Noah Vinter
The conversion of alcohols to alkanes is a complex process that often requires multiple steps. While there are various reduction methods, direct reduction is generally difficult. The Barton-McCombie reaction, for instance, involves removing hydroxyl groups under free-radical conditions, while other methods may use metal hydrides such as LiAlH4 or catalytic amounts of InCl3. The choice of method depends on the specific alcohol and desired alkane, with ongoing research developing more general techniques.
Characteristics and Values of Alcohol-to-Alkane Conversion Methods
| Characteristics | Values |
|---|---|
| General Difficulty | Direct reduction of alcohols to alkanes is generally difficult. |
| Classical Methods | Heterogeneous hydrogenation and the Birch reduction |
| Two-Step Sequence | Conversion of alcohols into leaving groups (halides, sulfonate esters) followed by reduction with metal hydrides (LiAlH4, LiHBEt3, Bu3SnH + radical initiator) |
| Reduction of Primary Alcohols | NaBH3CN-(PhO)3PCH3I |
| Popular Method | Barton-McCombie deoxygenation (nucleophilic displacement by hydride) |
| Iridium-Catalyzed Method | Direct dehydroxylation of benzylic, allylic, and primary alcohols (Li and coworkers, 2013) |
| Isao Saito's Method | Convert alcohol to m-trifluoromethylbenzoate, photolyze with a high-pressure Hg lamp in THF/water with N-methylcarbazole |
| Strong Reducing Agent | Red P + HI |
| Dehydration | Catalytic methods to convert aldehydes and keystones into alcohols |
| LuCl3/B(C6F5)3 Cocatalyzed Reductive Deoxygenation | Alkanes produced under mild conditions with tolerance for reactive amino, hydroxyl, nitro, halogen, vinyl, and ester functional groups |
| Primary Alcohol Deoxygenation | Reduction of derived diphenyl phosphate esters with lithium triethylborohydride in THF at room temperature |
| Direct Electrochemical Reductive Approach | Deoxygenation of alcohols in the presence of substoichiometric AlCl3 |
| Chlorodiphenylsilane as Hydride Source | High chemoselectivity for benzylic, secondary, and tertiary alcohols |
| Direct Electrolysis | Formation of deoxygenated product from primary alcohols in the presence of methyl toluate |
| Ir-Catalyzed Alcohol Deoxygenation | Efficient with activated alcohols under harsh reaction conditions |
| Ru-Catalyzed Aliphatic Primary Alcohol Deoxygenation | Good functional group tolerance and excellent efficiency under practical reaction conditions |
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What You'll Learn
Using an iridium complex with hydrazine as the terminal oxidant
While there are many methods to reduce an alcohol to an alkane, using an iridium complex with hydrazine as the terminal oxidant is a notable one. This method was introduced in a 2013 publication by Li and coworkers.
The method successfully reduced benzylic, allylic, and primary alcohols, although benzylic/allylic substrates worked much better. This method is also a catalytic process, which means that the iridium complex catalyst can be recovered and reused after the reaction.
The iridium-catalyzed direct dehydroxylation of alcohols involves using hydrazine as a reducing agent. This process involves the conversion of hydrazine to nitrogen gas, which is the driving force for the reaction. To achieve this, a strong base such as KOH is used in a high-boiling protic solvent like ethylene glycol (with a boiling point of 197 °C). The reaction occurs at a reasonable rate when heated to almost 200 °C.
It is important to note that this method is still a subject of ongoing research, and there may be variations or improvements to the procedure over time.
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Using an iridium-catalyzed alcohol deoxygenation
The direct reduction of alcohols to alkanes is generally a difficult process. The conversion usually requires a two-step sequence involving the conversion of alcohols into leaving groups, such as halides and sulfonate esters, followed by reduction with metal hydrides.
Developing a general method for this transformation is an ongoing area of research, with many methods published. This means that there is no single best method. A 2013 publication from Li and colleagues used an iridium complex with hydrazine as the terminal oxidant. Benzylic, allylic, and notably primary alcohols were successfully reduced, although benzylic/allylic substrates worked much better.
Iridium-catalysed direct dehydroxylation of alcohols can be achieved using the Barton-McCombie reaction. This involves converting the alcohol to a m-trifluoromethylbenzoate, followed by photolysis in the presence of an electron donor such as N-methylcarbazole.
Another method involves an Ir-catalysed alcohol deoxygenation based on dehydrogenation/Wolff-Kishner reduction. This method is efficient mainly with activated alcohols under harsh reaction conditions.
It is important to note that while these methods can reduce an alcohol to an alkane, they may also reduce other functional groups present in the molecule.
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Using a ruthenium hydride complex
There are several methods for reducing an alcohol to an alkane, and the choice of method depends on the specific alcohol in question and the presence of other functional groups. One method that can be used involves ruthenium hydride complexes, which are effective catalysts for hydrogenation and hydrogen transfer reactions.
One example of a ruthenium hydride complex that can be used for this purpose is Shvo's ruthenium hydride complex. This complex is a well-known hydrogen transfer catalyst that has been successfully applied in a broad range of hydrogen transfer processes. The reaction facilitated by this complex involves transferring a hydride (bonded to the metal centre) and a proton (bonded to a ligand) to a double bond. However, the exact mechanism of this process is still a subject of debate, with both inner- and outer-sphere mechanisms being proposed.
