Organozirconium complexes as catalysts for markovnikov-selective lntermolecular hydrothiolation of terminal alkynes: Scope and mechanism

Charles J. Weiss, Tobin J. Marks

Research output: Contribution to journalArticle

83 Citations (Scopus)

Abstract

The efficient and selective organozirconium(IV)-mediated, intermolecular hydrothiolation of terminal alkynes by aliphatic, benzylic, and aromatic thiols using CGCZrMe2 (CGC = Me2SiCp" NCMe3, Cp" = C5Me4), Cp*ZrBn 3 (Cp* = C5Me5, Bn = benzyl), Cp*ZrCI 2NMe 2, Cp* 2ZrMe 2, and Zr(NMe 2) 4 precatalysts is reported, These transformations are shown to be highly Markovnikov-selective, with selectivities up to 99%, and typically in greater than 90% yields. The reaction has been demonstrated on the preparative scale with 72% isolated yield and 99% Markovnikov selectivity. A mixture of anti-Markovnikov products is occasionally observed as a result of a known, non-organometallic, radical mechanism, which can be suppressed by addition of a radical inhibitor. Kinetic investigations show that the CGCZrMe2-mediated reaction between 1-pentanethiol and 1-hexyne is first-order in catalyst concentration, first-order in alkyne concentration, and also first-order in thiol at lower concentrations but transitions to zero-order at concentrations > 0.3 M, Deuterium labeling of the alkyne yields K H/K D = 1.3(0.1), along with evidence of thiol-mediated protonolytic detachment of product from the Zr center. Activation parameters for CGCZrMe 2mediated 1-pentanethiol hydrothiolation of 1-hexyne measured over the temperature range of 50-80 0C are AhP = +18.1(1.2) kcal/mol and δS‡ = -20.9(2.5) e.u. for [alkyne] and [thiol] at 0.2 M. These and other findings are consistent with turnover-limiting alkyne insertion into the Zr-SR bond, followed by a thiol-induced Zr-C protonolysis. Observed zirconium-thiolate dinners in the reaction medium suggest instances of dimeric catalyst resting states and possible aggregated, hydrothiolation-active species.

Original languageEnglish
Pages (from-to)10533-10546
Number of pages14
JournalJournal of the American Chemical Society
Volume132
Issue number30
DOIs
Publication statusPublished - Aug 4 2010

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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