Alkene Reactions: Hydroboration-Oxidation, Catalytic Hydrogenation, Syn-Dihydroxylation, and Ozonolysis

Hydroboration-Oxidation Reaction and Regiochemistry

• Bench-top synthetic utility:

  • Hydration of alkenes on a laboratory bench top avoids the extreme temperatures and specialized extraction equipment required by industrial hydration processes.

  • Provides an alternative reaction pathway that alters normal regiochemical addition of water across an alkene double bond.

• Structure and electronic properties of borane ($BH_3$):

  • Borane ($BH_3$) is an exception to the octet rule because the central boron atom does not achieve an eight-electron outer shell.

  • Possesses an unfilled, empty $p$ orbital.

  • The boron atom exhibits $sp^2$ hybridization.

  • The electron-deficient nature of boron allows it to undergo nucleophilic attack and form charged complexes, such as a winter iodine complex.

• Regioselectivity of hydroboration:

  • Adds water across a double bond with non-Markovnikov regiochemistry, also referred to as anti-Markovnikov (or designated in alternate textbook phrasing as "non republican", "anti republican", or "anti iron").

  • In a standard hydration, hydrogen adds to the less substituted carbon and the hydroxyl group ($-OH$) adds to the more substituted carbon; hydroboration inverts this selectivity so that the hydrogen adds to the more substituted carbon and the $-OH$ attaches to the less substituted carbon.

  • Pharmaceutical relevance: Flipping regiochemical orientation is critical in drug design and pharmaceutical synthesis, where placing an $-OH$ group at a specific constitutional position rather than its regioisomer can dramatically alter the therapeutic efficacy and biological activity of the drug.

• Mechanism and transition state of hydroboration:

  • Occurs through a concerted, geared four-membered ring transition state.

  • Simultaneous bond breaking and bond making:

    • The bond between boron and hydrogen ($B-H$) breaks concurrently with the carbon-carbon double bond ($C=C$).

    • The carbon-boron ($C-B$) bond forms simultaneously with the carbon-hydrogen ($C-H$) bond.

• Trialkylation sequence:

  • All three $B-H$ bonds present in $BH_3$ are capable of reacting sequentially with alkene molecules.

  • Reaction of $BH_3$ with one equivalent of alkene yields a monoalkylated borane intermediate ($RBH_2$).

  • The monoalkylated species reacts with a second equivalent of alkene to produce a dialkylated intermediate ($R_2BH$).

  • The dialkylated species reacts with a third equivalent of alkene, resulting in a trialkylborane complex ($BR_3$).

  • Stoichiometric requirement: $1$ equivalent of borane ($BH_3$) reacts completely with $3$ equivalents of starting alkene molecules ($BH_3 + 3\,\text{alkene} \rightarrow BR_3$).

• Oxidation step and byproducts:

  • The trialkylborane intermediate is treated with a mixture of peroxide and base (hydroxide ion, $OH^-$) in aqueous solution ($H_2O$).

  • Reactions involving peroxide operate via free radical pathways ("ugly reactions") involving species with a single unpaired electron, which makes drawing a tidy curved-arrow mechanism complex and difficult.

  • The organoboron species serves strictly as a temporary synthetic intermediate; after the $-OH$ group is installed, the remaining boron-containing side products are discarded and washed away during reaction workup.

• Stereochemical outcome:

  • The oxidation sequence yields the trans product ("transporb") as the dominant and exclusive species.

  • The $-OH$ group and the adjacent substituent (e.g., a methyl group) reside on opposite faces of the former double bond.

  • Represents an anti addition relative to the substituent, which is observed exclusively over the syn addition (cis) product (which yields essentially $0$).

Catalytic Hydrogenation of Alkenes

• Fundamental redox definitions:

  • Addition of hydrogen to an unsaturated molecule increases electron density around the carbon atoms, constituting a chemical reduction.

  • Mnemonic devices for reduction-oxidation:

    • "OIL RIG": Oxidation Is Loss of electrons, Reduction Is Gain of electrons ($\text{OIL} = \text{Oxidation Is Loss}$, $\text{RIG} = \text{Reduction Is Gain}$).

    • "LEO": Loss of Electrons is Oxidation ($\text{LEO}$).

  • The gain of electron density by carbon reduces the carbon-carbon double bond ($C=C$) back into a single bond ($C-C$).

• Substrate scope and selectivity:

  • Catalytic hydrogenation cleanly reduces aliphatic carbon-carbon double bonds ($C=C$).

  • Under standard conditions, it does not reduce carbon-oxygen double bonds ($C=O$) or carbon-nitrogen double bonds ($C=N$).

  • Benzene rings do not react easily: despite containing three conjugated double bonds, the aromatic system possesses immense resonance stability that renders standard catalytic hydrogenation virtually impossible without extreme conditions.

• Heterogeneous catalysis system:

  • The reaction is heterogeneous rather than homogeneous; it does not take place in simple solution.

