Alkenes stereoisomer

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

The products observed are those expected of attack occurs on A from the less hindered side i.e., between the two smaller groups, in this case H and Cl. Once this has occurred the later products arise by a sequence of trans displacements. As would be expected from the sequence outlined, the other alkene stereoisomer can be prepared by... 

The desired synthetic route becomes apparent when it is recognized that the Z alkene stereoisomer may be obtained from an alkyne, which, in turn, is available by carboxylation of the anion derived from the starting material. 

The Peterson Reaction allows the preparation of alkenes from a-silylcarbanions. The intermediate p-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric p-hydroxy silanes and subsequently performing two different eliminations. 

Mechanism The Peterson alkenation offers the synthesis of desired alkene stereoisomer by careful separation of the two diastereomeric intermediate P-hydroxysilanes and subsequently performing elimination under two different conditions. 

One limitation of the Wittig reaction is that a mixture of alkene stereoisomers sometimes forms. For example, reaction of propanal (CH3CH2CHO) with a Wittig reagent forms the mixture of E and Z isomers shown. 

E,Z System of nomenclature (Section I0.3B) A system for unambiguously naming alkene stereoisomers by assigning priorities to the two groups on each carbon of the double bond. The E isomer has the two higher priority groups on opposite sides of the double bond, and the Z isomer has them on the same side. 

Alkythioacetylenes also smoothly add stabilized lithio carbanions located on the same substrate to provide functionalized exocyclic thioenol ethers (Scheme 7-14) [17a]. It should be noted that only one alkene stereoisomer was obtained this results from an atypical trans-carbometallation reaction, or from a cw-carbometallation followed by an equilibration of the metallated intermediate [15]. 

In contrast, vinylsilane acetals (43a)-(43c), with a two-carbon tether between the vinylsilane and the MEM group, cyclize in the presence of dn tetrachloride to form tetrasubsdtuted alkylidenetetrahydro-furans (44a>K ). However, little or no stereochemical preference for either ( )- or (Z)-alkene stereoisomers was observed in these reactions (Scheme 20 and Table 4)." ... 

Such differences between alkene stereoisomers are common and may be exploited to prove stereochemistries. The geminal couplings in aromatics (H-C = C) are 4-8 Hz. For sp carbons, the coupling becomes quite large 49.3 Hz in ethyne (H-C = CH) and 61.0 Hz in (H-C = COPh). 

A different alkene stereoisomer is obtained from each diastereoisomer of the alkyl halide. 

A haloalkane of general structure RR CH-CHX-R" will have only one conformation with H ajiri to X. Therefore, only a single alkene stereoisomer can form. Its slercochentisiry will he determined by the. siereochemisiry of the two chiral carbons in the haloalkane. 

Another route to alkenyl radicals is by addition of radicals to alkynes. An application of this procedure, which serves as a model for the synthesis of the CD ring system of cardiac aglycones was reported by Stork and co-workers (4.55). The initial alkyl radical, formed selectively from the bromide 61 attacks the alkyne regioselectively to give an intermediate alkenyl radical, which reacts further with the alkene of the cyclohexene to give the product 62. A mixture of alkene stereoisomers is produced owing to the ease of E-Z alkenyl radical isomerization. 

Alkenes were introduced in Chapter 5 (Section 5.1). They are characterized by a C=C unit containing four groups or atoms. Because rotation about the C=C unit is not possible, atoms or groups attached to the C=C unit are effectively locked in space. Therefore, alkene stereoisomers are possible, but they are not enantiomers. 

Problem 8.6 While cis- and frans-2-butene differ in stability by only 3 kJ/mol, cis- and fra/7S-2,2,5,5-tetramethyl-3-hexene differ in stability by 44 kJ/mol. Explain why the difference in the stability of the second pair of alkene stereoisomers is so much larger. 

BUILD MODELS H3C—CH=CH—CH3 PART C ALKENE STEREOISOMERS (E/Z AND trans/cis)... 

As one of the earliest appUcatiOTis of RCM for macrocyclization in natural product synthesis, the Hoveyda group reported in 1995 that Schrock catalyst catalyzed Z-selective formation of the macrocycle in Fig. 24 as a single alkene stereoisomer, which is a late stage intermediate for the total synthesis of Fluvirucin B [54]. While the conformational control of the substrate was believed to be crucial for the selectivity, recent stodies showed that catalyst cmitrol also played a key role in the reaction outcome, as Ru-catalyzed RCM of a very similar substrate yielded a 1 1 mixture of Z E isomers [55]. 

Note that, as in R, S nomenclature, the E or Z is placed in parentheses. The E, Z system of nomenclature can be used for all alkene stereoisomers consequently, the lUPAC recommends that this system be used exclusively. However, many chemists continue to use the cis and trans designations for simple alkenes. 

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