation parameters for all of the bridged bicyclobutane systems known at that point of time. 

The bridged bicyclobutane (40 R = H) is attacked by silver(i) ion at the sterically least congested edge bond to give the transient allylic ion (345) which undergoes... 

NMR data are in accord with a bicyclobutane-type structure for B4R4H3 and the presence of two bridging hydrogen atoms. At elevated temperature the atoms B2 and B3 become equivalent and also the bridging hydrogen atoms. This enantio-merization is shown in Figure 2.1-20. 

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

More recently, the doubly lithiated derivatives of [4.1.1]- and [3.1.1]propellasilanes underwent bridging metathesis at the bicyclobutane ends with dihalides of germanium, tin and transition metal titanium, to give the corresponding highly strained metallacyclic propellanes (equation 7)29. 

The formation of nickela- and palladabicyclobutanes from ge/w-difluorocyclo-propabenzene has been recorded (Section V.A.4). Such bicyclobutane formation is diverted to a bridged nickelabicyclodecatriene with tris(ethene)nickel(0) when TEEDA is used in place of TMEDA (equation 34)277. However, the behaviour of l278 and its disilyl... 

In related tricyclo[2.1.0.0 jpentane (202) systems substituted at C1, C5 and C3, the dihedral angle between the three-membered rings is about 95 to 99°. The bridge bond Cl—C5 shortens as the bond angles C5—Cl—R and Cl—C5—R to the R substituents open, following a linear correlation . Irngartinger and Lukas have also found a linear relationship with the fold angle of the bicyclobutane unit ... 

The most thoroughly studied bicyclobutane system is a bridged derivative, tricy-clo[4.1.0.0 ]heptane (Moore s hydrocarbon, 47), which has revealed many facets of radical cation reactivity. Nucleophilic solvents (CH3OH, H2O) or ionic nucleophiles (CN ) capture 47 +, leading to products formally derived by addition across the transannular bond (53, 54, 55). Significantly, each product is derived by backside attack, as indicated clearly for the monomethyl derivative (54, 55, R = CH3) [180], In the absence of nucleophiles, a (dimeric) rearrangement product (57) was obtained (Scheme 2) [180, 181]. 

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