Kinetics and Mechanism of the Gold-Catalyzed Hydroamination of 1,1-Dimethylallene with N-Methylaniline.

The mechanism of the intermolecular hydroamination of 3-methylbuta-1,2-diene (1) with N-methylaniline (2) catalyzed by (IPr)AuOTf has been studied by employing a combination of kinetic analysis, deuterium labelling studies, and in situ spectral analysis of catalytically active mixtures. The results of these and additional experiments are consistent with a mechanism for hydroamination involving reversible, endergonic displacement of N-methylaniline from [(IPr)Au(NHMePh)]+ (4) by allene to form the cationic gold π-C1,C2-allene complex [(IPr)Au(η2 -H2 C=C=CMe2 )]+ (I), which is in rapid, endergonic equilibrium with the regioisomeric π-C2,C3-allene complex [(IPr)Au(η2 -Me2 C=C=CH2 )]+ (I'). Rapid and reversible outer-sphere addition of 2 to the terminal allene carbon atom of I' to form gold vinyl complex (IPr)Au[C(=CH2 )CMe2 NMePh] (II) is superimposed on the slower addition of 2 to the terminal allene carbon atom of I to form gold vinyl complex (IPr)Au[C(=CMe2 )CH2 NMePh] (III). Selective protodeauration of III releases N-methyl-N-(3-methylbut-2-en-1-yl)aniline (3 a) with regeneration of 4. At high conversion, gold vinyl complex II is competitively trapped by an (IPr)Au+ fragment to form the cationic bis(gold) vinyl complex {[(IPr)Au]2 [C(=CH2 )CMe2 NMePh]}+ (6).

Mechanochemical Regulation of Oxidative Addition to a Palladium(0) Bisphosphine Complex.

Here, we report the effect of force applied to the biaryl backbone of a bisphosphine ligand on the rate of oxidative addition of bromobenzene to a ligand-coordinated palladium center. Local compressive and tensile forces on the order of 100 pN were generated using a stiff stilbene force probe. A compressive force increases the rate of oxidative addition, whereas a tensile force decreases the rate, relative to that of the parent complex of strain-free ligand. Rates vary by a factor of ∼6 across ∼340 pN of force applied to the complexes. The crystal structures and DFT calculations support that force-induced perturbation of the geometry of the reactant is negligible. The force-rate relationship observed is mainly attributed to the coupling of force to nuclear motion comprising the reaction coordinate. These observations inform the development of catalysts whose activity can be tuned by an external force that is adjusted within a catalytic cycle.

Gold Sulfonium Benzylide Complexes Undergo Efficient Benzylidene Transfer to Alkenes.

The gold sulfonium benzylide complexes [(P1)AuCHPh(SR1 R2 )]+ {B[3,5-CF3 C6 H3 ]4 }- [P1=P(tBu)2 o-biphenyl; R1 , R2 =-(CH2 )4 - (1 a); R1 =Et, R2 =Ph (1 b); R1 =R2 =Ph (1 c)] were synthesized by reaction of the gold α-chloro benzyl complex (P1)AuCHClPh with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and excess sulfide. Complexes 1 undergo efficient benzylidene transfer to alkenes and DMSO under mild conditions without external activation. Kinetic analysis of the reaction of 1 c with styrene was consistent with the intermediacy of the cationic gold benzylidene complex [(P1)AuCHPh]+ (I).

Synthesis, Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes.

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η1 -C≡CC(OMe)ArAr'] (L=IPr, P(t Bu)2 (ortho-biphenyl); Ar/Ar'=C6 H4 X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at -78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr']+ OTf- in ≥85±5 % yield according to 1 H NMR analysis. 13 C NMR and IR spectroscopic analysis of these complexes established the arene-dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh2 ]+ OTf- reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.

Experimental Evaluation of (L)Au Electron-Donor Ability in Cationic Gold Carbene Complexes.

