Active sites cyclopropane

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

In order to study the nature of active sites generated over MeFSM-16 by the sulfiding with hydrogen sulfide, further study using cyclopropane isomerization, which is known to require strong Bronsted acid site, was performed. The effect of sulfiding of MeFSM-16 in the... 

A related allylic C-H insertion that has considerable promise for strategic organic synthesis is the reaction with enol silyl ethers [23]. The resulting silyl-protected 1,5-dicarbonyls would otherwise typically be formed by means of a Michael addition. Even though with ethyl diazoacetates vinyl ethers are readily cyclopropanated [l],such reactions are generally disfavored in trisubstituted vinyl ethers with the sterically crowded donor/acceptor carbenoids [23]. Instead, C-H insertion predominates. Again, if sufficient size differentiation exists at the C-H activation site, highly diastereoselective and enantioselective reactions can be achieved as illustrated in the reaction of 20 with 17 to form 21 [23]. 

Isomerization of cyclopropane was studied by Hall and co-workers (84, 103), who reported catalyst activity to increase with vacancy concentration. They postulated that the active site consisted of a vacancy and neighboring Al—OH group. Hydrogenation of ethylene at room temperature was found to increase with increase in catalyst reduction and irreversible hydrogen was apparently not involved in the reaction (104). 

Both the intramolecular and the intermolecular secondary metathesis reactions affect the polymerisation kinetics by decreasing the rate of polymerisation, because a fraction of the active sites that should be available as propagation species are involved in these non-productive metathesis reactions. The kinetics of polymerisation in the presence of metal alkyl-activated and related catalysts shows in some cases a tendency towards retardation, again due to gradual catalyst deactivation [123]. Moreover, several other specific reactions can influence the polymerisation. Among them, the addition of carbene species to an olefinic double bond, resulting in the formation of cyclopropane derivatives [108], and metallacycle decomposition via reductive elimination of cyclopropane [109] deserve attention. 

Recent work has demonstrated that the activity of Ga/H-MFI(Si,Al) catalysts for the activation of propane was due to the presence of dual active sites, consisting of highly dispersed (Ga+3,02) ions pairs, acting in synergy with neighbouring Brensted sites to form initially protonated pseudo-cyclopropane... 

Polymerization of the Bicyclo [w.1.0] alkanes. Some cyclopropane type hydrocarbons have already been polymerized—i.e., cyclopropane (28), 1,1-dimethyl- (10) and isopropylcyclopropane (14), particularly in presence of cationic catalysts. This work has established that a it complex is formed between the active site in the chain and a new molecule of monomer the complex then develops via opening of the cyclopropane ring in certain cases this opening is accompanied by transfer of a hydride ion (10). 

Cationic polymerization of XII may therefore be visualized in terms of Figure 9 according to which the ir complex initially formed between the active site and the monomer is converted into a carbocation with rupture of a C—C bond in the cyclopropane. This cation may be Xlla, b, or c, but only the latter can give rise to Structure M, alone compatible with the experimental data. This change necessitates the transfer of a hydride ion to transform the primary cation XIIc into the more stable tertiary cation Xlld. On this assumption, the termination reaction probably occurs as the result of the displacement of a proton in the alpha position with respect to the C+, which is relatively easy, whereas the steric hindrance around the active site does not favor continued poly-... 

Courtois E, Ploux O. Escherichia coli cyclopropane fatty acid synthase is a bound bicarbonate ion the active-site base Biochemistry 2005 44 13583-13590. 

It appears that this facile bond cleavage is the result of an assisted adsorption of the cyclopropane ring promoted by the adsorption of the unsaturated species on the catalyst surface adjacent to the hydrogenolysis active site as depicted in Fig. 20.3. Unsaturated substituents are not the only species that can assist in the adsorption of the cyclopropane ring. Cyclopropyl amines are hydrogenolyzed over palladium or Raney nickel at 80°C and 50 atmospheres by breaking one of the bonds adjacent to the amine group.26,27... 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

Cleavage of a C-H bond followed by reformation of the same bond on hydrolysis often takes place without formation of other products than the stereoisomer(s) of the starting material. Such reactions have therefore been used to perform isomerization and racemization of cyclopropanes on a preparative scale (Section 5.2.4.). The reaction sequence is also a very important tool in the studies of mechanisms of reactions involving cyclopropanes. By quenching the reactions with deuterium oxide rather than water, formation of one or several C-D bonds at active sites has been observed in numerous cases. However, these reactions... 

The solvent has also a great influence. In fact when the reaction was carried out without a solvent the conversion of styrene was very high, in spite of the fact this reagent was used in excess, but the yield of cyclopropanes was low. When acetonitrile was used as a solvent the reaction did not take place, which indicates that this solvent coordinates the active sites of the catalyst hindering the access of the reagents. 

Of various mechanisms that may be proposed, the only acceptable one is that summarized in Equation 5. It is assumed that the attack on the cyclopropane system by the active site leads to the formation of a 7r complex, which later rearranges to a carbo cation. The rupture of the bond of carbons 1 and 2 and the rotation between A and carbon 2 involves the appearance of a positive charge on carbon 1. The primary carbo cation formed will be able to rearrange into a more stable tertiary carbo cation by hydride shift. The polymers obtained by such a mechanism would have structures P2a to P4b. They are the only ones having one methyl group in the side chain per monomer unit and two in the case of l-methylbicyclo[n.l.O]alkanes. It must, therefore, be assumed that this is the mechanism to be considered, and that structures P a to P4b are the only ones that agree with the data. 

The proposed polymerization mechanism suggests that a tt complex is formed from the spirane cyclopropane and the carbo cation active site (Equation 10). 

Fig. 9. Mechanism for the opening of the cyclopropane ring of cycloeucalenol B and.N are active groups at the active site of enzyme the hydrogen which is transferred from B to C-19 can exchange with in the aqueous environment.

Cycloheptanes.—One synthesis of karahanaenone (231) depends upon thermal rearrangement of a 2-methylene-5-vinyltetrahydrofuran, and the conditions for this type of reaction have been examined on a simpler model (232), which arises from the dihydrofuran (233) at 140—200 °C. The reaction to the cycloheptenone (234) occurs rapidly at active sites on a glass surface, but is arrested in tubes coated with sodium hydroxide. Higher temperatures and lower pressures give two other compounds (235) and (236). All these reactions involve the biradical (237), as does the conversion of the cyclopropane (238) into the cycloheptenone (234). Karahanaenone (231) has also been made by isomerization of terpinolene epoxide (239) with boron trifluoride etherate. Eucarvone (240 R = H) should not be... 

Catalyst deactivation is a serious drawback to exploiting alkene metathesis for the production of olefins. There are many routes for the deactivation of a rhenium-based catalyst. Polar compounds, such as H2O, which might be present as an impurity in the reactants, are catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state, (ii) adsorption of polymeric by-products on the surface of the catalyst, blocking the active sites, and (iii) reductive elimination of the metallacyclobutane intermediate. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. Therefore, reductive elimination of the metallacyclobutane intermediate (to form cyclopropane, or 3-elimination to an alkene) is probably the main cause of deactivation and always seems operative [46,55,56]. 

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