Kinetische und mechanistische untersuchungen von übergangs-metallkomplex-reaktionen : X. Kinetische untersuchungen der insertion des schwefels von organylisothiocyanaten in die metall—carbenkolenstoff-bindung von (arylphenylcarben)pentacarbonylwolfram

Abstract

(Arylphenylcarbene)pentacarbonyltungsten complexes, (CO) 5W[C( p-C 6H 4R)-C 6H 5] (Ia, R  OCH 3; Ib, R  CH 3; Ic. R  H; Id, R  Br; Ie, R  CF 3), react with organylisothiocyanates, R′NCS (IIa, R′  CH 3; IIb, R′  C 2H 5; IIc, R′  C 6H 5), via insertion of the sulfur atom from R′ NCS into the metal—carbene bond to yield (arylphenylthioketone)pentacarbonyltungsten complexes, (CO) 5W[SC( p-C 6H 4R)C 6H 5] (III). The decrease of the starting compound I in dibutyl ether follows a second-order rate law: d[I]/d t  k [I] [II], but in octane, an additive law. d[I]/d t  k1 [I] [II] + k 2[Il[II] 2. Replacement of R  H in I by electron-withdrawing substituents (Br, CF 3) results in an increase, by electron-donating groups (CH 3, OCH 3) in a decrease of the reaction rate. The rate constant k increases in the series R′  C 6H 5, R′  CH 3, R′  C 2H 5. For the reaction of Ic with IIb in dibutyl ether the activation enthalpy is ΔH≠  40 kj mol -1, the activation entropy is ΔS≠  147 J mol -1 K -1. The results are discussed on the basis of an associative stepwise mechanism with a nucleophilic attack of the sulfur from R′NCS at the carbene carbon in the first reaction step.