Uncatalyzed reactions alkenes

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

The other catalytic approach to the 1,3-dipolar cycloaddition reaction is the inverse electron-demand (Fig. 8.17, right), in which the nitrone is coordinated to the Lewis acid, which for the reaction in Scheme 8.7 was found to be deactivated compared to the uncatalyzed reaction. In order for a 1,3-dipolar cycloaddition to proceed under these restrictions the alkene should be substituted with electron-donating substituents. 

The theoretical investigations of Lewis acid-catalyzed 1,3-dipolar cycloaddition reactions are also very limited and only papers dealing with cycloaddition reactions of nitrones with alkenes have been investigated. The Influence of the Lewis acid catalyst on these reactions are very similar to what has been calculated for the carbo- and hetero-Diels-Alder reactions. The FMOs are perturbed by the coordination of the substrate to the Lewis acid giving a more favorable reaction with a lower transition-state energy. Furthermore, a more asynchronous transition-structure for the cycloaddition step, compared to the uncatalyzed reaction, has also been found for this class of reactions. 

Nafion-H (144), a perfluorinated resin-sulfonic acid, is an efficient Bronsted-acid catalyst which has two advantages it requires only catalytic amounts since it forms reversible complexes, and it avoids the destruction and separation of the catalyst upon completion of the reaction [94], Thus in the presence of Nafion-H, 1,4-benzoquinone and isoprene give the Diels-Alder adduct in 80% yield at 25 °C, and 1,3-cyclohexadiene reacts with acrolein at 25 °C affording 88 % of cycloadduct after 40 h, while the uncatalyzed reactions give very low yields after boiling for 1 h or at 100 °C for 3.5 h respectively [95], Other examples are given in Table 4.24. In the acid-catalyzed reactions that use highly reactive dienes such as isoprene and 2,3-dimethylbutadiene, polymerization of alkenes usually occurs with Nafion-H, no polymerization was observed. 

The hydroboration of e%o-cyclic alkenes affords stereochemically complemental products between the catalyzed and uncatalyzed reaction (Scheme 1-16). The hy-... 

Unlike the late metal chemistry reviewed above, these reactions did not require Michael acceptor substrates, but the reactions were rather slow (turnover frequencies range from 2 to 13 h at 22°C). For phosphino-alkenes (Scheme 5-15, Eqs. 1-3), a competing uncatalyzed reaction gave six-membered phosphorinane rings (Scheme 5-15, Eq. 6) this could be minimized by avoiding light and increased temperature. For phosphino-alkynes (Scheme 5-15, Eqs. 4 and 5), the products were unstable and could not be isolated [14]. 

Diazomethane is also decomposed by N O)40 -43 and Pd(0) complexes43 . Electron-poor alkenes such as methyl acrylate are cyclopropanated efficiently with Ni(0) catalysts, whereas with Pd(0) yields were much lower (Scheme 1)43). Cyclopropanes derived from styrene, cyclohexene or 1-hexene were formed only in trace yields. In the uncatalyzed reaction between diazomethane and methyl acrylate, methyl 2-pyrazoline-3-carboxylate and methyl crotonate are formed competitively, but the yield of the latter can be largely reduced by adding an appropriate amount of catalyst. It has been verified that cyclopropane formation does not result from metal-catalyzed ring contraction of the 2-pyrazoline, Instead, a nickel(0)-carbene complex is assumed to be involved in the direct cyclopropanation of the olefin. The preference of such an intermediate for an electron-poor alkene is in agreement with the view that nickel carbenoids are nucleophilic 44). 

Hydroboration of alkenes with BH3 and R2BH proceeds at room temperature without a catalyst. As an exception, the reaction of the less reactive catecholborane (CBH, 529) proceeds at 70-100 °C. It was found that the slow reaction of CBH is accelerated by a Rh catalyst, and the catalyzed and uncatalyzed hydroborations give different products [200]. By the uncatalyzed reaction, the ketone in the enone 530 is attacked to form 531. On the other hand, the alkene is hydroborated to give 532 by the catalysis of RhCl(Ph3P)3 [201]. Different regio- and stereoselectivities are observed in the Rh-catalysed hydroboration of cyclohexenol (533) with catecholborane and uncatalysed reactions with 9-BBN-H [202],... 

The catalyzed hydroboration did not attract much attention until Sneddon in 1980 and Noth in 1985 reported that rhodium complexes significantly accelerate the addition of B-H bond to alkenes or alkynes. The protocol was proved to be an interesting strategy to realize the different chemo-, regio-, diastereo-, and enantioselectivities, relative to the uncatalyzed reaction. The reaction has been reviewed.132-135... 