29 Si NMR spectroscopy was employed to evaluate the electron donor properties of the (L)Au fragments in the cationic gold (ß,ß-disilyl)vinylidene complexes [(L)Au=C=CSi(Me)2 CH2 CH2 Si(Me)2 ]+ B(C6 F5 )4- [L=P(tBu)2 o-biphenyl or NHC] relative to the p-substituted aryl group in the α-aryl-(ß,ß-disilyl)vinyl cations [(p-C6 H4 X)-C= CSi(Me)2 CH2 CH2 Si(Me)2 ]+ B(C6 F5 )4- . Similarly, 19 F NMR was employed to evaluate the σ- and π-electron donor properties of the (L)Au fragments in the neutral gold fluorophenyl complexes (L)Au(C6 H4 F) and in the cationic (fluorophenyl)methoxycarbene complexes [(L)AuC(OMe)(C6 H4 F)]+ SbF6- [L=P(tBu)2 o-biphenyl or IPr] relative to the p-substituted aryl group of the protonated monofluorobenzophenones [(p-C6 H4 X)(C6 H4 F)COH]+ OTf- . The results of these studies indicate that relative to p-substituted aryl groups, the gold (L)Au fragments [L=P(tBu)2 o-biphenyl or NHC] are significantly more inductively electron donating and are comparable π-donors and for this reason, the extent of (L)Au→C1 electron donation in gold carbene complexes appears to exceed that provided by a p-(dimethyamino)phenyl group. Furthermore, the [L=P(tBu)2 o-biphenyl]Au fragment is a nominally stronger electron donor than the (IPr)Au fragment, and both are significantly more inductively electron donating than the (PPh3 )Au and [P(OMe)3 ]Au fragments.

Formal Synthesis of (+)-Laurencin by Gold(I)-Catalyzed Intramolecular Dehydrative Alkoxylation.

8-Membered cyclic ethers are found in a wide range of natural products; however, they are challenging synthetic targets due to enthalpic and entropic barriers. The gold(I)-catalyzed intramolecular dehydrative alkoxylation of ω-hydroxy allylic alcohols was explored to stereoselectively construct α,α'-cis-oxocenes and further applied in a formal synthesis of (+)-laurencin. The gold(I)-catalyzed intramolecular dehydrative alkoxylation may constitute an alternative method for the synthesis of molecular building blocks and natural products that contain highly functionalized 8-membered cyclic ethers.

Tandem gold/silver-catalyzed cycloaddition/hydroarylation of 7-aryl-1,6-enynes to form 6,6-diarylbicyclo[3.2.0]heptanes.

Mixtures of [{PCy2(o-biphenyl)}AuCl] and AgSbF6 catalyze the tandem cycloaddition/hydroarylation of 7-aryl-1,6-enynes with electron-rich arenes to form 6,6-diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involving gold-catalyzed cycloaddition followed by silver-catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate.

Synthesis and Characterization of a Gold Vinylidene Complex Lacking π-Conjugated Heteroatoms.

Hydride abstraction from the gold (disilyl)ethylacetylide complex [(P)Au{η(1) -C≡CSi(Me)2CH2CH2SiMe2H}] (P=P(tBu)2o-biphenyl) with triphenylcarbenium tetrakis(pentafluorophenyl)borate at -20 °C formed the cationic gold (ß,ß-disilyl)vinylidene complex [(P)Au=C=CSi(Me)2CH2CH2Si(Me)2](+)B(C6F5)4(-) with ≥90% selectivity. (29)Si NMR analysis of this complex pointed to delocalization of positive charge onto both the ß-silyl groups and the (P)Au fragment. The C1 and C2 carbon atoms of the vinylidene complex underwent facile interconversion (ΔG(≠)=9.7 kcal mol(-1)), presumably via the gold π-disilacyclohexyne intermediate [(P)Au{η(2)-C≡CSi(Me)2CH2CH2Si(Me)2}](+)B(C6F5)4(-).

Gold-catalyzed intermolecular anti-markovnikov hydroamination of alkylidenecyclopropanes.

The cationic gold phosphine complex [{PCy2 (o-biphenyl)}Au(NCMe)](+) SbF6 (-) (Cy=cyclohexyl) catalyzes the intermolecular, anti-Markovnikov hydroamination reaction of monosubstituted and cis- and trans-disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin-2-ones and other nucleophiles. This reaction forms 1-cyclopropyl alkylamine derivatives in high yield and with high regio- and diastereoselectivity. NMR spectroscopic analysis of gold π-ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction.

Kinetics and mechanism of the racemization of aryl allenes catalyzed by cationic gold(I) phosphine complexes.

The kinetics of the racemization of aromatic 1,3-disubstituted allenes catalyzed by gold phosphine complexes has been investigated. The rate of gold-catalyzed allene racemization displayed first-order dependence on allene, and catalyst concentration and kinetic analysis of gold-catalyzed allene racemization as a function of allene and phosphine electron-donor ability established the accumulation of electron density on the phosphine atom and the depletion of electron density on the terminal allenyl carbon atoms in the rate-limiting transition state for racemization. These and other observations were in accord with a mechanism for allene racemization involving rapid and reversible inter- and intramolecular allene exchange followed by turnover-limiting, unimolecular conversion of a chiral gold η(2)-allene complex to an achiral η(1)-allylic cation intermediate through a bent and twisted η(1)-allene transition state. With respect to proper ligand selection, these studies reveal that both electron-poor phosphine ligands and polar solvents facilitate racemization.

