Substitution tetrasubstituted olefins

This model prediets that tri-substituted and tetra-substituted olefins would also be poor substrates. Thus it was not until 1994 that a study in the epoxidation of higher substituted olefins appeared. Indeed Jaeobsen revealed that tri-substituted olefins, and even tetra-substituted olefins ean be excellent substratesA new model was put forth that encompasses a skewed side-on approach of tri-substituted olefins to the Mn-oxo eomplex. The observation that certain tetrasubstituted olefins undergo epoxidation with good enantioseleetivity suggests that further studies are needed in order to fully understand the transition state geometry of the catalyst and substrate. 

In similarly substituted olefins Kf is strongly influenced by steric effects, as shown by the comparison of tetraisobutylethylene with adamantylideneadamantane and (i,/-D3-trishomocubylidene-D3-trishomocubane. In particular, the comparison between cyclohexene and the two tetrasubstituted cage olefins indicates that Kf increases at least by a factor of 103 on passing from a 1,2 disubstituted to a tetrasubstituted olefin. This dependence is likely to be similar in other solvents, because solvent effects on Kf are modest. 

At first, the reaction was characterized as most effective in the epoxidation of cw-disubstituted olefins.86 Later, the scope of this reaction was expanded to include the highly enantioselective synthesis of hYwiv-dis ubstitutcd8 7 and tri-substituted epoxides,88 as well as certain monosubstituted epoxides.89 The first example of nondirect asymmetric epoxidation of tetrasubstituted olefins has also appeared.90... 

These results contradict Jacobsen s earlier mechanistic theories, which would have predicted a top-on" approach for the sterically demanding tetrasubstituted olefins (Figure 1) and thus inferior results compared to the less-substituted olefins, which were assumed to approach from a skewed side-on" disposition. Furthermore, his observation that trisubstituted olefins were epoxidized in an opposite stereochemical sense compared to other olefins required invoking a stepwise mechanism, wherein the radical intermediate is steered by the pendant chiral catalyst [94JOC4378], At the current time, these results fail to coalesce into a clear unified predictive model. 

The synthetic potential18 of these reactions is principally due to the possibility of nitrous acid (HN02) elimination, making it possible to synthesize tri- and tetrasubstituted olefins, and also due to the substitution of the nitro group by a nucleophile. 

Contrary to what was found in the reactions of 2-substituted-5-nitroimidazoles80 82, elimination of nitrous acid from the C-alkylation product in the presence of only the nitronate anion as a base was not observed. A good base is necessary to promote the elimination to give the corresponding tetrasubstituted olefin (equation 19). 

An interesting reaction of nitroso chlorides derived from tetrasubstituted olefins described by Closs is reduction to the chloroamino derivative and cyclization by base to an aziridine. The sequence is not applicable to less fully substituted olefins because the nitroso chloride decomposes during attempted reduction. 

Alcohols, including tertiary alcohols, react readily with the reagent. The reagent adds stereospecifically (Irons) to unhindered olefins, for example, to the 2-butenes tetrasubstituted olefins do not react. The reagent can be used for Friedel-Crafts substitution of aromatic compounds, including azulene. The 2,4-dinitrobenzene-sulfenyl derivatives formed are colored solids readily purified by crystallization or chromatography and hence are useful for purposes of identification. 

The enantioselectivities we have achieved in the hydrogenation of p,p-di-substituted a-enamides are significantly higher than any previously reported for this challenging class of substrates. The sterically demanding nature of enamides 12 (tetrasubstituted olefins) has presented difficulties in the past with regard to both rates and enantioselectivities. These results lucidly illustrate the advantage... 

Although transition metal catalysts can effectively promote the hydrosilylation of mono- and di-substituted olefins, these catalysts are inactive for the hydrosilylation of tetrasubstituted olefins. However, aluminum trichloride is found to be active for the reaction giving organosilanes bearing bulky tm-alkyl substituents175 An aluminum hydride, HA1C12, is assumed to be the active catalyst in this reaction (equation 21). 

