Bridgehead atom

For the purposes of nomenclature, the simple cryptands are assumed to be macro-bicyclic and nitrogen is assumed to be the bridgehead atom. The different cryptands are designated by assigning numbers according to the number of heteroatoms in each ethylenoxy chain. The three cryptands shown below are designated [ 1.1.1 ]-cryptand (9), [2.2.1 ]-cryptand (10) and [3.2.2]-cryptand (11), respectively. 

The exclusion of nitrogen-bridged bicyclic azines is purely arbitrary since the work on such azinoazines with a 10 7r-electron system (and the necessarily accompanying quaternary nitrogen-bridgehead atom) shows the activation expected of other aromatic azinium... 

Bridgehead atom (Section 4.9) An atom that is shared by more than one ring in a polycyclic molecule. 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

The bond between the bridgehead atoms is inverted in the [l.l.l]propellane system 59. The hybrid orbitals are directed away from each other, hi the geminal 0-0 interaction, the greater lobes of the geminal hybrid orbitals at the bridgehead atom can be in phase with each other, while the front lobes of the hybrid orbitals of the normal and inverted bonds at the 1,3-positions are in phase with each other... 

Reaction between 34a, chloropentafluorobenzene, and 1 equiv of water in polar aprotic solvents produced the tricyclic 45 by hydrolytic ring opening between the bridgehead atoms. Under similar conditions 34b afforded two isomeric compounds, 46 and 47 by ring opening between the N(9)-C(9 ) atoms (Scheme 2) <2003JOC3139>. 

The use of theoretical methods in the study of bicyclic systems with P-, As-, Sb-, or Bi- bridgehead atoms has contributed to an increased understanding of the geometry, stability, and ring-strain effects of these systems. In addition, important data relating to basicity and the interpretation of nuclear magnetic resonance (NMR) and X-ray data have been generated. A vast majority of the work done has focused on P. 

Compounds with two rings have three paths between the two bridgehead atoms. The numbers in brackets identify the number of carbon atoms between the two bridgeheads. Each additional ring is characterized by an additional path joining two atoms, which serve as additional bridgehead. These additional paths are identified by the number of carbon atoms between the two atoms, with a superscript that identifies the position numbers of the two bridgehead atoms. 

Figure 3.12 Active site of a reduced form of the Fe-only hydrogenase from Desulphovibrio desul-phuricans. The Fe atom on the right is defined as the proximal Fe (relative to the neighbouring [Fe-S] cluster), Fep the Fe atom on the left is defined as the distal Fe, FeD. The arrow indicates the potential hydron-binding site on FeD that is occupied by either HzO or an extrinsic CO in the structure of Cp I. Also shown is a close contact between the bridgehead atom X of the exogenous dithiolate ligand and the S atom of cysteine-178. (Reprinted with permission from Parkin et al., 2006. Copyright (2005) American Chemical Society.)...

Fig. 3 Typical ICT probes (left) and representative spectroscopic responses toward selected metal ions (right). Color code (left) coordinating atoms in blue, bridgehead atom of the fluorophore that takes part in complexation in orange, formal donor fragment in red, formal acceptor fragment in green (right) hypsochromic shifts in red, bathochromic shifts in green, fluorescence enhancement in violet, fluorescence quenching in blue. Symbols in table Aabs, 7em,

The replacement of the bridgehead atoms by planar tricoordinate skeletal units should then lead to an achiral molecule. As is seen from example 14 this is not true, i.e.,... 

According to Lonsdale et al., the four aromatic bridgehead atoms C8, Cg, Cu and C14 are bent out of the plane of the remaining benzene carbon atoms by about 0.168 A (14°) at 291 °K (see Fig. 1) the aromatic nuclei are deformed into a boat conformation. The distance between the aromatic bridgehead atoms C3, C14 and Cg, Cu has shrunk to 2.751 A. The intramolecular distance between the plane formed by atoms C4, C5, C7, Cg and that formed by atoms C12, C13, C15, Ci6 is only 3.087 A the van der Waals distance between two parallel benzene nuclei is usually at least 3.4 A. There must therefore be considerable transannular -overlapping in compound 2. To compensate for this the CH2-CH2 bond length is unusually large 1.630 A at 291 °K at 93°K it is only 1.558 A. 

The iH—NMR spectrum of [2.2](l,6)naphthalenophane (46) is not consistent with a symmetrical structure 46 b 71>. According to molecular models, the CH2—CH2 bridges in 46 a are not perpendicular but inclined toward the naphthalene planes and skewed with respect to each other. Rotation of the naphthalene nuclei about the axis through the bridgehead atoms can be ruled out. 

In order to illustrate the difference between these concepts, Corey analyses structure 26 in terms of the number of bridgehead atoms, the number of times bridged and the number of bridging atoms (the rings with the maximum number are indicated with bold numbers)... 

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