Single norbornenes

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

To date, only double Heck reactions have been described, but triple Heck reactions are also possible [39]. Reaction of the alkynyl aryl iodide 6/1-55 with norbor-nene in the presence of Pd(OAc)2, triphenylphosphane and triethylamine as base led to the cyclopropanated norbornene derivate 6/1-59 as a single diastereomer in 40% yield (Scheme 6/1.13). It can be assumed that the alkenyl Pd-species 6/1-56 is first formed stereoselectively, and this undergoes a Heck reaction with norbornene... 

A special class of synthesis is the utilization of retro-Diels-Alder (RDA) reactions. A double RDA sequence was used to prepare the pyrimido[l,2-A]pyridazin-3-one 118. In this versatile method both reactants of the parent compound were constructed from cyclopentadiene. The parent compound 117 contains two norbornene units and decomposes on heating in toluene in a double RDA reaction leaving two double bonds in the target heterocycle. Similarily, the parent compound 119 decomposes in a single RDA reaction to yield the benzologue, pyridazino[6,l-3]-quinazolin-10-one 120 (Scheme 13) <2000SL67>. 

The polymerization of several different protected monomers was executed with the molybdenum initiator. Two types of norbornene templates were generated as mixtures of diastereomers one class possessed a single saccharide substituent attached through either an endo or exo ester group, and the other was equipped with two carbohydrate residues appended to the bicyclic scaffold... 

The molybdenum-based catalyst MoOCl2(t-BuO)2 has been used to copolymerize norbornene and dicyclopentadiene (20). The polymeric product exhibits a single peak in gel permeation chromatography. 

The distribution of the cis and turns double bonds in a given polymer chain may be expressed in terms of the ratios rt = (tt)/(tc) and rc = (cc)/(ct). If the probability of formation of a cis double bond is independent of the configuration of the previous double bond, the distribution will be random (Bernoullian) and characterized by a single parameter rt = 1 /rc. This is the case for polymers of norbornene with less than 35% cis content, but for polymers with more than 50% cis content the distribution is generally somewhat... 

Hydrogenation of a mixture of styrenes ArCH=CH2 (or reactive alkenes, such as norbornene or ethylene) and symmetric or mixed carboxylic anhydrides [(RC0)20 or (RCO)O(COR )] in the presence of cationic rhodium catalysts ligated by triphenylar-sine (Ph3As), generates hydroacylation products ArCH(Me)COR as single regioiso-mers in high yields.108... 

Norbornene derivatives bearing two alkynes undergo cascade enyne metathesis reactions when treated with a first generation ruthenium carbene and ethylene, giving heterocyclic dienes [28]. The ROM of the norbornene moiety initiates the cascade enyne RCM reactions (Scheme 14). When ethylene is replaced by a monosubstituted alkene, a single enyne RCM takes place, after the initial ROM of norbornene. 

Single-site metallocene catalysts are also highly reactive vis-a-vis cycloolefins such as cyclobutene, cyclopentene or norbornene. While homopolymers of these cycloolefins have melting temperatures (>380°C), much too high for technical processability, ethylene-cycloolefin copolymers (COC s) - e.g. ethylene-norbornene copolymers - are amorphous materials with glass transition temperatures, above which they become soft and processable [W. Kaminsky, J. Polym. Sci. A, Polym. Chem., 2004, 42, 3911]. 

Ferrocene derivatives containing a single disulfide link are occasionally obtained from reactions of fc(SH)2, e.g., with norbornene [84] or with the oxonium salt [(Ph3PAu)30]BF4 (Scheme 5-18) [90]. Splitting of l,2,3-trithia-[3]ferrocenophane, fc(S3), by organolithium reagents followed by air oxidation is another pathway to disulfides (Scheme 5-51) [95]. 

In 7-isopropylidene-norbornene (INORl) hyper conjugation also has a significant influence on the geometrical parameters. All single bonds which interact with the 7r-systems... 

Recently, norbornene was reported to undergo ring-opening polymerization in the presence of molybdenum, tungsten, and rhenium halide catalysts (33). In each of these studies (33-35), the catalysis was interpreted by mechanisms involving the scission of the carbon-carbon single bond adjacent to the double bond, e.g., Eq. (12). 

Thus, for a highly active, single-component catalyst for the addition polymerization of norbornenes it seems that a necessary requirement is accessibility to the metal by the norbornene substrate. We refer to this family of catalysts as being naked since all of their ligands (olefins) are readily displaced by the norbornene monomer to afford a highly active cationic metal nucleus nickel catalysts being preferred. The envisaged active site is illustrated in Fig. 4.5. 

Moreover, the microstructure of the norbornene homopolymer that is formed using this multi-component catalyst is identical to that produced by naked nickel as evidenced by and NMR spectroscopy. Based on these observations, it is clear that the active catalyst formed in situ from the multi-component system is very similar to the naked nickel single component catalyst [50]. 

Thus, we discovered the unusual activation of nickel toward the polymerization of norbornene-type monomers by CgFs transfer from B(C6F5)3 to nickel [58], a reaction pathway that is typically a decomposition route for transition metal catalysts [59]. This discovery led to the development of a class of neutral, single-site nickel complexes containing electron-withdrawing group such as CgFs that are effective for the polymerization of norbornene-type monomers. 

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