Bridged systems

The balance between the controlling influence of steric effects and secondary orbital interactions in the chemistry of heterosubstituted propellanes has been analysed. In a discussion of intramolecular interactions in a variety of systems, Leonard probes the unusual hybridization of l-azabicyclo[3.3.3]undecane (1) 

Bredt s Rule is further assessed in a report by Shea, which covers both carbocyclic and heterocyclic bridged systems. 

A number of more general aspects concerning reactivity and the determination of structure are discussed here. Specific examples are more fully analysed in subsequent sections. 

The earlier structural assignments of the products of oxidation of tropine (4) with hydrogen peroxide have been corrected. Analysis by n.m.r. at 220 MHz establishes the major isomer to be (5) and the minor isomer to be (6). The shift reagent europium tris-(l, 1,1,2,2,3,3-heptafluoro-7,7-dimethyl-octane-4,5-dione) has been used with (7) but fails to establish conclusively whether contact shifts are important close to the co-ordination site. 

Ring inversion has been studied in the cyclic amide (8) and related compounds. Further n.m.r. studies concern l,5-diazabicyclo[3,3,l]nonanes, l,5-diazabicyclo[3,2,l]octanes, 3,7-diazabicyclo[3,3,l]nonanes, and 3-ethylidene-1 -azabicyclo [2,2,2]octane.  

Mass spectra have been reported for tropanes, 6-azabicyclo[3,2,1 ]-octanes, and 3-azabicyclo[3,3,l]nonanes.  

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It is now understood tliat inclusion of nearest and second-neighbour interactions is adequate for describing tunnelling interactions in many bridged systems [31, 32]. Moreover, tunnelling interactions have been dissected for various... 

Positively activated olefins have also been condensed with dienamines derived from aldehydes 321,330,347,348) and ketones. Of special interest is the formation of bridged systems from homoannular dienes (229-231) which has been applied to the isoquinuclidine system of the iboga alkaloids (137-140,349). 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

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Theoreticians have also considered the difference in the distance dependence of hole transport in G(T)nG sequences. Voityuk et al. [56] predict more rapid hole transport for Tn vs An bridges and a ca. 10-fold decrease in kt for each additional bridging base. Olofsson and Larsson [55] also predict more rapid hole transport via Tn vs An bridges. However, Troisi and Orlandi [54] and Rak et al. [57] have recently reached the opposite conclusion, going so far as to predict that hole transport in G(T)nG sequences would occur via the polyA sequence in the complementary strand. If hole transport does in fact occur via the complementary An bridge in Tn-bridged systems, then the... 

An intramolecular Mannich reaction of carboline derivative 352 afforded a complex bridged system containing an indolo[2,3-tf]quinolizidine moiety, as a mixture of two diastereomers. One of them, 353, was transformed into the alkaloid tacamonine 15 (Scheme 79) <2002T4969>. 

Fallis, in the synthesis of longifolene (53), a bridged sesquiterpene, performed the intramolecular cycloaddition of compound 51 as a key reaction in the construction of the bridged system (Scheme 9.13) [55]. 

If a tether is bound to the C(5) or C(6) atoms, bridged systems (215) and (216), respectively, are formed, and the number of atoms between the six-membered ring and the double bond can be reduced to one. 

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