  • Transition metal catalysts utilized: Platinum ($Pt$), palladium ($Pd$), or nickel ($Ni$).

  • The active metal is finely dispersed and embedded across solid carbon (charcoal) powder particles.

  • Mechanism on the solid catalyst surface:

    • $H_2$ gas is introduced over the solid catalyst powder.

    • Molecular hydrogen ($H_2$) chemisorbs to the surface of the metal substrate ($Pt$ or $Pd$), breaking the $H-H$ bond and binding individual hydrogen atoms directly to the metal surface.

    • The alkene double bond approaches the surface; a pair of pi electrons attacks a surface-bound hydrogen atom while transient bonds form between the metal substrate and the carbons of the alkene.

    • Electron density shifts, the substrate-to-carbon bonds cleave, and the fully saturated alkane ($RCH_2-CH_2R'$) desorbs from the surface.

• Industrial applications in food science:

  • Used extensively in the manufacturing of margarine and processed fats from liquid oils (such as unsaturated soybean oil).

  • Unsaturated fats contain natural double bonds that keep them in a liquid state at room temperature.

  • Partial hydrogenation selectively reduces double bonds, raising the melting point of the substance.

  • This converts a liquid oil into a soft, spreadable solid that remains firm at refrigerator temperatures but smooth enough to spread directly onto toast.

Syn-Dihydroxylation Using Osmium Reagents

• Reagent properties and toxicity:

  • Reagent: Osmium tetroxide (also referred to as osmium hydroxide or "positive hydrogen tetroxide"), followed by treatment with sodium bisulfide (sodium bisulfite).

  • Osmium ($Os$) is a dense, heavy transition metal located deep in the periodic table.

  • Osmium reagents exhibit extreme chemical toxicity, requiring careful handling.

• Chemical transformation:

  • Converts an alkene into a 1,2-diol (glycol).

  • Installs a hydroxyl group ($-OH$) onto both carbon atoms involved in the original double bond, rather than adding a hydrogen and a halogen, or a hydrogen and a hydroxyl group.

• Stereospecificity:

  • Operates exclusively via syn addition.

  • Both hydroxyl groups ($-OH$) are delivered simultaneously to the exact same face of the alkene double bond.

• Synthetic purpose:

  • Useful when preparing multifunctional molecules where solubility must be adjusted.

  • In the synthesis of agricultural chemicals or pesticides, adding two hydroxyl groups significantly increases the aqueous solubility of a hydrophobic hydrocarbon backbone compared to adding a single $-OH$ group.

Ozone Structure, Reactivity, and Ozonolysis Cleavage

• Lewis electronic structure and formal charges of ozone ($O_3$):

  • Standard diatomic oxygen ($O_2$) has formal charges of $0$ on each oxygen atom.

  • Ozone ($O_3$) possesses a total of $18$ valence electrons ($3 \times 6 = 18$ electrons).

  • The valid Lewis structure requires one single bond, one double bond, and assigned lone pairs across the three oxygen atoms ($O=O^+-O^-$).

  • Formal charge distribution:

    • The singly bonded terminal oxygen atom has $1$ single bond and $3$ lone pairs ($6$ unshared electrons), yielding a formal charge of $-1$ ($6 - (1 + 6) = -1$).

    • The central oxygen atom carries a formal charge of $+1$.

    • The doubly bonded terminal oxygen atom carries a formal charge of $0$.

  • Due to resonance, there is no possible resonance structure where all oxygen atoms achieve a formal charge of $0$.

  • This unavoidable charge separation and inability to neutralize formal charges explains why ozone is an extraordinarily unstable and reactive gas.

• Environmental and physical characteristics of ozone:

  • Beneficial in the upper atmosphere (stratosphere) where it blocks harmful radiation.

  • Harmful pollutant at ground level (troposphere), found in automobile exhaust emissions.

  • Generated by high-voltage electrical discharges and sparks.

  • Produces a sharp, pungent, acidic odor frequently described as smelling like "burning electricity", typically detectable in the air after lightning storms.

• Ozonolysis cleavage mechanism and products:

  • Reaction conditions: The alkene is treated first with ozone ($O_3$), followed by a reductive workup using zinc ($Zn$) and water ($H_2O$).

  • Net transformation: The carbon-carbon double bond is cleaved completely in half, inserting a carbonyl oxygen ($=O$) onto each carbon atom that formed the original double bond.

  • Mechanism proceeds through a cyclic "molosemide intermediate" (molozonide) that undergoes structural rearrangement prior to reductive cleavage by zinc.

  • Substrate examples:

    • Cyclic alkenes such as cyclohexene ("cyclohexane") are cleaved open into an acyclic dicarbonyl compound containing two aldehyde groups (a dialdehyde or "dialyte").

    • Polyunsaturated substrates (e.g., unsaturated fatty acids with multiple double bonds) undergo oxidative cleavage at each double bond site, generating up to $3$ distinct cleavage fragments.