A New Synthesis of Isocyanates and Isothiocyanates

Abstract

of the M-CO bond relate only t o t h e ground state; however, t h e reaction behavior is determined by the energy of the transition state. It is possible that ligands such as amines contribute t o the stabilization of the transition state, though the reason for this is still unclear. A fuller knowledge of the electronic structure of the excited states of carbonylmetals would be very useful.Finally, it should be mentioned that the kinetic studies have also led t o preparative successes. Thus the study of the mechanism of substitution reactions of hexacarbonylmetals and the application of the knowledge gained led t o the preparation of the first hexamethylborazine-transition metal complex B3N3(CH3)6-Cr(C0)3 11051, and other complexes of this type are .... ~_ _ [lo51 R . P r i m and H . Werner, Angew. Chem. 79, 63 (1967); Angew. Chem. internat. Edit. 6, 91 (1967); R. Prinz, Dissertation, Technische Hochschule Munchen, 1967.now also known 1106J. I t is not difficult to predict that a combination of kinetic and preparative work in this field will continue t o be fruitful in the future. The work described here was carried out with the assistance in particular ofDr. R. Prinz, as well as of Dipl.-Chem. Edith Deckelmann, Dipl.-Chem. K. Deckelmann, Dipl.-Chem. B. Heckl, RenatE Koneberg, and Hermine Rascher, to all of whom I am very grateful. The work was supported by grants ,froni the Deutsche Forschungsgerneinschaft and the Verband der Chemischen Industrie. Thanks are also due to Prof. Dr. E. 0. Fischer for his support of o w investigati?ns.Thermal cleavage of carbamoyl chloride and carbamic esters (urethanes) usually gives only poor yields of isocyanates because the high cleavage temperatures (200-300 "C) favor side reactions and the decomposition products undergo reverse reaction at slightly lower temperatures. We have found that chlorides, anhydrides, and esters ( I ) of carbamic acids, when N-silylated to (2) [e.g., by chlorotrimethylsilane/triethylamine or mono-or bis-(trimethylsily1)acetamide], decompose to isocyanate ( 3 ) and a silicon compound:(4):even:between -20land + 120 "C. 0 n 0 II R'-N=C C X-Si(CH3)3 (31 (41 X = CI, OCOR3, OR'; R1, R3 = alkyl, aryl, or cycloalkyl Rz = C1, CHsCON-, CH3CONSi(CH& Since the products ( 4 ) and (3) undergo neither back-reaction nor, to an appreciable extent, side reactions, even sensitive or very reactive isocyanates can be obtained in yields of between 70and95 %. Forurethanes Rl-NH-COOR3 the temperature for fission depends on the nature of R3 if R1 remains the same; it decreases rapidly with increasing stability of the anion R3-Oe(examples: R1= C6Hs; R3 = C2H5, C6H5, p-CI-C6H4, or p-N02-C6H4; fission at 80 110°C. 50 to 70 "C, 10-30 OC, or -20-0 "C, respectively).Preparative work starts with an amine, which is first acylated by a chloroformic or carbonic ester to the urethane and then silylated; for fission the reaction mixture is warmed. Instead of chlorotrimethylsilane, other chlorosilanes such as chlorotrialkyloxysilanes, and even Sic14 can be used. This method can also be use...

One-Component Nanocomposites Based on Polymer-Grafted Cellulose Nanocrystals

“…The degrafting of polymethacrylate grafts from HNPs through selective urethane cleavage was performing based on the procedure reported by Greber and Kricheldorf. 59 In brief, a 5 mL 2necked round-bottomed Schlenk flask fitted with a reflux condenser and a stirring bar, the HNPs (150 mg) were dispersed under nitrogen atmosphere in anhydrous THF (4 mL) and vigorous stirring. Then, dry trimethylamine (2.2 equivalent of urethane present in the HNPs) was added and chlorotrimethylsilane (2.2 equivalent of urethane present in the HNPs) was dropwise added.…”

“…The degrafting of polymethacrylate grafts from HNPs through selective urethane cleavage was performed based on the procedure reported by Greber and Kricheldorf. 59 In brief, a 5 mL twoneck round-bottom Schlenk flask fitted with a reflux condenser and a stirring bar was used, and the HNPs (150 mg) were dispersed under a nitrogen atmosphere in anhydrous THF (4 mL) and vigorous stirring.…”

Solid‐Phase Synthesis of Macrocyclic Peptides via Side‐Chain Anchoring of the Ornithine δ‐Amine

“…Therefore attention was changed to methods that transform carbamates directly into isocyanates [21–25] . As lysine 4 contains two carbamates ( N ‐α‐Fmoc and N ‐ϵ‐Boc), any successful method to turn 4 into 6 should transform the latter carbamate into the isocyanate while leaving the former untouched.…”

“…Therefore attention was changed to methods that transform carbamates directly into isocyanates. [21][22][23][24][25] As lysine 4 contains two carbamates (N-α-Fmoc and N-ɛ-Boc), any successful method to turn 4 into 6 should transform the latter carbamate into the isocyanate while leaving the former untouched. Cho and co-workers described a method to convert an N-Boc carbamate to the corresponding isocyanate (Scheme 1).…”

A New and Convenient Method of Generating Alkyl Isocyanates from Alcohols­, Thiols and Trimethylsilyl Ethers Using Triphenylphosphine/2,3-Dichloro-5,6-dicyanobenzoquinone/Bu4NOCN

“…1 The high yields and lack of by-products associated with isocyanate chemistry have led to their widespread application in the pharmaceutical, agrochemical and polymer industries. Typically, aliphatic and aromatic isocyanates are generated from amines by reacting with phosgene 2 or phosgene equivalents, such as diphosgene (trichloromethyl chloroformate) 3 or triphosgene [bis(trichloromethyl) carbonate], 4 or via thermal dissociation of carbamic acid derivatives using chloroformates, 5 diphenyl carbonate 6 or N,N¢-carbonyldiimidazole. 7 Aryl isocyanates can also be generated from non-amine precursors via the rearrangements of acyl azides (Curtius rearrangement) 8 and hydroxamic acids (Lossen rearrangement).…”