Olefin geometry, importance

A new procedure in which the rate of radical cychzation was accelerated was demonstrated by Ooi and Maruoka [213]. They used the cavity of ATPH that would be expected to hold substrates in a favorable conformation for the cyclization (Scheme 6.167). In fact, the radical cyclization of iodide 168 proceeded even at -78°C, for 1 h, to afford (Z)-169 in 99% yield. The selectivity in the olefinic geometry of the cyclized products was significantly enhanced by proper choice of radical propagation reagent. TTMSS afforded absolute Z-selectivity whereas E Z=14 86 was obtained with Bu3SnH. It seemed that steric constraint played an important role when a second radical eventuaUy generated by the cyclization was trapped by bulky hydride reagents. 

Table 7 summarizes several important aspects of substrate control of diastereoselectivity. Variation of either the relative configuration of the lactone or of the olefin geometry allows access to the opposite diastereochemical series (Table 7, entries 2-5)80. Since only the ( )-olefins are formed, a successful chirality transfer either requires ionization of the lactone from a single conformation B with nucleophilic attack being faster than stereorandomization or an involvement of solely the, mi,.n -7t-allyl complex generated via n-a-n rearrangement prior to C —C bond formation. The soft carbanion attacks the allyl complex charge directed distal to the carboxylate anion. 

The labeling experiments, therefore, provide strong support for the mechanism we proposed to account for the dependence of ozonide cis/trans ratios on olefin geometry 14). In this particular case, under conditions of added aldehyde, approximately 70-75% of the ozonide (5ab) was, by all indications, formed through the molozonide-aldehyde reaction. However, in a normal ozonolysis aldehyde is not present initially, and before the molozonide-aldehyde mechanism can become important, a sufficient quantity of aldehyde must be produced, presumably by fission of the molozonide to zwitterion and aldehyde. Under these conditions it would not be surprising to find the new mechanism somewhat less important than in the present study. Once sufficient aldehyde is obtained in the normal ozonolysis, production of zwitterion may well nearly cease since the molozonide-aldehyde reaction does not deplete aldehyde concentration, and at sufficiently high aldehyde concentrations this reaction competes well with molozonide fission. Reaction temperature should be important in this competition. 

For the practical reduction of these substrates, one must again confront the problem of olefin geometry. Most synthetic methods form a mixture of (Z)- and (E)-P-acetamido acrylic acid esters. Thus, catalysts that reduce both geometries of the olefins are important, and catalysts that reduce both geometries of the olefin to form the same enantiomer are most practical. Chiral phosphorus ligands, such as BINAP,- DuPhos, BICP, BDPMI, o-Ph-HexaMeO-BIPHEP, DuanPhos,- fcrf-Bu-BisP, TangPhos, - and... 

Piers and Morton have reported that lithium (phenylthio)(trimethyl-stannyOcuprate smoothly transfers one trimethylstannyl group to a,/3-acetylenic esters in a conjugate sense to give /3-trimethylstannyl-a,/3-olefinic esters. Importantly, products of almost exclusive E- or Z-geometry are formed under conditions of kinetic or thermodynamic control. Partial reduction followed by Wittig... 

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

On the other hand, from a spectroscopic point of view it is more correct to consider the diene moiety as a whole. Therefore, it seems that the allylic contribution can be evaluated using two different points of view the olefin-picture (the former) and the diene-picture (the latter). This difference has important consequences. In fact, in a vast majority of cases these two pictures lead to the same result, whilst in some instances the two predictions can be opposite. Let us consider the three possible different geometries of... 

Several methods achieving the debromination of v/c-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inermess of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threo-and eryf/iro-dibromides, respectively. 

The geometry of the Michael acceptor has been shown to play an important role in the intramolecnlar Stetter reaction [70,72], In the case of salicylaldehyde derived substrate 90, which contains a c -l,2-disubstituted aUcene, no reaction occurs under standard reaction conditions. The same is not true with trisubstituted olefin acceptors. Cychzation of p,p-disubstituted substrate E)-9 provides cyclized product in high yield and 91% ee Eq. 7. The corresponding (Z)-isomer gives a similar yield although the enantioselectivity is decreased to 86%. 

The most common oxidation state of palladium is H-2 which corresponds toa electronic configuration. Compounds have square planar geometry. Other important oxidation states and electronic configurations include 0 ( °), which can have coordination numbers ranging from two to four and is important in catalytic chemistry, and +4 (eft), which is octahedral and much more strongly oxidizing than platinum (IV). The chemistry of palladium is similar to that of platinum, but palladium is between 103 to 5 x 10s more labile (192). A primary industrial application is palladium-catalyzed oxidation of ethylene (see Olefin polymers) to acetaldehyde (qv). Palladium-catalyzed carbon—carbon bond formation is an important organic reaction. 

The ease with which the geometry of the metal carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction124. 

The general description of this reaction is given in Table II by Reaction 18. Many investigations, theoretical as well as experimental, have been carried out on this type of coupling (29a). The decrease in the number of valence electrons makes the reaction more plausible for saturated systems. The geometry of the complex is also very important, since the orientation of the olefins or alkynes is the factor which determines the feasibility of the reaction. 

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