1-Alkenes orientation

Another stereochemical feature of the Diels-Alder reaction is addressed by the Alder rule. The empirical observation is that if two isomeric adducts are possible, the one that has an unsaturated substituent(s) on the alkene oriented toward the newly formed cyclohexene double bond is the preferred product. The two alternative transition states are referred to as the endo and exo transition states ... 

In theoretical calculations (B3LYP/6-31G) different catalyst conformations and alkene orientation were taken into consideration. The best agreement of experimental and predicted isotope effects were achieved for asynchronous transition state with oxygen atom distances of 2.32 and 1.99 A to carbon atoms 1 and 2, respectively. 

The behaviour of the oxides of Group IV which contains thoria is quite complex. Titania can exist in two forms, anatase and rutile, the former usually giving selective dehydration with Saytzev alkene orientation from 2-alcohols. Hafnia also gives selective dehydration irrespective of any... 

Salt catalysts, mainly phosphates and carbonates, have also been studied using model butanols by Thomke and Noller and Thomke.The carbonate catalyts were fairly inactive, and even decomposed under reaction conditions. Phosphate catalysts gave Saytzev alkene orientation from 2-alcohols, the... 

The mechanism of electrophilic addition to alkynes is the same as with alkenes. Orientation of addition of unsymmetrical reagents to unsymmetrical alkynes is determined by the stability of the intermediate carbocation. 

The detailed mechanism of this cyclopropanation method is still unclear, however, copper and its chiral ligands must be present in the sterically determining step. A carbene complex type intermediate is assumed and metal-carbene and alkene orientations are depicted to minimize steric repulsions. Thus, preferred enantioselection and tram selectivity can be accounted... 

In early reports on nickel(0)-catalyzed (3 + 2] cycloaddition reactions of methylenecyclo-propane with alkenes, orientation and stereochemistry was investigated37. Additionally, palladium-catalyzed versions of this formal [2intramolecular mode of a palladium-catalyzed version, an additional chelation effect can be used for control of regio- and stereochemistry to yield the thermodynamically less favored frtwv-fused bicyclo[3.3.0] octanes 39. 

Light-activated charge transfer takes place with low energy between O2 and the alkene oriented by interaction with the reaction such that there is overlap between the resp>ective HOMO and LUMO orbitals. Adsorption on the cations overcomes the repulsive interaction between molecules when they approach so close that van der Waals radii overlap, with the result that a (photo) chemical reaction occurs. This can be considered again as an example of pre-transition state stabilization, that was discussed earlier in Section 4.2.1. 

Treatment of PtGl2(cod) with pent-4-en-l-ylmagnesium bromide yields rings with both 77 -bound alkenes oriented as to one another (X-ray). This complex has unusual volatility, which has proved useful for GVD of thin platinum films. Its thermolysis in the gas phase and in aromatic solvents has been thoroughly examined (Scheme 7). Kinetic studies reveal the presence of a platinum hydride complex, which is chemically trapped with PhG=GPh, to give the novel (r/, 77 -pentenyl)(l,2,3,4-tetraphenylbuta-l,3-dien-l-yl)platinum(ii) complex by successive insertion of two PhG=GPh molecules, as confirmed by X-ray, and PhG=GPh)2. ... 

Total syntheses of tazettine and 6a-epi-pretazettine have been carried out by application of the Pd-catalyzed cydization of 169, in which a single pentacyclic product 170 was obtained, establishing a preference for an eclipsed orientation of the Pd—C (7-bond and alkene 7r-bond in the key intramolecular insertion step[l40]. 

Hydrochlorination. The addition of hydrogen chloride to alkenes in the absence of peroxides takes place by an electrophilic substitution mechanism. The orientation is in accord -with Markovnikov s mle in -which the hydrogen atom adds to the side of the double bond that -will result in the... 

Chiral nitrones react with alkenes to produce 3,5-disubstituted isoxazolidines that are nonracemic diastereomeric mixtures and are oriented predominantly cis (equation 53) (77CC303, 79JOC1212). 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

Recently, Burger devised an improved method of carrying out mild, regiospecific cyclizations that involve an intermediate that acts as a synthon for a nitrile ylide of HCN [47 (equation 48). With this methodology, cycloadditions with activated alkenes, alkynes, and azo compounds were earned out [47] (equation 49). All such reported reactions were regiospecific and had the same orientational preference... 

In the case of the reaction between 2-diazopropane and diphenyldiacetylene, the reverse (as compared with other diynes) orientation of addition of the first molecule of the diazo compound with a predominant formation of 4-phenylethynylpyrazole is observed. Therefore, it is noteworthy that whereas the regioselectivity of the addition of diazoalkanes to alkenes is well studied audits products have, as a rule, the structure been predicted with respect to electron effects, the problem of orientation... 

Products 7a and 7c, with the substituent R a to the carbonyl group, are by far predominantly formed. This regioselectivity is a result of the preferential approach of the alkene 2 to the dicobalthexacarbonyl-alkyne complex 5 from the side opposite to the substituent R of the original alkyne. The actual incorporation of the alkene however is less selective with respect to the orientation of the olefinic substituent R, thus leading to a mixture of isomers 7a and 7c. 

Look carefully at the reactions shown in the previous section. In each case, an unsymmetrically substituted alkene has given a single addition product, rather than the mixture that might have been expected. As another example, 1-pentene might react with HC1 to give both 1-chloropentane and 2-chloropentane, but it doesn t. Instead, the reaction gives only 2-chloropentane as the sole product. We say that such reactions are regiospecific (ree-jee-oh-specific) when only one of two possible orientations of addition occurs. 

Problem 7.6 When an unsymmetrjcal alkene such as propene is treated with iV-biomosiiccin-imide in aqueous dimethyl sulfoxide, the major product has the bromine atom bonded to the less highly substituted carbon atom. Is this Markovnikov or non-I Markovnikov orientation Explain. 

We can picture a Diels-Alder addition as occurring by head-on (cr) overlap of the two alkene p orbitals with the two p orbitals on carbons 1 and 4 of the diene (Figure 14.7). This is, of course, a cyclic orientation of the reactants. 

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