Synthesis, structure, and reactivity of a gold carbenoid complex that lacks heteroatom stabilization.

Hydride abstraction from the neutral gold cycloheptatrienyl complex [(P)Au(η(1)-C7H7)] (P=P(tBu)2(o-biphenyl)) with triphenylcarbenium tetrafluoroborate at -80 °C led to the isolation of the cationic gold cycloheptatrienylidene complex [(P)Au(η(1)-C7H6)](+) BF4(-) in 52% yield, which was characterized in solution and by single-crystal X-ray diffraction. This cycloheptatrienylidene complex represents the first example of a gold carbenoid complex that lacks conjugated heteroatom stabilization of the electron-deficient C1 carbon atom. The cycloheptatrienylidene ligand of this complex is reactive; it can be reduced by mild hydride donors, and converted to tropone in the presence of pyridine N-oxide.

Photomechanical actuation of ligand geometry in enantioselective catalysis.

A catalyst that couples a photoswitch to the biaryl backbone of a chiral bis(phosphine) ligand, thus allowing photochemical manipulation of ligand geometry without perturbing the electronic structure is reported. The changes in catalyst activity and selectivity upon switching can be attributed to intramolecular mechanical forces, thus laying the foundation for a new class of catalysts whose selectivity can be varied smoothly and in situ over a useful range by controlling molecular stress experienced by the catalyst during turnover. Forces on the order of 100 pN are generated, thus leading to measurable changes in the enantioselectivities of asymmetric Heck arylations and Trost allylic alkylations. The differential coupling between applied force and competing stereochemical pathways is quantified and found to be more efficient for the Heck arylations.

Cationic Bis(Gold) Indenyl Complexes.

Reaction of (P)AuOTf [P=P(t-Bu)2o-biphenyl] with indenyl- or 3-methylindenyl lithium led to isolation of gold η1-indenyl complexes (P)Au(η1-inden-1-yl) (1 a) and (P)Au(η1-3-methylinden-1-yl) (1 b), respectively, in >65 % yield. Whereas complex 1 b is static, complex 1 a undergoes facile, degenerate 1,3-migration of gold about the indenyl ligand (ΔG≠ 153K=9.1±1.1 kcal/mol). Treatment of complexes 1 a and 1 b with (P)AuNTf2 led to formation of the corresponding cationic bis(gold) indenyl complexes trans-[(P)Au]2(η1,η1-inden-1,3-yl) (2 a) and trans-[(P)Au]2(η1,η2-3-methylinden-1-yl) (2 b), respectively, which were characterized spectroscopically and modeled computationally. Despite the absence of aurophilic stabilization in complexes 2 a and 2 b, the binding affinity of mono(gold) complex 1 a toward exogenous (P)Au+ exceed that of free indene by ~350-fold and similarly the binding affinity of 1 b toward exogenous (P)Au+ exceed that of 3-methylindene by ~50-fold. The energy barrier for protodeauration of bis(gold) indenyl complex 2 a with HOAc was ≥8 kcal/mol higher than for protodeauration of mono(gold) complex 1 a.

The regio- and stereospecific intermolecular dehydrative alkoxylation of allylic alcohols catalyzed by a gold(I) N-heterocyclic carbene complex.

A 1:1 mixture of [AuCl(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) and AgClO(4) catalyzes the intermolecular dehydrative alkoxylation of primary and secondary allylic alcohols with aliphatic primary and secondary alcohols to form allylic ethers. These transformations are regio- and stereospecific with preferential addition of the alcohol nucleophile at the γ-position of the allylic alcohol syn to the departing hydroxyl group and with predominant formation of the E stereoisomer. The minor α regioisomer is formed predominantly through a secondary reaction manifold involving regioselective γ-alkoxylation of the initially formed allylic ether rather than by the direct α-alkoxylation of the allylic alcohol.

Synthesis and study of cationic, two-coordinate triphenylphosphine-gold-π complexes.

Cationic, two-coordinate triphenylphosphine-gold(I)-π complexes of the form [(PPh3)Au(π ligand)]⁺SbF6⁻ (π ligand=4-methylstyrene, 1∙SbF6), 2-methyl-2-butene (3∙SbF6), 3-hexyne (6∙SbF6), 1,3-cyclohexadiene (7∙SbF6), 3-methyl-1,2-butadiene (8∙SbF6), and 1,7-diphenyl-3,4-heptadiene (10∙SbF6) were generated in situ from reaction of [(PPh3)AuCl], AgSbF6, and π ligand at -78 °C and were characterized by low-temperature, multinuclear NMR spectroscopy without isolation. The π ligands of these complexes were both weakly bound and kinetically labile and underwent facile intermolecular exchange with free ligand (ΔG(≠) ≈9 kcal mol(-1) in the case of 6∙SbF6) and competitive displacement by weak σ donors, such as trifluoromethane sulfonate. Triphenylphosphine-gold(I)-π complexes were thermally unstable and decomposed above -20 °C to form the bis(triphenylphosphine) gold cation [(PPh3)2Au]⁺SbF6⁻ (2∙SbF6).