Another ruthenium hydride complex that can be used for alcohol reduction is the cationic ruthenium hydride complex. This complex has been shown to exhibit high catalytic activity for the hydrogenolysis of carbonyl compounds, resulting in the formation of the corresponding aliphatic products. This reaction is particularly effective in the presence of a phenol ligand.
Additionally, ruthenium-based catalysts have been used in combination with other reagents for the reduction of alcohols. For example, a combination of a chiral Ru complex and KOtBu catalyses an asymmetric transfer hydrogenation of various benzaldehyde-1-d derivatives with 2-propanol to yield (R)-benzyl-1-d alcohols. Similarly, PhanePhos-ruthenium-diamine complexes are effective catalysts for the asymmetric hydrogenation of a wide range of aromatic, heteroaromatic, and α,β-unsaturated compounds.
Furthermore, ruthenium hydride complexes have been employed in the synthesis of primary amines through the direct amination of alcohols. Specifically, the Ru–MgO/TiO2 catalyst has been found to efficiently convert a variety of alcohols to primary amines at low temperatures of approximately 100 °C without the need for H2 gas. This catalytic system offers an efficient synthetic route for biomonomers such as 2,5-bis(aminomethyl)furan (BAMF).
In summary, ruthenium hydride complexes, such as Shvo's catalyst and cationic ruthenium hydride complexes, are effective tools for reducing alcohols to alkanes. These complexes facilitate hydrogen transfer and hydrogenolysis reactions, making them valuable reagents in organic synthesis. Additionally, ruthenium-based catalysts can be combined with other reagents to achieve specific reductions, such as the synthesis of primary amines or chiral alcohols.
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Using chlorodiphenylsilane as a hydride source
The direct reduction of alcohols to alkanes is generally a difficult process. The conversion usually requires a two-step sequence involving the conversion of alcohols into leaving groups, such as halides and sulfonate esters, followed by reduction with metal hydrides. However, a highly chemoselective reducing system for secondary or tertiary alcohols uses chlorodiphenylsilane with a catalytic amount of indium trichloride.
This method effectively reduces benzylic, secondary, and tertiary alcohols to their corresponding alkanes in high yields. The chlorodiphenylsilane/InCl3 system is highly selective for hydroxyl groups, even in the presence of other functional groups that are typically reduced by standard methods, such as esters, chloro, bromo, and nitro groups. This system operates under mild conditions, avoiding the need for high temperatures or pressures.
The reaction mechanism involves the initial formation of a hydrodiphenylsilyl ether, which then reacts with InCl3 acting as a Lewis acid to form an oxonium complex. This complex accelerates the desiloxylation step, leading to the formation of the desired alkane product.
The use of chlorodiphenylsilane as a hydride source offers a selective approach to reducing alcohols to alkanes while avoiding the reduction of other functional groups. This method provides a valuable tool for chemists in various synthetic applications, especially when dealing with sensitive functional groups.
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Using LiAlH4-AlCl3
Lithium aluminium hydride (LiAlH4) is a strong reducing agent that can be used to reduce alcohols to alkanes. It is prepared through the reduction of a solution of AlCl3 in ether by lithium hydride (LiH).
LiAlH4 is a white solid that is more reactive than its analogue, sodium borohydride (NaBH4). It is a powerful, readily available, and relatively cheap reducing agent that can be used to reduce almost any organic functional group to alcohols, amines, etc. However, it cannot reduce alkenes or alkynes to alkanes in general cases.
To reduce an alcohol to an alkane using LiAlH4, the following steps can be followed:
First, it is important to note that the reduction of an alcohol to an alkane using LiAlH4 involves breaking the C-O bond in the alcohol and forming a new C-H bond in the alkane. This is because LiAlH4 acts as a source of the hydride anion nucleophile (H:-).
Next, the hydride anion (H:-) from LiAlH4 undergoes nucleophilic addition to the carbonyl carbon of the alcohol, forming a C-H single bond and a tetrahedral alkoxide ion intermediate.
The alkoxide ion is then protonated to form the corresponding alkane. It is important to note that the alkoxide salts formed during this reaction are insoluble and need to be carefully hydrolyzed before the alkane product can be isolated.
Finally, water is typically added in a second step to complete the reduction process. The lithium, sodium, boron, and aluminium present in the reaction mixture end up as soluble inorganic salts at the end of the reaction.
It is worth mentioning that the reduction of unsymmetrical ketones with LiAlH4 can produce a pair of stereoisomers because the hydride ion can attack either face of the planar carbonyl group. This results in a racemic mixture of enantiomers if no other chiral centers are present in the molecule.
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Frequently asked questions
The reduction of alcohol to alkane is a difficult process and generally requires a two-step sequence. However, you can try the Barton-McCombie reaction, which involves the conversion of alcohols into leaving groups, followed by reduction with metal hydrides such as LiAlH4.
Yes, there are catalytic methods of adding hydrogen gas to aldehydes and keystones to convert them into alcohols. However, there are currently no purely catalytic methods for converting an alcohol to an alkane.
There are several other methods, including the use of an iridium complex with hydrazine as the terminal oxidant, direct electrolysis of primary alcohols, and the use of chlorodiphenylsilane as a hydride source in the presence of a catalytic amount of InCl3.
