Cyclic alkenes stereochemistry

The stereochemistry of radical addition of hydrogen bromide to alkenes has been studied with both acyclic and cyclic alkenes. Anti addition is favored.This is contrary to what would be expected if the s[p- carbon of the radical were rapidly rotating or inverting with respect to the remainder of the molecule ... 

The preferred stereochemistry of addition to cyclic alkenes is anti The additions are not as highly stereoselective as hydrogen bromide addition, however. 

The stereochemistry of oxymercuration has been examined in a number of systems. Conformationally biased cyclic alkenes such as 4-r-butylcyclohexene and 4-f-butyl-l-methycyclohexene give exclusively the product of anti addition, which is consistent with a mercurinium ion intermediate.17,22... 

With cyclic alkenes, the reaction proceeds with variable regiose-lectivity and stereoselectivity depending on the substituents, although the overall reaction proceeds in moderate to good yield. The approach is of particular value for those systems where regiochem-istry and stereochemistry are not variable. 

In the steric course of olefin metathesis, acyclic and cyclic alkenes exhibit opposite behavior. The stereoselectivity of the transformation of most acyclic olefins is low and usually goes to equilibrium. In some systems there is a strong initial preference for retention of stereochemistry, that is for the transformation of cis to cis and that of trans to tram isomers.88 This means that geometric isomers of an olefin give different isomeric mixtures of the same product ... 

Evidence for the existence of the bromonium ion is provided from the observation that bromine adds to cyclic alkenes (e.g. cyclopentene) in an anti-stereochemistry (Following fig.). Thus, each bromine adds to opposite faces of the alkene to produce only the trans isomer. None of the ds isomer is formed. If the intermediate was a carbocation, a mixture of cis and trans isomers would be expected as the second bromine could add form either side. With a bromonium ion, the second bromine must approach from the opposite side. 

Fig. Anti-stereochemistry of bromine addition to a cyclic alkene.

The anti addition of diethyl 7V,Ar-dibromophosphoramidates to acyclic and cyclic alkenes was achieved in the presence of boron trifluoride, which makes the ionic dissociation of the N —Br bond more labile94,9s. After reduction of the initial /J,lV-dibromo adducts with sodium bisulfite, the /i-bromo-A -hydrophosphoramides 3 precursor of / -bromo amine hydrochlorides 4 were obtained (Table 4). However, a mixture of diastereomers was obtained from (Z)-l-phenyl-l-propene and (E)- and (Z)-l,2-diphenylethylene. Direct assignment of stereochemistry by H NMR of the phosphoroamidates derived from 2-butenes was not possible. Detailed analysis is, however, possible for the H-NMR spectra of the /J-bromo amine hydrochlorides. As determined by 31P NMR spectroscopy all additions to unsymmetrical aliphatic alkenes were not regiospecific. The /f-bromo amine hydrochlorides were converted to 1,2-diamines95. 

Stereochemistry of Glycol Formation (a) From Cyclic Alkenes... 

As with amine oxides and sulfoxides, acyclic 1,2-disubstituted alkenes are usually obtained with the ( )-stereochemistry, although the formation of a,(3-unsaturated nitriles is reported to give a mixture of ( )- and (Z)-isomers. For cyclic alkenes, the stereochemistry of double bond formation depends upon ring size. However, it can be affected by conformational factors, e.g. cyclododecyl phenyl selenide gives a mixture of cis- and fra/is-cyclododecenes on oxidative elimination (equation 38) but only the (El-isomer (101) was obtained from the acetoxycyclododecyl selenide (100 equation 39). ... 

Loss of geometric integrity is not observed with cyclic alkenes which have ring sizes of five members or less since cis-trans isomerization is inhibited. Larger rings, however, can result in loss of alkene stereochemistry (equations 16, The cyclobutane ring juncture derived from the excited-state alkene is... 

