Partial bridging

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Methyl-2-norbornyl (66) and 1,2-dimethyl-2-norbornyl (67) cations have partially bridged structures. 

Since the products are optically active, this excludes the symmetric, resonance-stabilized allylic ion intermediate. The main (treats) product arises from the preferential attack of Br- on the more stable, less strained, partially bridged 30 intermediate (as compared to 31). Similar conclusions were drawn from the results of the iodination and haloiodination of 2,3-pentadiene256 and from bromination of 1,2-cycloalkadienes.257... 

For diorganotin dicarboxylates, the symmetrically chelated structure has been proposed from infrared and far-infrared spectra (9). A nonsymmetrically chelated configuration and a partial bridging of acetoxy groups have also been proposed based on infrared data (10, 11). 

Although the synthesis of each chromophore material is to some extent unique (and hence requires individual description which is beyond the scope of this review), most chromophores used for device prototyping involve amine donor moieties. Thus, once a large quantity of aldehyde-terminated amine do-nor/partial bridge material has been synthesized, it can be reacted via Knoeve-nagel condensation with a variety of acceptor groups to form different series of chromophores. The acceptor in Schemes 1-4 is 3-cyano-5,5-dibutyl-2-dicy-anomethylene-4-methyl-2,5-dihydrofuran (which we refer to as the cyanofuran, CF, acceptor). This is synthesized by a literature method [213]. 

It was possible to explain the stereochemical phenomena of Scheme 18 by assuming the intermediacy of partially bridged cations 3 [89]. A detailed analysis of the transition states, which explains the simultaneous formation of rearranged products, will be developed in Section III.B.2. 

As discussed in Section III.B.l, these reactions are assumed to proceed via partially bridged cations. Attack of Ph2CH+ at (Z)- and ( )-2-... 

Static classical ion [235] and the corresponding norbornyl- and cyclopentyl derivatives it was concluded that the charge delocalization through a-bridging is much less significant in [234] than in the partially bridged 2-methylnorbornyl cation. In this paper the classical structure [233] was also considered to be the major contributor to the observed nmr-shifts in the parent system. 

Mathematical modeling attempts to connect constructions made in the experiential world with those made in the world of ideas. Since we cannot perceive with infinite detail, we cannot measure with infinite resolution, and we cannot compute in the experiential world with infinite precision, there exists a gap between any system constructed in terms of experiential distinctions and a mathematical system that attempts to model it. While this gap cannot be fully avoided, it can be at least partially bridged by choosing appropriate mathematical constructions for our modeling purposes. 

Shtarev AB, Pinkhassik E, Levin MD, Stibor I, Michl J (2001) Partially Bridge-fluorinated Dimethyl Bicyclo[l.l.l]Pentane-l,3-Dicarboxylates Preparation and NMR Spectra. J Am Chem Soc 123 3484-3492... 

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