Allophanate derivatives are likely superior to many other more common methods to isolate and purify substances that are formed in reaction where there arise several products.
For an example that might be tested in the laboratory, consider the hypothetical reduction of 4-methyl-3-penten-2-one, (mesityloxide), bp 129 C, by hydrogenation. There are three theoretically possible products that are alcohols: 4-methyl-2-pentanol, bp 132 C, (ketone and double bond reduced); 4-methyl-4-penten-2-ol, bp 131.7 C, (only ketone reduced): and 4-methyl-3-penten-2-ol, bp 132 C, (ketone reduced and double bond isomerized). It is likely that reaction conditions can be found that lead to substantially one desired substance but such an enriched mixture still could not be separated based on boiling points. A solution that should be considered is the formation of the allophanate derivatives and the mixture’s recrystallization to get at the predominant compound’s allophanate in pure form, followed by hydrolysis back to the parent alcohol.
Another possible situation could arise in the practice of the Prins reaction. The Prins reaction is expected to produce a 1,3-diol from formaldehyde and an olefin. This is not necessarily a clean reaction. In fact, the infrequency of its application suggests that it may lead to multiple products. Formation of the bis-allophanates as a method to obtain a pure crystalline product seems to be worth investigating.
The literature suggests that allophanates are derivatives that can be expected to crystallize from even quite difficult mixtures. For example the method was useful in the isolation and purification of various vitamins from natural sources. Fieser & Fieser in Organic Chemistry, the Third Edition, Reinhold Publishing Company, 1956 wrote, “The isolation of two pure factors from wheat-germ oil concentrates in 1936 was simplified by the discovery of crystalline derivatives, allophanates, resulting from esterification of the factors with cyanic acid….. on hydrolysis of the derivatives, the two pure active factors were obtained as highly active pale yellow oils named alpha and beta tocopherol.”
Similarly the allophanate derivative was used by Windaus and coworkers in isolating Vitamin D3 from an irradiation mixture. This is reported in Fieser & Fieser’s, Reagents for Organic Chemistry Vol. 1 pg. 171:
“Treatment of the crude, oily mixture with isocyanic acid afforded directly a solid product easily purified by recrystallization from acetone and converted into pure vitamin by hydrolysis.” Vitamin D3 has mp 82-84 C while the allophanate had mp 173-174 C, so one can see the inherent advantage. The co-products of the hydrolysis are conveniently totally water soluble!
Another possible situation could arise in the practice of the Prins reaction. The Prins reaction is expected to produce a 1,3-diol from formaldehyde and an olefin. This is not necessarily a clean reaction. In fact, the infrequency of its application suggests that it may lead to multiple products. Formation of the bis-allophanates as a method to obtain a pure crystalline product seems to be worth investigating.
The literature suggests that allophanates are derivatives that can be expected to crystallize from even quite difficult mixtures. For example the method was useful in the isolation and purification of various vitamins from natural sources. Fieser & Fieser in Organic Chemistry, the Third Edition, Reinhold Publishing Company, 1956 wrote, “The isolation of two pure factors from wheat-germ oil concentrates in 1936 was simplified by the discovery of crystalline derivatives, allophanates, resulting from esterification of the factors with cyanic acid….. on hydrolysis of the derivatives, the two pure active factors were obtained as highly active pale yellow oils named alpha and beta tocopherol.”
Similarly the allophanate derivative was used by Windaus and coworkers in isolating Vitamin D3 from an irradiation mixture. This is reported in Fieser & Fieser’s, Reagents for Organic Chemistry Vol. 1 pg. 171:
“Treatment of the crude, oily mixture with isocyanic acid afforded directly a solid product easily purified by recrystallization from acetone and converted into pure vitamin by hydrolysis.” Vitamin D3 has mp 82-84 C while the allophanate had mp 173-174 C, so one can see the inherent advantage. The co-products of the hydrolysis are conveniently totally water soluble!
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