For the photoadducts derived from cyclic enones and alkenes, stereochemistry at the ring junction is influenced by the structure and especially by the ring size of the starting reagents. For steric reasons, only cis-fused cycloadducts can be formed on photocycloaddition of cyclopentenones (n = 0). From cyclohexenone derivatives (n = 1), cis- and transfused adducts can be isolated, even if the cis-fused structure is thermodynamically more stable. This indicates that trans-fused cycloadducts result from a kinetic rather than a thermodynamic control. Fortunately, trans-fused cycloadducts can be epimerized easily to the more stable cis stereoisomers (Scheme 11). 

Stereochemistry of products from nucleophilic attack at alkyne and cyclic alkene ligands in 18-electron iron complexes [(j7 -C5H5)(CO)(L)Fe(Un)]+ has been extensively examined [35]. It was deduced that most nucleophiles including (MeOOC)2CH , Ph , RNH2 and PhS attacked directly at the carbon of the unsaturated ligand from the side opposite to the iron metal (e.g. Scheme 8.22, path A). 

If a more reactive alkene (in this case the electron-donating O makes the enol ether more reactive) is available, the ketene adds to that instead. Note that the alkene must be present as the ketene is generated. The mechanism and part of the stereochemistry are simple. Because the cyclic alkene has cis stereochemistry, the two hydrogens on the six-membered ring must be cis in the product. The regiochemistry arises because the alkene is an enol ether and the large coefficient in its HOMO interacts with the central atom of the ketene, the one with the larger LUMO coefficient. 

The olefin retains its cri-configuration, i.e., the two cri-hydrogens of cyclic alkene must remain cis in the product. The stereochemistry at the remaining center comes from the way the two molecules approach each other. (1) As the alkene is unsymmetrical, the less sterically demanding carbonyl part of the ketene will be oriented above the larger alkene substituents, i.e., will be in the middle of the ring. (2) As the ketene is unsymmetrical, favored transition state must have the larger of the two substituents oriented away from the plane of the alkene. 

The stereochemistry of 43 (38) is easy to see because the cyclohexane ring cannot rotate about individual C-C bonds. Does this same stereochemical bias occur with acyclic alkenes When an acyclic alkene reacts with a halogen, the product is an acyclic dihalide, and free rotation is possible about those bonds. However, the answer is yes Acyclic alkenes react with the same stereochemical bias because the mechanism of reaction of an alkene and diatomic halogen is the same for both acyclic and cyclic alkenes. This selectivity is demonstrated with the simple acyclic alkene, 2-butene however, an analysis requires an examination of each stereoisomer, cts-2-butene and rans-2-butene (see Chapter 9, Section 9.4). When cts-2-butene reacts with bromine, the product is a racemic mixture, (2S,3iS)-dibromobutane along with (2i2,3i2)-dibromobutane (see 44). Two new stereogenic centers are created by this reaction. (See Chapter 9, Section 9.3, to review absolute configuration.) When rans-2-butene reacts with bromine, however, the product is a racemic mixture of (2S,3i2)-dibromobutane and (2i ,3S)-dibromobutane (see 45), which are drawn a second time as the eclipsed rotamer (Chapter 8, Section 8.1) to show their relationship to 44. Dibromides 44 and 45 are diastereomers (Chapter 9, Section 9.5). 

The overall cis addition of two OH units to the C=C unit of the alkene is easy to see when a cyclic alkene is used, but the reaction with an acyclic alkene is also diastereospecific. If hex-2 -ene (133) reacts with OSO4, osmate ester 134 is formed where the stereochemistry of the two alkyl groups attached to the C=C unit has been preserved in the cis addition. There is no facial bias, so both enantiomers of the osmate ester are formed 134 and 135. In other words, formation of the osmate ester can occur from either the top or the bottom, as shown, so the final product will be racemic. Subsequent reaction with NMO leads to diols 136 and 137, which have the (2S,3S) and 2R,3R) absolute configurations. Similarly, the reaction with permanganate via the manganate ester gives the cis diol. 

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