Frans-disubstituted olefinic

The enantiomeric excesses obtained to this point for the catalytic AD of monosubstituted olefins (seeTable6D.2 [16,26,29,31,40,44-46,49]) are lower than for frans-disubstituted olefins (Table 6D.3). The entries in Column 9 show enantiomeric purities ranging from 54% ee to 97% ee for dihydroxylations with the (DHQD)2-PHAL and (DHQ)2-PHAL pair of chiral ligands. Several monosubstituted olefins with branching at the a-position (e.g., entries 2-4 and 11) are dihy-droxylated with higher enantioselectivities when DHQD-PHN is used as the chiral ligand instead of (DHQD)2-PHAL. Recently, a new ligand for terminal olefins has been discovered [48b],... 

On the other hand, these reactions are also limited by the typical reactant preferences of porphyrin catalysts. For instance, ds-disubstituted olefins react much faster than frans-disubstituted olefins as a consequence of the... 

The stereoselectivity of the HWE olefination reaction depends on the nature of the RO groups on phosphorus, structural features of the ylide, the solvent, and the reaction temperature. Generally, the HWE reactions give preferentially the more stable fran -disubstituted olefins. 

A Cu(I)-7V-heterocyclic carbene complex, IPrCu(DBM), catalyzes alkene aziridination with TcesNH2, Phl=0, and 4 A molecular sieves (eq 4). Reactions are performed with limiting alkene as- and frans-disubstituted olefins give isomeric product mixtures. In one example with styrene, TcesNH2, and PhI(OAc)2, 3 mol % of an Au(I) catalyst, [Au( Bu3tpy)](OTf), is found to catalyze aziridine formation. ... 

There are two possible transition states spiro and planar. Nearly every example of frans-disubstituted and trisubstituted olefins which were studied with Shi s catalyst is consistent with the spiro transition state. The extent of the involvement of the competing planar transition state depends on the nature of the substituents on the olefins. 

Two successful strategies for the enantioselective synthesis of trans-epoxides by means of oxo-metal catalysis have been discovered. The stereospecific epoxidation of frans-olefins offers a direct route to trans-epoxides, although progress in this area has been limited (see Sect. 2.2.1). Alternatively, the [Mn(salen)]-catalyzed epoxidation of cis-disubstituted olefins in the presence of alkaloid-derived phase-transfer catalysts such as 24 resulted in the formation of the transepoxide as the major, and in some cases nearly exclusive, product (Scheme 6)... 

Given the difficulties encountered in the epoxidation of frans-olefins by Mn(salen) complexes, it is intriguing that a wide range of trisubstituted olefins are outstanding substrates for asymmetric epoxidation (Scheme 7) [62,76]. The absolute stereochemistry of the epoxide products is inverted at the benzylic carbon when compared with the sense of induction seen with cfs-disubstituted olefins. A qualitative transition state model has been suggested wherein the trisubstituted substrate reacts with the metal-oxo complex via a skewed side-on approach (Fig. 12). The distortion of trisubstituted olefins from planarity resulting from Aj 2 or Aj 3 interactions may be critical in this context. 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

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