Sep 062016
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Diphenhydramine Hydrochloride
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Diphenhydramine hydrochloride is the active pharmaceutical ingredient in several widely used medications (e.g., Benadryl, Zzzquil, Tylenol PM, Unisom), and its worldwide demand is higher than 100 tons/year.
In 2013, Jamison and co-workers developed a continuous flow process for the synthesis of minimizing waste and reducing purification steps and production time with respect to existing batch synthetic routes (Scheme 1).
In the optimized process, chlorodiphenylmethane 1 and dimethylethanolamine 2 were mixed neat and pumped into a 720 μL PFA tube reactor (i.d. = 0.5 mm) at 175 °C with a residence time of 16 min. Running the reaction above the boiling point of and without any solvent resulted in high reaction rate. Product 3, obtained in the form of molten salt (i.e., above the melting point of the salt), could be easily transported in the flow system, a procedure not feasible on the same scale under batch conditions.
The reactor outcome was then combined with preheated NaOH 3 M to neutralize ammonium salts. After quenching, neutralized tertiary amine was extracted with hexanes into an inline membrane separator. The organic layer was then treated with HCl (5 M solution in iPrOH) in order to precipitate diphenhydramine hydrochloride 3 with an overall yield of 90% and an output of 2.4 g/h.
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Snead, D. R.; Jamison, T. F. Chem. Sci. 2013, 4, 2822, DOI: 10.1039/c3sc50859e

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In 2013 the Jamison group reported the flow synthesis of the important H1-antagonist diphenhydramine·HCl (92) showcasing the potential of modern flow chemistry to adhere to green chemistry principles (minimal use of organic solvents, atom economy etc.) . The synthetic strategy relied on reacting chlorodiphenylmethane (93) with an excess of dimethylaminoethanol (94) via a nucleophilic substitution reaction (Scheme ).

Scheme : Flow synthesis of diphenhydramine.HCl (92).

As both starting materials are liquid at ambient temperature the use of a solvent could be avoided allowing direct generation of the hydrochloride salt of 92 in a high temperature reactor (175 °C) with a residence time of 16 min. Conveniently at the same reaction temperature the product was produced as a molten paste (m.p. 168 °C) which enabled the continued processing of the crude product circumventing any clogging of the reactor by premature crystallisation. Analysis of the crude extrude product revealed the presence of minor impurities (<10%) even when stoichiometric amounts of 94 were used, consequently an in-line extraction process was developed. Additional streams of aqueous sodium hydroxide (3 M, preheated) and hexane were combined with the crude reaction product followed by passage through a membrane separator. The hexane layer was subsequently collected and treated with hydrochloric acid (5 M in IPA) leading to the precipitation of diphenhydramine hydrochloride (92) in high yield (~90%) and purity (~95%). Furthermore, options to further reduce waste generated during the purification sequence are presented by combining hot IPA with the crude flow stream leading to the isolation of the target compound (92·HCl) by direct crystallisation in the collection vessel (yield 71–84%, purity ~93%, productivity 2.42 g/h).




David Snead

David Snead

    dsnead at mit dot edu
Ph.D. The University of Florida, 2010
with Prof. Sukwon Hong
B.S. The University of North Carolina at Chapel Hill, 2005
with Prof. Joseph DeSimone


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Timothy F. Jamison

Professor of Chemistry
Massachusetts Institute of Technology
Department of Chemistry
77 Massachusetts Ave., Bldg 18-590
Cambridge, MA 02139

Phone: (617) 253-2135
Fax: (617) 324-0253
Email: tfj at mit dot edu

Curriculum Vitae
Tim Jamison was born in San Jose, CA and grew up in neighboring Los Gatos, CA. He received his undergraduate education at the University of California, Berkeley. A six-month research assistantship at ICI Americas in Richmond, CA under the mentorship of Dr. William G. Haag was his first experience in chemistry research. Upon returning to Berkeley, he joined the laboratory of Prof. Henry Rapoport and conducted undergraduate research in his group for nearly three years, the majority of which was under the tutelage of William D. Lubell (now at the University of Montreal). A Fulbright Scholarship supported ten months of research in Prof. Steven A. Benner’s laboratories at the ETH in Zürich, Switzerland, and thereafter he undertook his PhD studies at Harvard University with Prof. Stuart L. Schreiber. He then moved to the laboratory of Prof. Eric N. Jacobsen at Harvard University, where he was a Damon Runyon-Walter Winchell postdoctoral fellow. In July 1999, he began his independent career at MIT, where his research program focuses on the development of new methods of organic synthesis and their implementation in the total synthesis of natural products.




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//////////////////////Diphenhydramine Hydrochloride,  Flow Synthesis, FLOW CHEMISTRY

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