Bridgehead bond

Scheme 21 Inverted angle calculated from the X-ray geometrical parameters and bond lengths of the bridgehead bond...

Tetraphosphabicyclo[1.1.0]butane 58 (Scheme 24) has been reported [60, 61]. The SE is much higher than cyclotriphosphane 14 or P (see Sect. 3.3.4), but stiU lower than bicyclo[1.1.0]butane 45 [62, 63], in agreement with the lone pair effect (see Sect. 2.1.4). Interestingly, the bridgehead bonds have been observed to be shorter than a normal P-P bond (2.218 A in diphosphine) [64]. The related... 

The authors suggest the isomerization proceeds via the rupture of the weak bridgehead bond, viz. ... 

The values of these Arrhenius parameters contrast dramatically with those obtained for the bicyclo[2,2,0]hexane isomerization. In this compound there is no weak bridgehead bond, and hence the reaction path is more closely akin to that for cyclobutane itself. The similarity of the A factors for this reaction and that for other simple cyclobutanes supports this contention. If this is so, then the lowering of the energy of activation in this bicyclic compound by some 7 kcal mole from that observed in the alkylcyclobutanes is to be attributed to extra strain energy in this molecule. 

As in the case of the bicyclo-octane the isomerizations must occur by a disrotatory process. It is clear that, owing to the rigid nature of these bicyclobutenes, considerable stretching of the bridgehead bond is necessary before appreciable twisting of the cyclobutene ring can... 

The validy of e in classifying bonds is borne out by the analysis of theoretical densities the ellipticity of the C—C bonds in the series ethane, benzene, ethylene increases from 0.0 to 0.23 to 0.45 (Bader et al. 1983). For the very long bridgehead bonds in propellanes, which are shared by three rings, the ellipticity can be quite large, as in [2.1.1] propellane,... 

The propellanes are a class of compounds with three condensed rings, either three- or four-membered, sharing a bridgehead bond. In two [1.1.1] propellane derivatives studied by Seiler et al. (1988), no peaks were observed in the deformation density of the central bridgehead bonds of lengths 1.58 A, but peaks at the apex of the inverted (i.e., pyramidal) bridgehead carbon atoms are in agreement wih electrophilic attack at these positions. 

Even stronger polarizations of double bonds in alkenes are induced by electron withdrawing substituents, as present in enol ethers, enones, and enamines (Sections 4.6.2, 4.7, and 4.9.2). Deshielding of C-7 in norbornadiene (75.5 ppm, Table 4.12) is understood as arising from interaction of antibonding n orbitals at the olefinic carbon atoms with o orbitals of the bridgehead bonds [214, 216]. Spiroconjugation in spiro[4.4]nonatetraene is interpreted similarly [242]. 

The cycloadduct of tetrafluoroethene and cyclopentadiene, 6,6,7.7-tetrafluoro-bicyclo[3.2.0]hept-2-ene (5). undergoes ring expansion by cleavage of the bridgehead bond at 700-750 C and 2-5 Torr in a glass tube filled with quartz rings.35 A mixture of double-bond isomers 6 and 7 is produced in 54% yield at 86% conversion. 

The C1C3 bridgehead bond is the most strained part of the molecule (CSE = 68.6 kcal mol-1258). According to Newton and Schulman260, it is formed from hybrid orbitals of nearly pure p character inclined at an angle of ca 30° with respect to the bond vector a n char-... 

The differences in the hybridizations of the exocyclic bridgehead bonds of adamantane and norbomane are reflected by the bridgehead C13-H coupling constants observed for the two systems Jq1 3-H = 120 lHz for all hydrogens in adamantane 1S°) Jq 3 h = 139 1 Hz for norbomane 172). These coupling constants correspond to 24 and 28 % s character for the exocyclic bridgehead C orbitals of adamantane and norbomane, respectively 173). 

Fig. 3.8. Relief maps of the electronic charge density in the symmetry plane bisecting the bridgehead bond in the propellanes, shown on the right, and in the corresponding symmetry plane in each of ihe related bicyclic structures, shown on the left, (a) [2.2.2]propeliane and bicyclooctane (b) [2.2.1]propellane and norborane (c) [2.1.1]propellane and bicyclohexane (d) [l.l.ljpropellane and bicyclopentane. Contrast the presence of a central maximum in p in [2.2.2]propellanc with its absence in the bicyclic compound. There is a bridgehead bond in the former, but not in the latter.

Attempted peroxy acid epoxidation of the bicyclic ketone (31 equation 13) gave the lactone (33), instead of several possible rational alternatives. The epoxide (32) was implicated as an intermediate when it was independently synthesized from the epoxy alcohol, and shown to give (33) on treatment with aqueous acid.- A mechanism involving scission of the acyl bridgehead bond via the hydrated 1,1 -diol form of the ketone was proposed to account for the formation of this unexpected product. The rearrangement of the isolongifolene derivative (34 equation 14) appears to be mechanistically related. The product (35) is formed by brief treatment with dilute HCIO4 in dioxane as a mixture of isomers believed to arise by acid-catalyzed epimerization of the carbinol center. ... 

Theimolysis of tricyclo[3.2.1.0 ]octane (1) in diphenyl ether produced only polymer, but in the gas phase at 318°C, l,3-bis(methylene)cyclohexane was formed. " The highly strained bridgehead to bridgehead bond in [1.2.3]propellane 1 was readily cleaved by several different reagents to give molecules 2 containing seven-membered rings. 

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