Alkenes steric congestion

Dubois and co-workers (119,410-415) characterized alkyl (R) substituent effects in simple and sterically congested alkanes, alkenes, carboxylic acid derivatives, ketones, amines, alcohols, and so on by the use of topological parameters XR ... 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

Isolable sterically congested cyclobutadienes react with electrophilic alkenes under mild conditions. tert-Butyl 2,3,4-tri-tert-butylcyclobutadienecarboxylate undergoes cycloadditions with both tetracyanoethene17 and maleic anhydride1S at — 20 to — 30 °C to give the correspondingly substituted bicyclo[2.2.0]hexenes. 

In marked contrast to that of cyclobutanes, the cycloreversion of cyclobutanones to ethene and ketene88 is most probably a concerted process.89-94 For example, the fact that the pyrolysis of 2-propylcyclobutanone (13) at 350 °C gives ethene and pent-l-ene in the ratio of 3.8 1 is not easily explained by a diradical mechanism.90 This transformation presumably involves two competing cycloreversion reactions. As expected for a concerted process, the conversion through the less sterically congested transition structure is favored. For this reason, ethene is generated as the major alkene.90... 

The cyclopropanation is sensitive to steric effects, adding from the less-hindered face. Having a neighboring hydroxyl group will generally accelerate the reaction, and will direct the cyclopropanation syn to the hydroxyl group even into sterically congested alkenes. 

Diels-Alder reactions of (Z)-A -substituted-4-methylene-5-propylidene-2-oxazoli-dinone dienes with methyl vinyl ketone, methyl propiolate, and captodative alkenes yield the highest regio- and stereo-selectivities in mixtures of H20 and MeOH or under BF3.Et20 catalysis.198 The asymmetric Diels-Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone is catalysed by a new Cu(II) catalyst containing a chiral sterically congested roofed (2-diphenylphosphino)phenylthiazoline ligand (169).199... 

Because of the different degree of steric congestion in the interstices between the NO, Cp, and PPhs ligands (NO/PPhs most congested > Cp/PPhs > NO/Cp least congested), the chiral (Re)+ fragment is able to discriminate between both faces of prochiral alkenes and aldehydes with high kinetic and thermodynamic selectivities. For example, the thermodynamic... 

Photochemically induced [2 + 2] cycloaddition is of extraordinary importance in organic synthesis,as this is a method ideally suited for the preparation of sterically congested compounds. The reaction may occur by a concerted mechanism allowed by rules of orbital symmetry, or, more often, via a biradical pathway. For preparative purposes, the most widely exploited is the enone-alkene photochemical [2 + 2] cycloaddition. This reaction proceeds with high regioselectivity, although its stereoselectivity might be low. The first example of the utilization of this reaction for the synthesis of a natural compound, a-cariophyllene 385, was described by Corey (Scheme 2.129). Adduct 386, formed as a mixture of stereoisomers in high yield from simple precursors, was further transformed via the tricyclic intermediate 387 into the... 

Cyclic Alkenes. The directing effect of alcohol substituents is perhaps most dramatically demonstrated with cyclic alkenes in which products are formed via hydrogenation from the more hindered face (Table 1). Entries 1-3 serve to illustrate that the directing alcohol may reside in either the allylic, homoallylic, or bis(homoallylic) position relative to the alkene undergoing hydrogenation, while synthetically useful levels of selectivity are retained. Entry 4 is included to illustrate the dramatic steric congestion which can be overcome in directed hydrogenation... 

The influence of the steric congestion on the catalytic performance of Ti(iv) active centers in the epoxidation of alkenes has been probed using a range of soluble-based silesquioxane species.1934 Bis-homoallylic alcohols are diastereoselectively converted, in good yields, into tetrahydrofuranols and tetrahydropyranols, catalyzed by mono-Gp and bis-Gp Ti derivatives in the presence of B -hydroperoxide activated with 4 A molecular sieves.1935... 

---