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Reduction of alkynes is a useful method for the stereoselective synthesis of disubstituted alkenes. If the cis -alkene is desired, hydrogenation in the presence of Lindlar's catalyst (a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead) is commonly used, though hydroboration ...
In organic chemistry, terminal alkenes (alpha-olefins, α-olefins, or 1-alkenes) are a family of organic compounds which are alkenes (also known as olefins) with a chemical formula C x H 2x, distinguished by having a double bond at the primary, alpha (α), or 1-position. [1]
The Pauson–Khand (PK) reaction is a chemical reaction, described as a cycloaddition.In it, an alkyne, an alkene, and carbon monoxide combine into a α,β-cyclopentenone in the presence of a metal-carbonyl catalyst [1] [2] Ihsan Ullah Khand (1935–1980) discovered the reaction around 1970, while working as a postdoctoral associate with Peter Ludwig Pauson (1925–2013) at the University of ...
Discrimination between terminal and disubstituted double bonds is often low, however. (5) Allenes are reduced to the more highly substituted alkene in the presence of diimide, although yields are low. [8] (6) Iodoalkynes represent an exception to the rule that alkenes cannot be obtained from alkynes.
As a result, the separation between alkyl groups is greatest in the most substituted alkene. [ 7 ] Hyperconjugation , which describes the stabilizing interaction between the HOMO of the alkyl group and the LUMO of the double bond, also helps explain the influence of alkyl substitutions on the stability of alkenes.
In the example shown below, the para-methoxybenzoyl substituent serves primarily as a source of steric bulk to allow the catalyst to differentiate the two faces of the alkene. [23] SAD scheme 3. It is often difficult to obtain high diastereoselectivity on cis-disubstituted alkenes when both ends of the olefin have similar steric environments.
The functional groups on the thiol and alkene compounds can affect the reactivity of the radical species and their respective rate constants. The structure of the alkene determines whether the reaction will be propagation or chain-transfer limited, and therefore first order with respect to alkene or thiol concentration respectively.
Lastly, the insertion of the carbonyl group of 3 in the residual carbon-titanium connection forms a cyclopropane ring. In the transition state of this elementary stage, which is the limiting stage of the reaction, an agostic interaction stabilizing between the beta hydrogen and the R2 group and the titanium atom was called upon to explain the ...