Acyclic olefins selectivity

Unlike the oxymercuration of acyclic olefins, oxymercuration of bicyclic olefins often gives jy -addition products. Norbornenes 93, for example, show exclusive fvo-oxymercuration. In this reaction, the ratio between the isomers depends on the nature of the fvo-substituent (R1) and tro/o-substituent (R2) (Equation (36)). The presence of electron-withdrawing fvo-substituents always leads to a much greater selectivity in favor of 94a-d over 95a-d.116 117 As indicated by extensive theoretical calculations, the charge distribution in the transition states governs the selectivity of these reactions.118... 

In summary, the interrelation of cis-selective catalysts and their lack of metathesis activity with acyclic olefins are rationalized by a speculative scheme that incorporates the concept of a tridentate cagelike complex as the active species of cis-directing catalysts. 

The same authors later on presented their results of the epoxidation of various cyclic and acyclic olefins employing a heterogeneous catalyst with an oxovanadium(IV) ion incorporated on a sulfonic acid ion-exchange resin and TBHP as oxidant . Selectivities... 

Whereas only limited stereoselectivity is characteristic of the metathesis of acyclic olefins, ring-opening metathesis polymerization of cycloalkenes may be highly stereoselective provided the proper catalysts and reaction conditions are selected. Cyclopentene, for instance, is transformed to either all-cis [Eq. (12.25)] or all-frans polypentenamers [Eq. (12.26)] in the presence of tungsten catalysts 21 92... 

In order to obtain high yields, the cyclic partner should be strained so that it < compete with the acyclic olefin to yield a selective cross-reaction (Eq. g 3 jgqj... 

The azomethine ylides that exhibit high stereoselectivity to symmetrical acyclic olefins are again ester- or cyano-stabilized types, and also N-unsubstituted or N-metallated types. The only exception is the N-substituted ylides with a general formula ArCH=N R"—CH COOR, whose poor endo selectivity to maleimides has been discussed above. 

Scheme 6.13 Improved selectivity in the asymmetric transfer hydrogenation of acyclic olefins using P(O)Ph2 protecting groups.

The reactions of aryl-substituted carbenes are extensively covered in Houben-Weyl Vol. E 19b, pp 824-1021, where procedures for all typical carbene transfer methods are reported. The simple diastereoselectivity is highly dependent on the carbene source and the reaction conditions, as demonstrated by the additions of phenylcarbenoids, generated by different methods, to cyclohexene. The highest endo selectivity is obtained with the diiodo(phenyl)methane/diethylzinc reagent3. Other cyclic or acyclic olefins show a similar moderate to high preference for endo- or cw-cyclopropane formation. 

Oxonium ylides have only recently been described in the context of [2,3] sigmatropic reactions 106 10 8, and the small number of acyclic substrates examined precludes the identification of a general trend regarding the olefin selectivity for these systems, Ylide 10 has been postulated as an intermediate in the trimethylsilyl triflate catalyzed rearrangement of allyloxy acetate 9, which yields an excess of the Z-olefin 11109. 

This selectivity means that although for experiments like the single cross to work, the substituents at the ends of the acyclic olefin s double bond must be different, for otherwise there would be no cross products like C12 or Cjb, they must not be very different, or no cross products would be observed either, since one path or the other in Eq. (6) would be preferred. 

Compared to conventional catalysts, (diphenylcarbene)pentacar-bonyltungsten has three advantages (a) it converts cycloalkenes stereo-selectively into cw-polyalkenamers (b) it contains no metal alkyl bonds, and therefore no acyclic olefins can form to decrease molecular weights by chain transfer and (c) it contains no metal halide to induce side reactions or corrode equipment. 

Hydration of acyclic olefins can be carried out readily with NAFION In a fixed bed tube reactor at 150°C. Because of thermodynamic consideration, propene conversion Is 16% with a Isopropyl alcohol selectivity of 97% (39). 

Cu(OTf)2 in the presence of the ligand ( )-N-((naphthalen-7-yl)methylene) benzenamine and C6H5NHNH2 catalyses selective oxidation of benzylic C(sp )-H bonds to C(sp )-0 bonds with t-butyl perbenzoate (TBPB) in acetone. Cu(MeCN)4PF5 catalyses asymmetric allylic oxidation of acyclic olefins by TBPB in acetone in the presence of spiro bisoxazoline ligands (8) the product allyl esters are formed with excellent regioselectivity (>20 1 in most cases) and up to 67% ee. °°... 

Selective hydrogenation of a -unsaturated carbonyl compounds can be carried out by reduction with iron pentacarbonyl and a small amount of base in moist solvents. The method is applicable to oc -unsaturated aldehydes, ketones, esters, and lactones with negligible over-reduction of the carbonyl group and is susceptible to the steric environment of the olefin. Spectroscopic evidence suggests that solutions of equimolecular amounts of iodine and thiocyanogen contain an appreciable concentration of iodine thiocyanate. Addition of alkenes results in tran.r-addition to yield jS-iodo-thiocyanates which in base suffer rapid hydrolysis of the thiocyanate followed by ring closure to the episulphide (516). As a synthetic procedure this does not appear to be applicable to acyclic olefins. ... 

Brown s discovery of the hydroboration reaction in the 1950s opened new avenues for the selective functionalization of olefins. The recognition that acyclic olefins can have well-defined conformational biases allowed the development of diastereoselective, substrate-controlled processes and the advent of the principles of acyclic stereocontrol. Given the central role the hydroboration of olefins has played, it is hardly surprising that the earliest examples in this field involve such transformations. For the organic chemist, diastereoselective and enantioselective hydrometalation reactions have rapidly become an indispensable tool equally useful in simple olefin functionalizations and in the stereoselective construction of highly complex molecules. 

As is apparent from the preceding discussion, a full understanding of the observed diastereoselectivity in dihydroxylation reactions of acyclic allylic alcohols remains elusive. Thus, the use of any one model is insufficient, and a careful analysis of the steric and electronic particulars of a given substrate must be conducted. Nevertheless, an impressive number of diastereoselec-tive dihydroxylations of acyclic olefins in complex molecule synthesis attest to the central role of this transformation [42, 43], Selected examples of stereodivergent dihydroxylations reported by Danishefsky are showcased in Schemes 9.36 and 9.37 [201]. Dihydroxylation of ( )- and (Z)-unsaturated esters 287 and 290, respectively, thus proceeded with excellent diastereoselectivity. Danishefsky has proposed a transition state model based on the ground state conformations of the starting materials as determined by X-ray analysis. The dihydroxylations were thus postulated to occur from the sterically less hindered faces of the olefins, as depicted in 288 and 291. Diol 292 was subsequently converted into N-acetylneuraminic acid (293). 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

It is of no benefit to analyze, stereoselectivity, the reaction under consideration in the general case. However, it should be noted that the exo approach of olefin is preferable in more cases. As can be seen from Chart 3.11, if olefin molecules contain two different double bonds or a double bond and a triple bond, only one double bond can be selectively involved in the reaction with acyclic nitronates. 

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