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Hydroboration-Oxidation of Alkenes03:08

Hydroboration-Oxidation of Alkenes

11.9K
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
11.9K
Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation02:47

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

21.7K
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
21.7K
Regioselectivity and Stereochemistry of Hydroboration02:36

Regioselectivity and Stereochemistry of Hydroboration

9.6K
A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
9.6K
Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide02:44

Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide

13.2K
Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.
13.2K
Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

5.0K
By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
5.0K
α-Alkylation of Ketones via Enolate Ions01:10

α-Alkylation of Ketones via Enolate Ions

4.0K
Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the...
4.0K

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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species
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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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Enzyme-Catalyzed Intramolecular Enantioselective Hydroalkoxylation.

Shu-Shan Gao1, Marc Garcia-Borràs1, Joyann S Barber1

  • 1Department of Chemical and Biomolecular Engineering and ⊥Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.

Journal of the American Chemical Society
|February 28, 2017
PubMed
Summary

Researchers discovered an enzyme, PhnH, that performs enantioselective hydroalkoxylation, efficiently creating cyclic ethers. This enzyme significantly accelerates the reaction, offering new possibilities in synthetic chemistry and biosynthesis.

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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

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Area of Science:

  • Synthetic chemistry
  • Enzymology
  • Natural product biosynthesis

Background:

  • Hydroalkoxylation is a key reaction for forming C-O bonds and cyclic ethers.
  • The biosynthesis of fungal natural products like herqueinone involves complex enzymatic pathways.

Purpose of the Study:

  • To identify and characterize enzymes involved in herqueinone biosynthesis.
  • To investigate the catalytic mechanism and efficiency of the identified enzyme in hydroalkoxylation reactions.

Main Methods:

  • Enzyme discovery through biosynthesis studies.
  • Biochemical assays to determine enzyme activity and kinetics.
  • Structural analysis of the enzyme and its catalytic domain.

Main Results:

  • Identification of the enzyme PhnH, capable of intramolecular enantioselective hydroalkoxylation.
  • PhnH catalyzes the addition of a phenol to a terminal olefin, forming a dihydrobenzofuran.
  • The enzyme exhibits a remarkable 3 × 10^5-fold acceleration compared to uncatalyzed reactions.
  • PhnH belongs to the DUF3237 protein superfamily, previously lacking functional annotation.

Conclusions:

  • Enzymes can effectively catalyze enantioselective hydroalkoxylation reactions.
  • PhnH represents a novel enzyme with significant potential in synthetic organic chemistry.
  • The discovery expands the known functions within the DUF3237 protein superfamily.