Abstract:Dineopentylbis(triethylphosphine)platinum(II) reacts with H2 (34 psi) at 32 °C in hydrocarbon solvents and yields neopentane and tra«i-dihydridobis(triethylphosphine)platinum(II). The addition of triethylphosphine (L) changes the rate-limiting step of the reaction. When [L] = 0 M, the overall rate-limiting step is the dissociation of triethylphosphine from L2PtR2: no isotope effect is observed on substitution of D2 for H2, and the rate is independent of the pressure of H2. When [L] > 0.1 M, phosphine loss is r… Show more
“…It is tempting to argue that the equilibrium is driven by formation of stronger Pt−B bonds in 1 ; however, a stronger B−B bond in PinB−BPin relative to CatB−BCat would also be consistent with the experimental observations − 6 …”
Section: Resultsmentioning
confidence: 62%
“…The reactivity of square-planar, d 8 systems has received a great deal of attention in the chemical literature, and mechanistic studies have revealed that alkylbis(phosphine)M(II) systems (M = Pd, Pt) often react by dissociation of phosphine to give three-coordinate reactive intermediates. − Although boryl ligands are topologically distinct from alkyl or aryl counterparts, the exclusion of the mechanisms in Scheme suggested dissociative pathways involving three-coordinate intermediates as possible alternatives. To test these possibilities the reaction in eq 1 was monitored at various phosphine concentrations.…”
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].
“…It is tempting to argue that the equilibrium is driven by formation of stronger Pt−B bonds in 1 ; however, a stronger B−B bond in PinB−BPin relative to CatB−BCat would also be consistent with the experimental observations − 6 …”
Section: Resultsmentioning
confidence: 62%
“…The reactivity of square-planar, d 8 systems has received a great deal of attention in the chemical literature, and mechanistic studies have revealed that alkylbis(phosphine)M(II) systems (M = Pd, Pt) often react by dissociation of phosphine to give three-coordinate reactive intermediates. − Although boryl ligands are topologically distinct from alkyl or aryl counterparts, the exclusion of the mechanisms in Scheme suggested dissociative pathways involving three-coordinate intermediates as possible alternatives. To test these possibilities the reaction in eq 1 was monitored at various phosphine concentrations.…”
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].
“…This methodology has also allowed for the synthesis of cyclopropyl derivative 5 . Though few cyclopropyl-substituted transition-metal complexes have been reported, most of those that are known have been prepared by one of two general methods: Grignard (or Grignard-like) substitutions of metal halides − and C−H activation of cyclopropane by the transiently generated Cp‘(PMe 3 )M (Cp‘ = Cp, Cp*; M = Rh, Ir) fragment. ,− …”
Reactions between Cp*(PMe 3 )Ir(Me)OTf (1) and aldehydes (RCHO) proceed with high selectivity to give the hydrocarbyl carbonyl salts [Cp*(PMe 3 )Ir(R)(CO)][OTf] (R ) Me (2), Et (3), n-Pr (4), c-Pr (5), Ph (6), 1-ethylpropyl (7), p-tolyl (8), mesityl (9), 2-(Z)-1-phenylpropenyl (10), vinyl (13), t-Bu (14), 1-adamantyl ( 17)). This tandem C-H bond activation/decarbonylation reaction provides access to the first isolated tertiary alkyl complexes of iridium. X-ray diffraction studies were performed on mesityl complex 9, tert-butyl derivative 15, and 1-adamantyl compound 18. Decarbonylation of cyclopropyl complex 5 results in irreversible opening of the cyclopropyl ring. This reaction has provided useful information concerning the mechanism of C-H activation of cyclopropane by 1. Hydride reduction of the p-tolyl carbonyl salt 8 has provided an example of a rare transition-metal formyl complex, Cp*-(PMe 3 )Ir(p-tolyl)(CHO) (28).
“…C, H analyses were performed by Galbraith Laboratories. The compounds cis -dimethyl(bis(triisopropylphosphine))platinum(II) and trans -dichlorobis(triisopropylphosphine)platinum(II) (compound 6 ) were prepared following literature methods.…”
The compound
trans-(methyl)(trifluoromethanesulfonato)(bis(triisopropylphosphine))platinum(II) (1) is stable in solution at room
temperature, but when treated with a catalytic
amount of HCl, it undergoes rapid loss of methane and forms the
metalated compound
(triisopropylphosphine)(trifluoromethanesulfonato)(2,2-diisopropyl-3-methyl-1-platina-2-phosphacyclobutane) (2). The key intermediate is
trans-chloro(trifluoromethanesulfonato)(bis(triisopropylphosphine))platinum(II) (3), which
rapidly undergoes metalation with loss of
trifluoromethanesulfonic acid.
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