Alkene-displacement approach

Thus, using an alkene-displacement approach, a range of P-diketiminato- and NHC-stabilized K -alane complexes of [M(CO)4]... 

While alkyne metathesis is not going to displace alkene methathesis as a synthetic method, it is a complementary approach that can offer advantages. 

There thenfollowed reports by Katz [13] and Grubbs [14] and their co-workers on studies that aimed to simplify and confirm the analysis. The key remaining issue was whether a modified pairwise mechanism, in which another alkene can coordinate to the metal and equilibrate with the product prior to product displacement, would also explain the appearance of the anomalous cross-over products early in the reaction evolution. However, a statistical kinetic analysis showed that for a 1 1 mixture of equally reactive alkenes, the kinetic ratio of cross-metathesis should be 1 1.6 1 for the pairwise mechanism and 1 2 1 for the Chauvin mechanism. Any equilibration (substrate or product) would, of course, cause an approach towards a statistical distribution (1 2 1) and thus allow no distinction between the mechanisms. 

Electron-deficient alkenes, either these with conjugating substituents or of enones, undergo Michael-like additions with regiospecific introductions of the nucleophiles, and this approach has been successful for the formation of cyclopropyl derivatives—notably from some nucleosides. An example involves the addition of the anion of bis(phenylsulfonyl)methane to the phenylselenone 173 which gives the adduct 174 in 35% yield. This reaction presumably occurs by Michael addition to C-2 followed by Sn2 ring-closure reaction at C-3 with displacement of phenylselenic acid. Reductive desulfonylation affords the 2,3,-dideoxy-2,3 -cyclopropayuridine 175.199... 

Many intermediate dialkylboranes derived from hydroboration with IpcBH2 can be recrystallized to enantiomeric purities approaching 100%, thus giving alcohols of 98-99% ee upon oxidation. If, instead of being oxidized in situ, the dialkylbo-rane intermediate is treated with Acetaldehyde, a-pinene is displaced for recovery and a chiral boronate bearing the R group of the alkene is obtained (eq 7). 

In both cases we must consider the danger of enolate formation from the ketone. In the first case the alcohol might displace the bromide or attack the ketone and in the second the allylic bromide might be attacked at the alkene though this makes no difference as the allylic system is symmetrical. The first approach is easier as the bromoketone is easily made from acetone (Chapter 21) and the allylic halide in the second approach would probably be made from the alcohol used in the firs synthesis. 

Another approach is the nucleophilic displacement of chloride from chloro-propyl-silica with a pyridine-substituted porphyrin. These materials are active in the epoxidation of alkenes, where iodosylbenzene is the preferred oxidant, and in the oxidation of alkanes to alcohols and ketones. The copolymerisation of a porphyrin containing four attached trimethoxysilane groups with tetra-ethoxysilane, leading to an active hybrid silica-porphyrin, offers another route to these important catalysts. 

Displacement reactions. 1,3-Dibromo-l-trimethylsilylpropene is available from allyltrimethylsilane by reaction with NBS. The dibromo compound couples with Grignard reagents to effect chain elongation. With further displacement of the vinylic bromine and protodesilyation, it constitutes an intriguing approach to (Z)-alkenes. ... 

As a bromine molecule approaches an alkene, the electron density of the alkene it bond repels electron density in the closer bromine, polarizing the bromine molecule and making the closer bromine atom electrophilic. The alkene donates a pair of electrons to the closer bromine, causing displacement of the distant bromine atom. As this occurs, the newly bonded bromine atom, due to its size and polarizability, donates an electron pair to the carbon that would otherwise be a carbocation, thereby stabilizing the positive charge by delocalization. The result is a bridged bromonium ion intermediate. 

Both alkyl and alkenyl amino acids can be prepared by this approach. A common method for introducing the halide into an alkene-bearing molecule is illustrated by the reaction of -pent-2-enoic acid with N-bromosuccinimide to form 1.12. Subsequent treatment with ammonia led to displacement of the bromine moiety to give 4-aminopent-2-enoic acid (J.I3). An alternative method reacted 1.12 with sodium azide and then reduced the azide with zinc and acetic acid (see section l.l.B.iv). Allylic halogenation in systems such as 1.12 are well known. 

The Jt electrons of the alkene act as a nucleophile to displace bromide ion from bromine. The resulting cychc bromonium ion can be viewed as the addition of Br to the double bond. Bromine has two covalent bonds and a formal 1+ charge in this intermediate. Attack of the nucleophilic bromide ion occurs from the opposite face because the bromine atom that is already there blocks approach from the same face. 

Hoveyda is one of the pioneers in the use of combinatorial chemistry techniques to discover novel ligands for catalytic asymmetric processes. A brilliant illustration is the identification of small oligopeptides as ligands for Cu-catalyzed Sn2 displacement reactions (Scheme 14.17) [95, 96). The copper complexes prepare from Schiff base-derived dipeptides such as 100 catalyze asymmetric displacement reactions of a wide range of allylic phosphates. Significantly, the method can also be utilized to prepare alkenes incorporating quaternary allylic stereogenic centers (cf 101). This approach was ele-... 

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