Subject olefin isomerization

A recent review (106) deals with the subject of olefin isomerization, which will therefore be dealt with only briefly here. 

Free radical promoted hydrosilation of olefins typically gives poorer conversions than are produed by hydrosilation mediated by metal catalysts. However, they are not subject to isomerization of the olefin, a side reaction that can be a nuisance in metal-catalyzed hydrosilations. This is due to the extremely efficient hydrogen atom transfer reaction from Si-H to carbon radicals and the resulting very short lifetimes of the latter. The generally accepted mechanism for these processes is illustrated in Scheme 1. 

Halide anions affect the rate of hydroformylation of internal olefins as well as the regioselective properties of the catalyst [136]. The rate of hydroformylation of thermally equilibrated internal higher alkenes increased by a factor of 6-7 by adding substoichiometric amounts (with respect to palladium) of Cl" or Br" and by a factor of 3-4 with I" [137]. When a thermally equilibrated mixture of internal Cg-Cj g olefins was subjected to isomerization-hydroformylation, a reverse effect on regioselectivity was observed [136e]. Thus, the formation of the linear aldehyde increased in the following order iodide > bromide > chloride. 

In an alternative approach, the isomeric unsaturated pyrrolidine or piperidine aldoximes 245 a and 245b were prepared and subjected to lOOC reaction affording 246a and 246b, respectively (Eq. 28). Esterification of 240 followed by N-tert-BOC protection and DIBALH reduction provided aldehyde 244 (X = 0) which was subjected to Wittig olefination. Introduction of a two carbon aldoxime chain on N in 244 (X = CH2) was carried out by alkylation with Et a-bromoacetate after deprotection of the N atom in 244. Reduction and oxima-tion led to 245. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

When the RelSi terminology (29) was introduced (28), its use was suggested also for describing the faces of double bonds. This was to be accomplished by citing the descriptors of their trigonal atoms when seen from the same side. Although this extension seems an obvious choice, it is subject to a possible misinterpretation. If one characterizes a face of the olefinic bond of 2-butenoic acid as Re-Re, one can deduce that the compound in question must be the Z and not the E isomer. It might be inferred from this that the cis-trans isomerism of these acids can be factored into two elements of prostereoisomerism. This... 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

The question as to whether these pathways involve an intermediate or are concerted has major implications for the stereochemistry of the product expected. This subject will be discussed more fully in the next section. The diradical obviously does not exist long enough to react with another molecule of olefin since no tetrahydropyrans have ever been detected from these reactions. It should also be pointed out that if the formation of the intermediate is reversible, olefin (or diene) isomerization may, in some cases, involve this pathway rather than triplet-triplet transfer. 

The geometric isomerization of olefins via photochemical electron transfer is well known28,29 and can be divided into two categories (a) isomerization via the radical cation, in which case the olefin is the donor in the presence of an excited electron acceptor (b) isomerization via the radical ion pair, which leads to the triplet-excited olefin, and in this mechanism the olefin is the acceptor. This subject is not discussed in this chapter because of space limitations. However, several reviews30 can be consulted in this regard. 

When treated with aluminum chloride in refluxing hexane, perhydrotriquinacene was rapidly isomerized to adamantane.382 Exposure of 1-, exo-2, or endo-2-per-hydrotriquinacenol to fluorosulfonic acid, followed by aqueous quenching, has been shown to produce 1-adamantanol. McKervey and his co-workers have found that 358 exchanges ten hydrogen atoms when subjected to exchange with deuterium and palladium.385 On this basis, the prediction was made that olefin 409 (presently an unknown molecule) should be a compound of moderate stability. 

In the case of the 7-lactols 12, the EfZ-isomeric olefination products ( )- and (Z)-13 were isolated and subjected to base-catalyzed cyclization separately38. 

The formation of olefinic nitriles from allylic halides is best accomplished with dry, powdered cuprous cyanide rather than with alcoholic alkali cyanides, with which side reactions such as isomerization and alcoholysis of the double bond are particularly bothersome. With cuprous cyanide the yields in the synthesis of allyl cyanide and methal-lyl cyanide are 84% and 86%, respectively. Higher allylic halides are subject to allylic rearrangements thus cuprous cyanide acts on crotyl halide (CH,CH CHCHjX) and methylvinylcarbinyl halide (CH,CHXCH = CH,) to produce the same mixture of isomeric nitriles (9 1) regardless of which halide is treated. Numerous cyanides of the allylic type (Cj-Ci4) have been prepared, although the possibility of an isomerization has not been considered. A similar isomerization has been observed in the reaction of sorbyl chloride and potassium cyanide. ... 

Cyclo-olefins are subject to structural isomerization in contact with acidic catalysts, as Bloch and Thomas (27) and Greensfelder and Voge (21) have shown. Therefore, such catalytic activity when intimately coupled to the aromatization reaction may direct the reaction path to products having five-carbon ring structures which cannot aromatize ... 

First the lower C4-C10 a-olefins are stripped off. In a heavy ends column the C20+ a-olefins are removed from the desired C12-C20 a-olefins. Finally the middle-range products meeting the market needs are separated into the desired cuts and blends. The very high flexibility of the SHOP results from the following steps. The C4-CJ0 and the C2o+-fractions are combined to be isomerized to internal linear olefins (eq. (4)) and then subjected to a metathesis reaction (eq. (5)). Both steps require about 80-140 °C and 0.3-2 MPa. Isomerization is accomplished by a typical isomerization catalyst such as Na/K on AI2O3 or a MgO catalyst in the liquid phase [42], where about 90% of the a-olefins are converted to internal... 

The research team of J. Tadanier prepared a series of C8-modified 3-deoxy-P-D-manno-2-octulosonic acid analogues as potential inhibitors of CMP-Kdo synthetase. One of the derivatives was prepared from a functionalized olefinic carbohydrate substrate by means of the Wohl-Ziegler bromination. The stereochemistry of the double bond was (Z), however, under the reaction conditions a cis-trans isomerization took place in addition to the bromination at the allylic position (no yield was reported for this step). It is worth noting that the authors did not use a radical initiator for this transformation, the reaction mixture was simply irradiated with a 150W flood lamp. Subsequently the allylic bromide was converted to an allylic azide, which was then subjected to the Staudinger reaction to obtain the corresponding allylic amine. 

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