Figure 8.5. Ligands for the Sharpless AD process, (a) The phthalazine ligand (PHAL class), recommended for most substitution types [74,75]. (b) The diphenylpyrimidine ligand (PYR class), used for mono- and tetrasubstituted olefins...

The hydrogenation of simple olefins by lr(0) NPs dispersed in ILs depends on steric hindrance at the C=C double bond, the reactivity follows the order terminal > disubstituted > tri-substituted > tetrasubstituted (Fig. 6.9). Such an order of reactivity for olefins is the same as that of classical iridium complexes in homogeneous conditions [128]. 

Recently, Zhou and Larock [57] have also used a domino Mizoroki-Heck/Suzuki process in their syntheses of several tamoxifen analogues. They described a three-component coupling reaction of readily available aryl iodides, internal alkynes and arylboronic acids to give the expected tetrasubstituted olefins in good yields. For instance, the treatment of a mixture of iodobenzene, alkyne 92 and phenylboronic acid with catalytic amounts of PdCl2(PhCN)2 gave 93 in 90% yield. In this process, substituted aryliodides and... 

The synthesis of trisubstituted alkenes has scarcely been disclosed some exanples may be found in the work of Najera and coworkers using electron-deficient aryl sulfones 47 combined with phosphazene tScheme 19.21T The scope is even more limited for tetrasubstituted olefins as the yields are quite low. Acylsilanes afford trisubstituted vinylsilanes such as 48 mainly as the E isomers, but the stereoselectivity can be controlled by modulating the nature of the silyl group tScheme 19.22T E-configured conjugated esters and Weinreb amides have been synthesized successfully as well by Najera and coworkers starting from conveniently substituted arylsulfone 49 (Scheme 19.23). 

The remote activation of C-H bonds is an important synthetic goal and some especially crowded tetrasubstituted olefins undergo such a reaction. Furthermore, in all the examples studied, this activation proceeds with high diastereoselectivity. Thus, the two tert-hutyl substituted olefins 65 and 66 undergo, after the initial hydroboration and migration, an insertion into a remote C-H bond forming the boracycles 67 and 68 which after oxidation furnish the two diols 69 and 70 as one diastereoisomer (Scheme 16) (24). 

The C=C stretching frequency near 1640 cm" in vinyl hydrocarbons is a medium intensity band which becomes inactive in the infrared region in a symmetrical trans- or symmetrical tetrasubstituted double bond compound, both of which have centers of symmetry. Even when the substituents are not exactly alike in trans-md tetrasubstituted olefins, the infrared absorption may be quite weak. These double bond vibrations all appear strongly in the Raman effect, however, where the C=C stretch vibration in all types of ethylenes gives rise to a strong Raman band in the region 1680-1630 cm The trans, tri, and tetra alkyl-substituted ethylenes appear at 1680-1665 cm which are strong in Raman but weak or absent in the infrared. The cis, or... 

Several other reductive-elimination procedures to give olefins have been developed this year. Marshall and Bierenbaum have identified a reductive-decyanation technique to give tetrasubstituted olefins. Hence, alkylation of the readily available mixture of nitriles (25) leads to a mixture of the (j8,y)-double-bond isomers (26) which contains principally the E-isomer. Reduction of the mixture with sodium in liquid ammonia gives the isomerically pure olefin (27) in 92% overall yield (Scheme 15). Generally, the most highly substituted olefin is formed on reduction of the intermediate j8,y-unsaturated nitriles. 

The alkylation of enamines with nitroolefins, which gives intermediates for reductive cyclization (6S2), also provided an example of a stable cycliza-tion product derived from attack of the intermediate imonium function by the nitro anion (683). A previously claimed tetrasubstituted enamine, which was obtained from addition of a vinylsulfone to morpholinocyclohexene (314), was shown to be the corresponding cyclobutane (684). Perfluoro-olefins also gave alkylation products with enamines (685). Reactions of enamines with diazodicarboxylate (683,686) have been used diagnostically for 6-substituted cyclohexenamines. In a reaction of 2-penten-4-one with a substituted vinylogous amide, stereochemical direction was seen to depend on solvent polarity (687). 

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