An important aspect of the metal catalyzed hydroboration reaction is its ability to selectively reduce certain functionalities within a molecule. For instance, a key step in the synthesis of a tripeptide derivative containing the Phe-Arg hydroxyethy-lene dipeptide iosostere is the selective rhodium-catalyzed hydroboration of a lactone. The use of disiamylborane, 9-H-BBN, dicyclohexylborane, and (.9)-alpmeborane, however, gave only low to variable yields of the alcohol due to competitive reduction of the y-lactone to the hemiacetal (equation 8). In another example, hydroboration of the diene illustrated in equation (9) with HBcat and RhCl(PPh3)3 gave exclusive formation of the terminal alcohol derived from reaction of the less substituted alkene. Interestingly, uncatalyzed reactions failed to hydroborate this substrate selectively. ... 

Alkene- and alkyne-substituted Fischer carbenes participate as dienophiles in Diels-Alder reactions. The conditions are usually mild and the reaction proceeds smoothly at room temperature. Similar isomeric ratio and rate acceleration is observed to that of Lewis acid-promoted Diels-Alder reactions between methyl acrylates and dienes when compared to the uncatalyzed reactions. The reactions are endo-selective. Asymmetric Diels-Alder reactions are... 

Complexes of cationic rhodium compounds with asymmetric phosphane ligands catalyze the hydroboration of prostereogenic alkenes with catecholborane (see Section D.2.5.2.1.4.). The product alcohols are of 7-96% cc (Table 3). Enantioselectivity is excellent for the hydroboration/oxidation of styrenes but low for stilbenes. The small number of examples studied to date precludes generalizations, however, compared to the uncatalyzed reaction, opposite regioselec-tivity is observed for the addition to styrenes. 

With alkenes having internal C=C bonds, hydroalumination is disfavored by both kinetic and thermodynamic factors, and the uncatalyzed reaction is generally unfeasible. The hydroalumination of internal alkenes can be catalyzed by the addition of titanium(IV) alkoxides but the same catalysts also promote the isomerization of the secondary aluminum alkyls generated into their primary isomers (equation 16). ... 

From a mechanistic point of view, two different ionic mechanisms have to be considered (due to the presence of oxygen the radical chain mechanism plays no role in the technical process) first, the uncatalyzed reaction of ethylene and chlorine and second, the metal halide catalyzed reaction. Both routes compete in this process. The uncatalyzed halogenation was studied extensively for the bromina-tion of olefins [14, 15] (Scheme 4). It is commonly accepted that the halogenation of olefins starts with formation of a 1 1 -complex of halogen and alkene followed by formation of a bromonium ion. Subsequent nucleophilic attack of a bromine anion leads to the dibromoalkane. However, when highly hindered olefins (such as tetraneopentylethylene) are used, formation of a 2 1 r-complex, as an intermediate between 1 1 ir-complex and a bromonium ion, is detectable by UV spectroscopy. In the catalyzed reaction the metal halide polarizes the chlorine bond, thus leading to formation of a chloronium or carbonium ion. Subsequent nucleophilic attack of a chloride anion gives the dichloroalkane [12] (Scheme 5). 

Although the process is exothermic, there is usually a high free energy of activation for uncatalyzed alkene hydrogenation, and therefore, the uncatalyzed reaction does not take place at room temperature. However, hydrogenation will take place readily at room temperature in the presence of a catalyst because the catalyst provides a new pathway for the reaction that involves lower free energy of activation (Fig. 7.9). 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

In concert with the general observation that electron withdrawing groups in the dienophilic portion of the Diels-Alder electrocyclic Jt4s-Fjt2s addition reaction facilitate the process (raising both the HOMO and LUMO of the alkene), haloal-kenes can serve in this capacity. As shown in Equation 7.69,1-bromoethene (ethylene bromide, H2C=CHBr) serves as a dienophile in the reaction with cyclopentadiene to provide both endo-2-bromobicyclo[2.2.1]heptane and the corresponding exo-isomer. As expected on the basis of the Alder Endo rule (Chapter 4), the former predominates in the uncatalyzed reaction. 

Ketene-alkene cycloadditions are found to be catalyzed by Lewis acids with an acceleration compared to the uncatalyzed reaction, and with a high diastereoselectivity. " In an exceptional example the catalyzed reaction favors... 

Figure 7.9 Free-energy diagram for the hydrogenation of an alkene in the presence of a catalyst and the hypothetical reaction in the absence of a catalyst. The free energy of activation for the uncatalyzed reaction (AG (i)) is very much larger than the largest free energy of activation for the catalyzed reaction (AG (2)) (the uncatalyzed hydrogenation reaction does not occur).

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