Norbomenes strain

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

New kinds of living polymer systems result from the reactions of transition metals with cyclic, strained olefins 16). These polymerizations proceed through the intermediacy of metal carbenes and are exemplified by the polymerization of norbomene initiated by bis(cyclopentadienyl)-titane-cyclobutane described recently by Grubbs17>. 

In anti addition to a cyclic substrate, the initial attack by the electrophile is also from the less-hindered face. However, many (though not all) electrophilic additions to norbomene and similar strained bicycloalkenes are syn additions." In these cases attack is always from the exo side, for example," ... 

Reactive unsaturated nitroso compounds such as 1112 can also be readily prepared from a-halooximes such as 1111 on treatment with powdered Na2C03 in diethyl ether to give, in the presence of enoltrimethylsilyl ether 1113 or strained olefins such as norbomene and other dienophiles, hetero-Diels-Alder adducts such as 1114 and 1115 in moderate yields [150-155] (Scheme 7.47). 

Norbomene derivatives are very popular monomers for ROMP due to a comparably high ring strain and good functionalisability. The latter is needed to append any desired functional unit to the monomer [98, 99]. 

Fig. 3.31 Steric control in alternating ROMP Tendencies of norbomene and cyclooctene to give productive olefin metathesis upon coordination are illustrated by a thick arrow (preferred monomer) or a thin arrow (less favoured monomer) (a) only minor steric hindrance SlMes greatly favours the polymerisation of the strained norbomene (b) the rotating phenylethyl-group induces a steiically more congested active site, leading to preferred incorporation of the smaller cyclooctene (c) the flexible and small cyclooctene derived polymer fragment permits the incorporation of the bulky norbomene...

Selective aromatic functionalization has been a permanent object of research since the ninenteenth century. Catalysis has offered a powerful tool to achieve this goal. Over the years we have worked out a complex catalytic system consisting of an inorganic compound such as a palladium salt and an organic molecule containing a strained double bond such as norbomene (1,2). We have seen that these two catalysts cooperatively react with an aromatic iodide, an alkyl iodide and a terminal olefin. The following equation reports an example (L = solvent and/or olefin) (3). 

Rh2(OAc)4 is the most effective and versatile of the three catalysts used. Terminal and non-terminal olefins, strained olefins (norbomene, norbomadiene) and conjugated olefins (styrene) all react in good yield. 

Gunter and co-workers have applied this methodology to the condensation of norbomene derivative 90 (block A, Scheme 32) with epoxide 94 (block B, Scheme 33) <98S593>. Thus, using the Crossley porphyrin-a-dione 47, condensation with 1,2,4,5-benzenetetramine tetrahydrochloride and then the product with a strained dione, gave 90 as block A. 

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

It was found that 2-propenyloxymagnesium bromide reacts much more readily with nitrile oxides than other known dipolarophiles of electron-deficient, electron-rich, and strained types, including 3-buten-2-one, ethyl vinyl ether, and norbomene, respectively (147). Therefore, this BrMg-alkoxide is highly effective in various nitrile oxide cycloaddition reactions, including those of nitrile oxide/Lewis acid complexes. 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

As a last example we mention polydicyclopentadiene (DCPD). Dicyclopentadiene is the Diels-Alder adduct of cyclopentadiene, an abundant product from the cracker in the refinery. It contains a strained norbomene ring and a less strained cyclopentene ring. ROMP leads to opening of the norbomene ring, but some reaction of the cyclopentene ring also takes place, which leads to cross-linking (Figure 16.26). 

The optimum catalyst for a given reaction depends primarily on (a) the energetics of the reaction and (b) the functional groups present in the substrate. If, for instance, a strained cycloalkene such as norbomene or cyclobutene is to be polymerized, a catalyst of low activity will be sufficient to attain acceptable reaction rates. RCM... 

It appears that oxiranes known to give predominantly a-deprotonation in basic media (cyclopentene, cyclooctene and exo-norbomene oxide) are also the more strained (Table 1 entries 3, 6, 7). On the other hand, oxiranes that give mainly -deprotonation (butene, cyclohexene oxide) have lower strain energies and higher a-anion stabilities (Table 1 ... 

Ring strain enhances alkene reactivity. Norbomene, for example, undergoes rapid addition at 0°C.12... 

