Bridged-head atoms

Clearly, however, this induced conformational change will, by contrast, dramatically decrease the accessibility of the 7r-system of the heterocycle and thus addition reactions at a bridge head atom will be considerably hindered. 

The FE-NBMO and HMO-NBMO coefficients of it radicals corresponding to a- and 0-substitutions in naphthalene are derived in Chart I to serve as examples. In the case of a-substitution, the FEMO model offers two possibilities for treating the effective potential at a bridge head atom adjacent to a substitution site, that is, either as a joint or a nonjoint atom. In the first case, an appendage of the free-electron pathway is considered to be pointing toward the a-position. In this chapter, these types of atoms in PAH fragments are taken to be joint positions. 

A bridged-ring system is one in which two alicyclic rings have two or more atoms in common. These atoms bridge-head atoms) do not have to be adjacent to one another. Such compounds have a unique numbering pattern and nomenclature. 

A-Boc-4-tosyloxypiperidine (161) undergoes, upon (—)-sparteine-mediated deprotonation, cycloalkylation to form via the lithium compound 162 the l-azabicyclo[3.1.0]hexane 163. 163 is subsequently deprotonated at the bridged-head carbon atom and lithium compound 164 is trapped by silylation the yield of 165 and the e.r. are low (equation 37)" " . 

In condensed heteroaromatic systems with a bridge-head pyrrolic nitrogen atom, tr-electron density is always shifted from the electron-rich six-membered ring (formally contains 7 Tr-electrons) towards the five-membered ring (formally has 6 Tr-electrons). As a result electrophiles are directed to carbon atoms of the latter. Thus, imidazo[ 1,2-a]pyridines (140) unsubstituted at C-3 almost always react with electrophiles at that position. 

Saturated and Partially Saturated Systems with Bridge-head Nitrogen Atoms and Saturated Five- and Six-membered Ring Heterocyclic Compounds , T. A. Crabb, in MTP International Review of Science, Organic Chemistry Series One , ed. D. H. Hey, Butterworth, London, 1973, vol. 4, pp. 291-317. 

Figures 16.8.2(e) and (f) show two other isomeric forms of Teg+. In TcgiWCIe )2> the Teg+ cation is composed of two flve-membered rings, each taking an envelope conformation, with an average Te-Te bond length of 275 pm and a relatively short transannular Te- Te bond of 295 pm. In (Te6)(Te8)(WCl6)4, Teg+ exhibits a bicyclo[2.2.2]octane geometry with two bridge-head Te atoms.

In order to avoid an overloading, this review is limited to bicyclic cryptands possessing nitrogen atoms in bridge-head fashion. A more detailed discussion, concerning the complexing behaviour of chiral cryptands will be given in the end of this article. 

As an example of an amine of unusually high catal3dic activity, Farkas et al. studied the 1,4-diazabicyclooctane catalyzed reaction of phenyl isocyanate with 2-ethylhexanol 39). This amine is a di-tertiary base, N (C2H4)3N, with the N atoms at the bridge heads. The reaction was found to follow the second-order kinetics, and the rate of reaction was proportional to the diazabicyclooctane concentration. The temperature dependence of the uncatalyzed and the catalyzed reaction between the 23° and 47° corresponds to an energy activation of 11.1 and 5.5 kcal./mole for the uncatalyzed reactions, respectively. 

D) Bicyclo[3.3.0]octane one hydrogen in each of the methylene groups adjacent to the bridge-head hydrogen is eclipsed widi it, so all hydrogen atoms are exchangeable. 

The pyridinium / -toluenesulfonate (PPTS) generates a mild acidic medium that allows the selective hydrolysis of the oxazoline. This happens by protonation of the nitrogen, followed by an electronic movement that begins in the electron pair in one of the oxygen atoms in the bridge-head acetal, and produces the expulsion of the oxazoline oxygen. 

The exchange of the anionic Si bridge-head in the tripodal ligand systems by a neutral boron atom was evaluated in a next step. Moret and Peters introduced the neutral TPB ligand scaffold (where TPB=tris-(o-diisopropylphosphinophenyl)borane= B(o-C6H4P Pr2)3 ), which leads to a remarkable reactivity (Scheme 56). The TPB ligand is elec-... 

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