Mechanistic Investigation of Stoichiometric Alkyne Insertion into Pt−B Bonds and Related Chemistry Bearing on the Catalytic Diborylation of Alkynes Mediated by Platinum(II) Diboryl Complexes

Abstract

The insertion reactivity of alkynes with the diboryl complex (Ph 3 P) 2 Pt(BCat) 2 (1, Cat ≡ {C 6 H 4 O 2 } 2-) has been investigated. Under stoichiometric conditions 1 mediates cisdiborylation of alkynes and the (PPh 3 ) 2 Pt fragment is trapped by alkyne to give the corresponding Pt-alkyne complexes. Kinetic studies under pseudo first-order conditions of alkyne indicate that the reaction is first order in 1. In the absence of added phosphine, no alkyne dependence is observed. The stoichiometric reaction is inhibited by phosphine addition, and under these conditions, a first-order dependence on alkyne concentration is observed for the disappearance of 1. The stoichiometric results exclude simple, bimolecular insertion of an alkyne into Pt-B bonds of 1, and the observed dependence on phosphine and alkyne strongly favors a mechanism where phosphine dissociation generates a threecoordinate intermediate that mediates alkyne insertion. Activation parameters for the stoichiometric alkyne insertion were derived from the temperature dependence of k obs (70-110 °C). An Eyring plot yielded the following: ∆H q ) 25.9(7) kcal/mol and ∆S q ) 4(2) eu. The rates of alkyne diborylation are also sensitive to the nature of the alkyne as 4-octyne reacts much more readily than diphenylacetylene. For para-substituted diarylacetylenes, the rate for the bis(p-trifluoromethyl) derivative is accelerated and the rate for the bis(pmethoxy) derivative is retarded relative to diphenylacetylene. The reactivity of the related diboryl complex, (PPh 3 ) 2 Pt(BPin) 2 (9, Pin ≡ {(CH 3 ) 2 CO-CO(CH 3 ) 2 } 2-), is much more complex as reductive elimination of PinB-BPin is observed before the onset of the diborylation reaction. This appears to be a general feature for this compound as elimination is promoted by various reagents (e.g., CO, PPh 3 , Me 3 Sn-SnMe 3 , and CatB-BCat). The catalytic diborylation of alkynes mediated by 1 (in the presence of added triphenylphosphine) was investigated. Kinetics experiments revealed many similarities to the stoichiometric reaction as an inverse dependence on [PPh 3 ] and first-order dependence on [alkyne] and [1] were observed. Expressions that directly relate the catalytic and stoichiometric observed rate constants were derived, and the measured values for these two systems were identical within experimental error. Thus, the data are consistent with a catalytic manifold that is identical to that observed in the stoichiometric reaction. Under catalytic conditions, the rate of alkyne diborylation exhibited no dependence on [CatB-BCat].

Transition Metal−Boryl Compounds: Synthesis, Reactivity, and Structure

“…Kinetic investigations revealed that the process was firstorder overall in concentration of the platinumbisboryl species. 29 A number of alkynes 25,26,29,30 and diynes 30 43 In addition, a detailed examination of the alkyne diboration reaction was also reported 42 which showed that insertion of acetylene into the Pt-B(OH) 2 bond was exothermic by 28.1 kcal mol -1 with the overall reaction being exothermic by 60.8 kcal mol -1 . With the rate-determining step in the catalysis calculated to be phosphine dissociation from [Pt(PH 3 42 it is not surprising that the insertion proceeds readily when a monophosphine catalyst was prepared using [Pt(cod) 2 ].…”

“…[25][26][27][28][29][30][31][32][33][34][35][36][37] In addition, related theoretical studies regarding M-B bond energies of metal-boryl compounds and the mapping of reaction coordinates for the proposed catalytic cycles have also been reported. [38][39][40][41][42][43][44] Initially, Nöth and Schmid examined four general synthetic routes for the preparation of metal-boryl compounds, namely: (i) the nucleophilic reaction of anionic metal species with haloboranes (X-BR 2 , X ) Cl, Br, I), (ii) the reaction of metal hydride compounds with haloboranes, (iii) σ-bond metathesis reactions with cobalt bisboryl compounds, and (iv) oxidative addition of haloboranes by metal compounds.…”

Structure and Bonding in Hartwig Stretched η3-Hydridoborate σ-Complex of Niobium(III)

“…1-18 These complexes have been invoked as intermediates in a number of catalytic systems including metal-catalyzed hydroboration of unsaturated substrates and diboration of alkynes, alkenes, dienes, and allenes. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Although theoretical studies regarding metal-boryl bond energies and the mapping of the reaction coordinate for the proposed catalytic cycles have been reported, 35-42 accurate theoretical investigations which address the nature of the chemical bond (1) Wadepohl, H. Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman, N. C.; Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scitt, A. J.; Souze, F. E. S.; Stringer, G.; Whittell, G. R. Johann, T.; Marder, T. B.; Norman, N. C.; Orpen, A. G.; Peskman, T.; Quayle, M. J.; Rice, C. R.; Scott, A. J. …”

Platinum(0)-catalyzed diboration of allenes with bis(pinacolato)diboron

“…The slow reaction of 2 having strong electron-donating groups (Entries 7 and 8) may reflect the rate of insertion step, which was consistent with the relative reactivity observed on the insertion of diarylacetylenes to the B-Pt bond. 10 The selective formation of 3a in the reaction of 1,1-disubstituted 2 may be due to the steric requirement; however, the observed regio-and stereoselectivity for monosubstituted and heteroatom-substituted 2 remains controversial. The ready availability of various 3 from 2 now offers a simple route to substituted homoallyl alcohols.…”