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The periselectivity of a particular reaction depends on the structure of both the ketene and the substrate. Although the reaction is predominantly used to form four-membered rings, a limited number of substrates undergo [3+2] or [4+2] reactions with ketenes. Examples of all three modes of cycloaddition are discussed in this section.
The reaction has been applied to alkenes of virtually every substitution, often high enantioselectivities are realized, with the chiral outcome controlled by the choice of dihydroquinidine (DHQD) vs dihydroquinine (DHQ) as the ligand. Asymmetric dihydroxylation reactions are also highly site selective, providing products derived from reaction ...
Chiral phosphine ligands, especially C 2-symmetric ligands, are the source of chirality in most asymmetric hydrogenation catalysts. Of these the BINAP ligand is well-known, as a result of its Nobel Prize-winning application in the Noyori asymmetric hydrogenation. [3] Chiral phosphine ligands can be generally classified as mono- or bidentate ...
Akin to the dihydroxylation, the oxyamination involves the cycloaddition of the alkene to an imido Os(VIII) intermediate of the type OsO 3 (NR). Such species are generated by treatment of osmium tetroxide with the sodium chloramines. Typical procedures combine chloramine-T, alkene, an osmium catalyst, and a chiral ligand. [2]
The Sharpless asymmetric dihydroxylation [7] was developed by K. Barry Sharpless to use catalytic amounts of OsO 4 along with the stoichiometric oxidant K 3 [Fe(CN) 6]. [1] [2] [8] The reaction is performed in the presence of a chiral auxiliary.
In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift.
Because of the large size of the α-pinenyl substituents, diisopinocampheylborane only hydroborates unhindered alkenes. These reactions proceed with high enantioselectivity. 2-Butene, 2-pentene, 3-hexene are converted to the respective chiral alcohols in high ee's. [4] Norbornene under the same conditions gave an 83% ee.
Chiral auxiliaries, [26] chiral boron enolates, [27] and asymmetric phase transfer catalysis [28] have been used successfully to effect asymmetric induction in the Darzens reaction. (12) Diastereoselective epoxidations of chiral, non-racemic alkenes suffer from the limitation that removal of the auxiliary without disturbing the epoxide is often ...