The Geometry of Alkenes

Chapter 19 Elimination Reactions Chapter 31 Controlling the Geometry of Alkenes... 

The geometry of alkene coordination appears to affect the reactivity toward nucleophiles. 7-Methylenenorbomene reacts with PdCl2 to give the product from Pd-Cl addition across the exo-methylene double bond (equation 37). In the intermediate PdCl2-diene complex, which could not be isolated, the 7-exo double bond would be coordinated so as to lie in the ring plane, while the other aUcenic group would lie perpendicular to the palladium square plane, as is normal. From the structure of the product, trans addition of Pd and Cl occurs. 

The cycloadditions proceed stereoselectively with the geometry of ( )-alkenes being preserved, but that for (Z)-alkenes being partially lost (equations 1 and 2), Excellent diastereofacial selectivity has been observed in cycloadditions to cyclic alkenes, the major product resulting from the addition of the TMM-PdL2 complex to the least-hindered face of the alkene (equations 3-5). Modest (4 1) to high (>99 1) diastereoselectivity is also observed when enantiomerically pure acyclic alkenes are used (equations 6,7). ... 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

The geometry of bicyclic rings can also cause distortion of the alkene bond from coplanarity. An example is bicyclo[2.2.1]hept-l-ene ... 

The isomerization of alkenes is believed to take place via an excited state in which the two sp carbons are twisted 90° with respect to one another. This state is referred to as the p (perpendicular) state. This geometry is believed to be the minimum-energy geometry for both the singlet and triplet excited states. 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

For El eliminations, if there is a free carbocation (25), it is free to rotate, and no matter what the geometry of the original compound, the more stable situation is the one where the larger of the D-E pair is opposite the smaller of the A-B pair and the corresponding alkene should form. If the carbocation is not completely free, then to that extent, E2 type products are formed. Similar considerations apply in ElcB eliminations. ... 

Gycloisomerization of a disubstituted alkyne sometimes required activation of the alkyne by the addition of a conjugated carbonyl and performing the reaction at a higher temperature as in Equation (38). The geometry of the alkene determines the regioselectivity of the /3-hydride elimination, as ( )-60 gave predominantly 61 (Equation (38)), while 62 was the major product of the cycloisomerization of (Z)-60 (Equation (39)). 

In the case of tri-substituted alkenes, the 1,3-syn products are formed in moderate to high diastereoselectivities (Table 21.10, entries 6—12). The stereochemistry of hydrogenation of homoallylic alcohols with a trisubstituted olefin unit is governed by the stereochemistry of the homoallylic hydroxy group, the stereogenic center at the allyl position, and the geometry of the double bond (Scheme 21.4). In entries 8 to 10 of Table 21.10, the product of 1,3-syn structure is formed in more than 90% d.e. with a cationic rhodium catalyst. The stereochemistry of the products in entries 10 to 12 shows that it is the stereogenic center at the allylic position which dictates the sense of asymmetric induction... 

Among methods of preparing optically active cyclopropane compounds, the Simmons-Smith reaction, first reported in 1958, is of significance. This reaction refers to the cyclopropanation of alkene with a reagent prepared in situ from a zinc-copper alloy and diiodomethane. The reaction is stereospecific with respect to the geometry of the alkene and is generally free from side reactions in contrast to reactions involving free carbenes. 

Substitution has no significant effect on the geometry of the n complex and RuHCl(PH3)2(CH2=CH2) and RuHCl(PH3)2(H2C=CH(OMe)) have very similar shape. Of interest the OMe group cannot reach the second empty coordination site (trans to H) of Ru. Remarkably, the Ru-alkene bond dissociation energy is also not affected by the presence of OCH3 (less than 3 kcal.mol 1 difference in binding dissociation energy). The methyl vinyl ether... 

Absorption of a photon by an alkene produces a (tt,Jt ) vertical (Franck-Condon) excited state in which the geometry of the ground state from which it was formed is retained. Since the (it,it ) state has no net n bonding, there is little barrier to free rotation about the former double bond. Thus, relaxation takes place rapidly, giving a nonvertical (it,it ) state with a lower energy and different geometry to the vertical excited state. 

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