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A new route for the synthesis of thiocarbamates using a transition metal complex and CO as a replacement for phosgene has been discovered. The complex (PPh 3 ) 2 PdCl 2 reacts with N-benzylmethylamine, 4-chlorobenzenethiol, and CO to selectively generate a thiocarbamate. Kinetic studies and the isolation of several intermediates in this reaction allow for the proposal of a mechanistic scheme.

The reaction of Li2[C4H4B-N(CHMe2)2]·THF with 2 equiv of AlCl3 and 1 equiv of TaCl5 gives mononuclear [C4H4B-N(CHMe2)2]TaCl3 (1) in 47% yield. Alkylation of 1 with 3 equiv of MeMgCl gives methylated species in conjunction with the triple decker complex [C4H4B-N(CHMe2)2]Me2Ta[μ-C4H4B-N(CHMe2)2]TaMe4 (2). Monoalkylation is possible with LiCH(SiMe3)2 to give [C4H4B-N(CHMe2)2]TaCl2[CH(SiMe3)2] (3) which contains a Ta−Cα−H agostic interaction. Addition of H2NAr (Ar = 2,6- i Pr2-C6H3) and triethylamine to 1 affords [C4H4B-NH(CHMe2)2]Ta(NAr)Cl2 (4). When 2 equiv of acetone are added to 1, the result is [C4H4B-NH(CHMe2)2]TaCl3[Me2C(O)CH2C(O)Me] (5). Reaction with LiCp* (Cp* = C5Me5) gives Cp*[C4H4B-N(CHMe2)2]TaCl2 (6). Reduction of 6 with Mg under an atmosphere of CO produces Cp*[C4H4B-N(CHMe2)2]Ta(CO)2 (7) which can be protonated with [H(OEt2)2][B(C6H3(CF3)2)4] to form {Cp*[C4H4B-NH(CHMe2)2]Ta(CO)2}{[B(C6H3(CF3)2)4]} (8). Reaction of 1 with excess LiCp‘ (Cp‘ = C5H4Me) affords Cp‘2[η2-C4H4B-N(CHMe2)2]TaCl (10) in which the borole ligand is η2-bound. Addition of Li[C5H5B-R] to 1 results in the formation of [C4H4B-N(CHMe2)2][C5H5B-R]TaCl2 (11, R = Ph; 12, R = NMe2). Methylation of 11 affords [C4H4B-N(CHMe2)2][C5H5B-Ph]TaMe2 (14), which reacts with H2 in the presence of PMe3 to give [C4H4B-N(CHMe2)2][C5H5B-Ph]Ta(PMe3)2 (16). For PEt3, the product is [C4H4B-N(CHMe2)2][C5H5B-Ph]Ta(H)2PEt3 (17). Reduction of 1 in the presence of PMe3 under nitrogen gives {[C4H4B-N(CHMe2)2](Me3P)2ClTaN}2 (18). Under an argon atmosphere the reduced product is [C4H4B-N(CHMe2)2]Ta(PMe3)3Cl (19). Complex 19 reacts with hydrogen to give the asymmetric dinuclear complex [([C4H4B-N(CHMe2)2]Ta(H)(PMe3)Cl)μ-H([C4H4B-N(CHMe2)2]Ta(PMe3)2Cl) (20). The crystallographic characterization of complexes 1, 3, 4, 5, 7, 10, 11, 12, 16, 17, 18, 19, and 20 is also presented. These data give important insight into the metal−borollide relationship under a variety of ligand environments and different oxidation states. They also allow for an estimation of the contribution from the possible resonance forms.

The hydrogenation of Cp*[C4H4B-N(i-Pr)2]TaMe2 (1) (Cp* = C5Me5) in the presence of PMe3 affords Cp*[C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (2) in essentially quantitative yield. Similarly, the hydrogenation of Cp*[C4H4B-Me]TaMe2 (3) in the presence of PMe3 affords Cp*[C4H4B-Me]Ta(H)2(PMe3) (4). Hydrogenation of 1 and 3 is accompanied by the reversible formation of side products. The most important of these complexes, Cp*[C4H4B-N(i-Pr)2]Ta(PMe3)2 (5) and Cp*[C4H4B-Me]Ta(PMe3)2 (6), react slowly with dihydrogen forming 2 and 4, respectively. In the early stages of the hydrogenation of 1, the C−H activation product Cp*[C4H4B-N(i-Pr)2]Ta(H)(CH2PMe2) (7) is also present. Mechanistic details of the hydrogenation of 1 and 3 are discussed. Hydrogenation of [C5H5B-Ph][C4H4B-N(i-Pr)2]TaMe2 (8) in the presence of PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PMe3)2 (9) as the exclusive product. The use of a bulkier phosphine, P(i-Pr)3, gives [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2[P(i-Pr)3] (10). Changing the phosphine to one of intermediate bulk, PEt3, leads to the formation of trans-[C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PEt3) (11t). The cis isomer (11c) is observable during early reaction times. 11c is a classical dihydride, perturbed by an unsymmetric three-center/two-electron interaction with the boron of the boratabenzene ligand. Isomerization of 11c to 11t proceeds via phosphine loss followed by kinetically detectable rearrangement of the unsaturated intermediate prior to phosphine recoordination. Treatment of 11c with excess PMe3 results in the formation of 9 via a mixed-phosphine intermediate, [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(PEt3)(PMe3) (12). The addition of [H(OEt2)2][B(C6H3(CF3)2] to 11c results in the protonation of the nitrogen atom of the borollide ligand (H-11c + ). H-11c + is stable at room temperature for over a week. Treatment of 10 with excess PMe3 affords [C5H5B-Ph][C4H4B-N(i-Pr)2]Ta(H)2(PMe3) (13). Upon thermolysis in the presence of a large excess of PMe3, 13 is converted to 9. A mechanistic scheme for the hydrogenation of complexes such as 1 is proposed.