Stereochemistry of alkenes

Interactive to predict the products and stereochemistry of alkene addition reactions. 

Cis- and trans- designations the stereochemistry of alkene diasteroemers are unambiguous only when applied to disubstituted alkenes. 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

Wender and co-workers focused their attention on the selectivity of the reaction with dienynes mediated by rhodium catalysts. Disubstituted alkynes give excellent yields of PK products, whereas terminal alkynes react less efficiently. Substituents at 2- and 3-positions in the diene moiety tolerate well. The stereochemistry of alkenes is well conserved during the transformation. ... 

Further details on the stereochemistry of alkene hydrogenation can be found in earlier books and review papers62,63,67. They treat the effect of polar substituents... 

Table 1 Scope and Stereochemistry of Alkenes from Reductive Elimination of -Substituted Sulfones... 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

In all cases, Grubbs found that the cis-to-trans ratio was always 1 1, thus demonstrating that a-activation does not influence the rate or stereochemistry of alkene insertion. The result of the experiment was the key piece of evidence supporting the Cossee mechanism for Z-N polymerization long sought after by chemists. The experiment allowed researchers to make a clear distinction between metathesis and Z-N polymerization, the former involving the chemistry of the M=C bond and the latter that of the M-C bond.87... 

In the same reaction stereochemistry may be generated two stereogenic centres are in fact produced in this reaction and in the epoxidation of 3. The sense of the stereochemistry produced in these reactions depends critically on two factors the inherent stereoselectivity of the reactions (anti in the iodolactonisation and syn in the epoxidation) and the geometry of the alkene. Inside a six-membered ring of 1 the alkene must of course by cis or Z. In the open chain allylic alcohol 3 it can be E or Z depending on the method of synthesis. This chapter explores ways to control the stereochemistry of alkenes as an essential preliminary to the control of three-dimensional stereochemistry in chapter 25 where you will meet a method to make single enantiomers of 4 from the alkenes in E- and Z-3. 

Scheme 2.8. Summary of Stereochemistry of Alkene Dihydroxylation by OSO4-Catalysis... 

From what you know about the stereochemistry of alkene addition reactions, predict the configurations of the products that would be obtained from the reaction of 2-butyne with the following ... 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Although not all Wittig reactions proceed by exactly the same mechanism, an oxa-phosphetane intermediate is common to all of the mechanisms (Scheme 2). TTie overall rates of the reactions depend upon the rates of formation of the oxaphosphetane and of its conversion to alkene and phosphine oxide. The stereochemistry of alkene formation depends on the rates of formation and conversion to product of the stereoiso-meric oxaphosphetanes, and upon equilibration of the oxaphosphetane with starting materials and with zwitterionic intermediates in some cases f3-5k... 

Stereochemistry of Alkene Hydrogenation 230 6.15 Stereochemistry of Halogen Addition 250... 

In Section 4.5 we learned to use the terms cis and trans to designate the stereochemistry of alkene diastereomers (cis-trans isomers). These terms are unambiguous, however, only when applied to disubstituted alkenes. If the alkene is trisubstituted or tetrasubstituted, the terms cis and trans are either ambiguous or do not apply at all. Consider the following alkene as an example ... 

The stereochemistry from chain-end control and catalyst-control are not necessarily different. More details on the stereochemistry of alkene pol3mnerization are provided in Chapter 22. In brief, however, a symmetric catalyst like the ones used for the copolymerization of carbon monoxide with styrene will generate isotactic polymer, but polymerizations with stereochemistry dictated by chain-end control can form either syndiotactic or isotactic material. Catalysts displaying other symmetries have been designed for other types of polymerizations to generate polymer architectures beyond isotactic. 

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