%0 Journal Article %1 hlwoodcock:Q.1998e %A Cui, Q. %A Musaev, D. G. %A Morokuma, K. %D 1998 %J Organometallics %K atoms elimination complexes basis core functions gaussian first-row atomic addition oxidative reductive sets effective hydroboration bonds bibtex-import potentials molecular calculations %N 7 %P 1383--1392 %T Density functional study on the mechanism of palladium(0)- catalyzed thioboration reaction of alkynes. differences between pd(0) and pt(0) catalysts and between thioboration and diboration %V 17 %X The hybrid density functional (B3LYP) calculations have been carried out to study the mechanism of acetylene thioboration reactions catalyzed by Pd(O)/Pt(O) complexes, and the results are compared to those for the diboration reactions. The main findings can be summarized as follows. (i) No oxidative- addition product of the S-B bond to the Pd(PH3)(2) was found, therefore, the oxidative-addition mechanism proposed by Suzuki et al. is not the proper one for the thioboration reaction of alkynes. (ii) We propose a new mechanism involving the following steps: (a) acetylene coordination to Pd(PH3)(2), (b) dissociation of a phosphine ligand, (c) addition of the S-B bond to the metal center via a metathesis-like transition state, (d) isomerization of the resultant complex, accompanied by recoordination of the phosphine ligand, and (e) the reductive elimination of the alkenyl-thioboron product, R'S- CH=CH-B(OR)(2). The rate-determining stage is found to be the addition of the S-B bond to the metal center via a metathesis- like transition state. (iii) The Pd(O) complexes catalyze alkyne thioboration but not diboration, because the rate- determining barrier at the metathesis-like transition state is much higher for the latter, which has been explained in terms of the lack of hypervalency character of boron compared to sulfur. Due to the weaker C-S bond energy compared to C-B, the reductive-elimination step is highly exothermic for diboration but only slightly exothermic for thioboration. Consequently, the reductive-elimination step proceeds with a high barrier for thioboration but is barrierless for diboration. (iv) The Pt(O) complex is not expected to be a good catalyst for thioboration but is efficient for diboration. The reason behind this is the high barrier for reductive elimination, which in part comes from the promotion energy required for the metal during the process.

@article{hlwoodcock:Q.1998e, abstract = {The hybrid density functional (B3LYP) calculations have been carried out to study the mechanism of acetylene thioboration reactions catalyzed by Pd(O)/Pt(O) complexes, and the results are compared to those for the diboration reactions. The main findings can be summarized as follows. (i) No oxidative- addition product of the S-B bond to the Pd(PH3)(2) was found, therefore, the oxidative-addition mechanism proposed by Suzuki et al. is not the proper one for the thioboration reaction of alkynes. (ii) We propose a new mechanism involving the following steps: (a) acetylene coordination to Pd(PH3)(2), (b) dissociation of a phosphine ligand, (c) addition of the S-B bond to the metal center via a metathesis-like transition state, (d) isomerization of the resultant complex, accompanied by recoordination of the phosphine ligand, and (e) the reductive elimination of the alkenyl-thioboron product, R'S- CH=CH-B(OR)(2). The rate-determining stage is found to be the addition of the S-B bond to the metal center via a metathesis- like transition state. (iii) The Pd(O) complexes catalyze alkyne thioboration but not diboration, because the rate- determining barrier at the metathesis-like transition state is much higher for the latter, which has been explained in terms of the lack of hypervalency character of boron compared to sulfur. Due to the weaker C-S bond energy compared to C-B, the reductive-elimination step is highly exothermic for diboration but only slightly exothermic for thioboration. Consequently, the reductive-elimination step proceeds with a high barrier for thioboration but is barrierless for diboration. (iv) The Pt(O) complex is not expected to be a good catalyst for thioboration but is efficient for diboration. The reason behind this is the high barrier for reductive elimination, which in part comes from the promotion energy required for the metal during the process.}, added-at = {2006-06-16T05:03:46.000+0200}, author = {Cui, Q. and Musaev, D. G. and Morokuma, K.}, biburl = {https://www.bibsonomy.org/bibtex/2d8972f9f37371d9d309dbfd8e99b7860/hlwoodcock}, citeulike-article-id = {569407}, comment = {ZG590 ORGANOMETALLICS}, interhash = {1f8d4b7bf55d4d9bd9a645f7f04ce401}, intrahash = {d8972f9f37371d9d309dbfd8e99b7860}, journal = {Organometallics}, keywords = {atoms elimination complexes basis core functions gaussian first-row atomic addition oxidative reductive sets effective hydroboration bonds bibtex-import potentials molecular calculations}, number = 7, pages = {1383--1392}, priority = {2}, timestamp = {2006-06-16T05:03:46.000+0200}, title = {Density functional study on the mechanism of palladium(0)- catalyzed thioboration reaction of alkynes. differences between pd(0) and pt(0) catalysts and between thioboration and diboration}, volume = 17, year = 1998 }