In addition to the role of substituents in determining regioselectivity, several other structural features affect the reactivity of dipolarophiles. Strain increases reactivity. Norbomene, for example, is consistently more reactive than cyclohexene in 1,3-dipolar cycloadditions. Conjugated functional groups also usually increase reactivity. This increased reactivity has most often been demonstrated with electron-attracting substituents, but for some 1,3-dipoles, enamines, enol ethers, and other alkenes with donor substituents are also quite reactive. Some reactivity data for a series of alkenes with a few 1,3-dipoles are given in Table 6.3. Scheme 6.5 gives some examples of 1,3-dipolar cycloaddition reactions. 

Although it is generally accepted that the exo approach to norbomene is favored over the endo, the magnitude of the AAE is rarely measurable experimentally unless some fraction of the endo product can be detected. The exo approach to norbomene is favored over the endo orientation by nearly 3 kcalmol for both PFA and DMDO, while the exo TS for benzonorbomadiene is favored by nearly 5 kcalmol (Figure 27). The preferred exo approach does not appear to result from steric interactions. It is of particular interest that the AE for norbomene is only 0.9 kcalmol greater than that for cyclohexene despite the SE = 19.2 kcalmol" for the strained bicyclic alkene. Thus, in the absence of twist-strain we observe little rate enhancement due to strain energy. 

Polycycloolefins are prepared by ring opening metathesis polymerization (ROMP) using transition metal catalysts [31], By far the most commonly studied monomer is dicyclopenta-diene (Fig. 1.7). Cycloolefins with high ring strains like norbomenes and their analogs polymerize very fast and the polymerizations are quite exothermic. Metathesis catalyst systems tend to be sensitive to the presence of polar compounds and the polymerization rates... 

Highly strained hydrocarbons such as quadricyclane (structure see below) may serve as high-performance aviation fuels (Hill etal., 1997). It is, therefore, important to know the environmental behavior of such compounds, particularly with respect to spills. In this context, Hill et al. (1997) have studied the hydrolysis of quadricyclane in aqueous solution at pH values between 3 and 4 as well as in soil slurries exhibiting pH values between 4.6 and 6.4. They found that in homogeneous aqueous solution at a given pH, the disappearance of quadricyclane followed pseudo-first-order kinetics, and that two major products (i.e., nortncyclyl alcohol and exo-5-norbomen-2-ol) were formed at a ratio of about 15 ... 

Ruthenium complexes mediate the hydroamination of ethylene with pyridine.589 The reaction, however, is not catalytic, because of strong complexation of the amine to metal sites. Iridium complexes with chiral diphosphine ligands and a small amount of fluoride cocatalyst are effective in inducing asymmetric alkene hydroamination reaction of norbomene with aniline [the best enantiomeric excess (ee) values exceed 90%].590 Strained methylenecyclopropanes react with ring opening to yield isomeric allylic enamines 591... 

Aryl azides react with olefins to give 1,2,3-A2-triazolines.201-203 Whereas unactivated olefins are sluggish toward aryl azides, strained bicyclic systems, on the contrary, are particularly reactive.204-208 The rate of phenyl azide addition to a series of olefins parallels the heat of hydrogenation (Table Vffl).107 This thermodynamic parameter is a measure for the degree of strain associated with these molecules.200 Bicyclo[2.2.1]hep-tenes, such as norbomene (37), react readily at room temperature, and the addition occurs at the less hindered exo side (39).,06 1°7 210 211 A striking example of the increased... 

As noted earlier, many bicyclic and polycyclic alkenes undergo hydroxymeicuration-demeicuration. Norbomene and derivatives328 give cis adducts, but less strained alkenes afford the expected trans adducts. 

Strained alkenes such as norbomene, norbomadiene and bicyclo[2.2.2]octenes readily insert into ir-allyl complexes with the less substituted allyl terminus linked to the alkene (equation 81).233-261 This reaction has been utilized to prepare interesting prostaglandin analogs.233 234... 

Because the norbomene double bond of dicyclopentadiene is more strained, its selective hydrofbrmyl-ation can be achieved under mild reaction conditions (entry 9). Under more vigorous reaction conditions both double bonds are hydroformylated. The exo faces of both rings are the least hindered, accounting for the selectivity. 

Hydroformylation of norbomene proceeds slowly in spite of the expected reactivity of the strained double bond. Racemization of the product aldehyde does not occur under the reaction conditions. The hydroformylation of methyl methacrylate gives only one isomer, which is a useful chiral synthon. Dimethyl itaconate also can be hydroformylated using (12) as the chiral ligand to give aldehyde product in 82% ee accompanied by hydrogenated ester.64... 

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