Cationic, two-coordinate gold π complexes.

Cationic, two-coordinate gold π complexes that contain a phosphine or N-heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold-catalyzed functionalization of C-C multiple bonds. Although neutral two-coordinate gold π complexes have been known for over 40 years, examples of the cationic two-coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well-defined examples of two-coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.

Allosteric control of olefin isomerization kinetics via remote metal binding and its mechanochemical analysis.

Allosteric control of reaction thermodynamics is well understood, but the mechanisms by which changes in local geometries of receptor sites lower activation reaction barriers in electronically uncoupled, remote reaction moieties remain relatively unexplored. Here we report a molecular scaffold in which the rate of thermal E-to-Z isomerization of an alkene increases by a factor of as much as 104 in response to fast binding of a metal ion to a remote receptor site. A mechanochemical model of the olefin coupled to a compressive harmonic spring reproduces the observed acceleration quantitatively, adding the studied isomerization to the very few reactions demonstrated to be sensitive to extrinsic compressive force. The work validates experimentally the generalization of mechanochemical kinetics to compressive loads and demonstrates that the formalism of force-coupled reactivity offers a productive framework for the quantitative analysis of the molecular basis of allosteric control of reaction kinetics. Important differences in the effects of compressive vs. tensile force on the kinetic stabilities of molecules are discussed.

Mechanistic analysis of gold(I)-catalyzed intramolecular allene hydroalkoxylation reveals an off-cycle bis(gold) vinyl species and reversible C-O bond formation.

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.

Structures and dynamic solution behavior of cationic, two-coordinate gold(I)-π-allene complexes.

A family of seven cationic gold complexes that contain both an alkyl substituted π-allene ligand and an electron-rich, sterically hindered supporting ligand was isolated in >90% yield and characterized by spectroscopy and, in three cases, by X-ray crystallography. Solution-phase and solid-state analysis of these complexes established preferential binding of gold to the less substituted C=C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C=C bond. Kinetic analysis of intermolecular allene exchange established two-term rate laws of the form rate=k(1)[complex]+k(2)[complex][allene] consistent with allene-independent and allene-dependent exchange pathways with energy barriers of ΔG(≠)(1)=17.4-18.8 and ΔG(≠)(2)=15.2-17.6 kcal mol(-1), respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG(≠)=8.9-11.4 kcal mol(-1)) intramolecular exchange of the allene π faces through η(1)-allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)(2)o-binaphthyl}Au(η(2)-4,5-nonadiene)](+)SbF(6)(-) (5), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG(≠)(298K)=17.4 kcal mol(-1)), which ruled out the participation of a η(1)-allylic cation species in the low-energy π-face exchange process for this complex.

Stereochemistry and mechanism of the Brønsted acid catalyzed intramolecular hydrofunctionalization of an unactivated cyclic alkene.

Through employment of deuterium-labeled substrates, the triflic acid catalyzed intramolecular exo addition of the X-H(D) (X=N, O) bond of a sulfonamide, alcohol, or carboxylic acid across the C=C bond of a pendant cyclohexene moiety was found to occur, in each case, with exclusive formation (≥90%) of the anti-addition product without loss or scrambling of deuterium as determined by (1)H and (2)H NMR spectroscopy and mass spectrometry analysis. Kinetic analysis of the triflic-acid-catalyzed intramolecular hydroamination of N-(2-cyclohex-2'-enyl-2,2-diphenylethyl)-p-toluenesulfonamide (1a) established the second-order rate law: rate=k(2)[HOTf][1a] and the activation parameters ΔH(++)=(9.7±0.5) kcal mol(-1) and ΔS(++)=(-35±5) cal K(-1) mol(-1). An inverse α-secondary kinetic isotope effect of k(D)/k(H) =(1.15±0.03) was observed upon deuteration of the C2' position of 1a, consistent with partial C-N bond formation in the highest energy transition state of catalytic hydroamination. The results of these studies were consistent with a mechanism for the intramolecular hydroamination of 1a involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C3' position of 1a coupled with intramolecular anti addition of the pendant sulfonamide nitrogen atom to the alkenyl C2' position.