Catalyst Simplifies Amines Production

ChemicalProcessing.com

Pincer complex spurs single-step synthesis of primary amines from ammonia and alcohols

Primary amines play a crucial role in making pharmaceuticals, fine chemicals, polymers, emulsifiers and other materials. However, terminal primary amines, which are the most useful, boast high reactivity, making their selective synthesis challenging because reaction can continue to secondary and tertiary amines, note researchers at the Weizmann Institute of Science, Rehovot, Israel. Fortunately, a new catalyst promises significant improvement, says David Milstein, a professor in the Department of Organic Chemistry there — it enables direct production at high yields under mild conditions, obviating the multiple steps and toxic reagents now required.

He and co-worker Chidambaram Gunanathan have discovered a ruthenium-based catalyst that facilitates the reaction of ammonia with alcohols to form primary amines and water without byproducts. Small-scale laboratory tests have provided amine yields reaching up to 96% at 135°C and 7.5 atm. “The reported yields are not optimized, and it is likely that they can be improved by variation of reaction conditions such as temperature and reaction time,” notes Milstein, adding that no catalyst poisoning has been observed. “In commercial processes, much higher temperatures and pressures are employed, and the yields are lower, due to large amounts of secondary and tertiary amines as well as alkene (by alcohol dehydration) and alkane side products,” he says.

Figure 1.  Single-step synthesis: Ruthenium-based catalyst enables direct production of primary amines under mild conditions. Source: Weizmann Institute of Science.

The catalyst is a ruthenium complex — an organic ligand with phosphorus arms holds the ruthenium in pincer-like fashion (Figure 1). The catalyst is stable in air for several months and is readily prepared, he notes.

The reaction can take place in solvents such as toluene, without solvents, in water and, for alcohols immiscible in water, on water. Details were published online in October by Angewandte Chemie.

“The next step is mechanistic studies of the reaction, aiming at defining the rate-determining step of the reaction. Structural modification of the catalyst, in order to reduce the barrier of the rate-determining step, may follow. This may lead to higher rates and perhaps higher selectivity in the case of simple alkanes. We also plan to study the scope of the reaction for the production of functionalized primary amines,” notes Milstein.

“We believe that it [the catalyst] can be utilized by industry even as it is now, after optimization for the specific industrial product,” he says.

The school has applied for a patent and is interested in licensing the technology. It has already received inquiries from operating companies. “There are plans to make the catalyst commercially available by a company,” adds Milstein.
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