Acid chlorides, Curtius rearrangement, acyl

Modified Curtius rearrangement. Acyl azides, preferably prepared in situ under phase-transfer conditions by reaction of acid chlorides and NaN, are converted to tri-fluoroacetamides by reaction with CF,COOH in refluxing CH CI, solution. These products are cleaved to primary amines under mild conditions (equation I). ... 

Carboxylic acid chlorides may be converted to isocyanates via the Curtius rearrangement of the acyl azide. A solution of tetrabutylammo-nium azide (prepared by CH Cl extraction from aqueous sodium azide and tetrabutylammonium hydroxide) in toluene or benzene reacts readily with acid chlorides to give acyl azides. These are heated in solution to afford the isocyanates and nitrogen (Eq. 6.63) [101]. 

The Curtius rearrangement, like the Hofmann rearrangement, involve migration of an -R group from the G-O carbon atom to the neighboring nitro gen with simultaneous loss of a leaving group. The reaction takes place on heat ing an acyl azide that is itself prepared by nucleophilic acyl substitution of m acid chloride. 

Warren, J. D., Press, J. B. Trimethylsilylazide/KN3/18-crown-6. Formation and Curtius rearrangement of acyl azides from unreactive acid chlorides. Synth. Commun. 1980, 10, 107-110. 

Another interesting aspect is the elegant one-step synthesis of manifold substituted isocyanates achieved simultaneously by Washburne et al. and Kricheldorf who both used acyl chlorides acid anhydrides and esters to prepare isocyanates (457) via a Curtius rearrangement. This synthesis is especially useful since it enables for instance long chain isocyanates to be obtained (from fatty acid chlorides) and perfluorated compounds If a perfluoro-... 

The sodium azide pathway (Pathway A), Figure 4, begins with a thionylchloride treatment of the free acid to form the acyl chloride. Subsequent treatment with sodium azide may involve a non-aqueous environment (1,2-dimethoxy ethane, "dry method"), or an aqueous medium ("wet method"). The organic azide is recovered from the reaction mixture and converted into the isocyanate by the Curtius rearrangement. This may be accomplished in solid form ("dry method"), or in a non-aqueous solvent like dioxane or DMF ("solution method"). 

Acyl azides isocyanates. The reagent undergoes metathesis readily with carboxylic acid chlorides in benzene or toluene. If the solution is heated to 50-90°, Curtius rearrangement leads to isocyanates in 60-90% yield. 

Reaction of acid chlorides with sodium azide/ NaN3, yields acyl azides/ RCON3. When heated, these undergo the Curtius rearrangement to amines, RNH2, or, in a non-hydroxylie solvent, to isocyanates, RNCO. Using the structure... 

Tertiary alcohols are converted directly to azides using NaN3/ Sulfuric Acid or HN3/Boro Trifluoride or TUanium(IV) Chloride (eq 5), and the carboxylic acid to acyl azide transformation (often en route to Curtius rearrangements to isocyanates) occurs with DPPA or via activation with DMF/Thionyl Chloride. ... 

The Curtius rearrangement accomplishes the same synthetic goal as the Hofmann rearrangement, and it takes place by a similar mechanism. An acid chloride reacts with azide ion to give an acyl azide, which undergoes Curtius rearrangement when heated. 

Activation of the carboxylic acid as the acyl chloride permits direct reaction with azide anion to form the acyl azide substrates for Curtius rearrangement. Sodium azide is commonly used, and the reaction has been used on the process chemistry scale for the synthesis of benzyl-A-vinyl carbamate. Acryloyl chloride was combined with sodium azide in a biphasic system with phase-transfer catalysis (PTC), providing acyl azide 25. Upon heating, Curtius rearrangement provided vinyl isocyanate, which was distilled directly into benzyl alcohol containing phenothiazine (27) to inhibit polymerization of 26 and triethylamine to catalyze addition of the alcohol to the isocyanate. The vinyl carbamate product 28 was isolated by crystallization. As the autiior clearly pointed out, preparation and reaction of acyl azides, particularly on large scales, require appropriate safety precautions. 

Curtius rearrangement with various acid chlorides,including phenoxy acetyl chloride. Heating in dioxane provided 32 the trimer of isocyanate 31, formed by rearrangement of acyl azide 30. 

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