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...
