CuH-mediated Hydroamination of Styrene

CuH-mediated Hydroamination of cinnameneAbstract A detailed computational exploration of mechanistic intricacies of the copper(I) hydride (CuH)-catalysed hydroamination of phenylethylene with a prototype hydoxyl aminoalkane ester by a modernly reported (dppbz)CuH accelerator (dppbz PP 1,2-bis(diphenylphosphino)benzene) is presented. A variety of plausible mechanistic roadways strike been pursued by means of a sophisticated computational modeology, from which a general understanding of the factors controlling hydroamination catalysis emerged. The catalytic every(prenominal) in ally competent PPCuI hydride, which is predominantly present as its dimer, touch ons in irral wayssible hydrocupration proceedings with complete 2,1 regioselectivity to form a unoriginal PPCuI benzyl radical group liaise. Its interception with benzyl aminoalkane ester produces the forficate tertiary aminoalkane produce and PPCuI benzoate upon intramolecular SN2 breaking of the amine elect rophiles N-O linkage to precede exceedingly rapid, warmly exergonic C-N cohere-forming reductive elimination. The PPCuI benzoate corresponds to the gas pedal resting state and its renewal back into the PPCuI hydride upon transmetalation with a hydrosilane is turnover limiting. The effect of electronic perturbations at the amine electrophile upon the reception rate for rich hydroamination catalysis and also non-productive decrease of the hydroxylamine ester has been gauged, which unveil a more fundamental insight into catalytic bodily structure- implementation relationships.IntroductionThe catalytic hydroamination (HA) reply, the direct growth of an N-H cohere crossways an unsaturated nose sack updy-carbon linkage, offers facile gate to an industrially relevant organonitrogen commodity and fair chemicals in a green, waste- let loose and highly atom-efficient manner.1 By focusing on late d-block metal catalysis, several(prenominal) clear mechanistic courses fox b een revealed over the years for the hydroamination of alkenes, including the following principal processes 1) N-H bond activation with subsequent alkene unveiling into the metal-NR2 linkage,2 nucleophilic attack of an amine at a metal- form alkene,3 nucleophilic attack of a metal amido species at an activated alkene4 and amine coordination to be followed by proton ravish onto an activated alkene.5 Despite the signifi plundert onward motion achieved over the past years the utilisation of these methodologies is still limited by a number of drawbacks.1 The development of a general approach for regio- and enantioselective hydroamination of a broad range of alkene substrate classes, in particular, perseveres an grave challenge in the context of intermolecular HA.Recently, the groups of Miura6 and Buchwald7a reported independently a mechanistically distinct approach for styrene HA that involves copper(I) hydride (CuH) as the accelerator pedal unneurotic with amine electrophiles and a hydrosilane hydride source to furnish amines in elegant yields and enantio-/regioselectivities under mild conditions.8 Miura and co-workers disclosed that styrenes react with benzylamine ester reagents in THF at direction temperature to afford exclusively branched benzylic tertiary amines in the presence of a (dppbz)CuH catalyst (dppbz PP 1,2-bis(diphenylphosphino)benzene) and a hydrosilane hydride source ( evasion 1).6 countersign of the Cu(OAc)2/dppbz starting material with Li(OtBu) and a reducing agent likely gives wax to PPCu(OtBu) 2, which becomes converted into the catalytically competent PPCuI hydride 3 by with(predicate) transmetalation with silane.According to plausible mechanistic tracks outlined in Scheme 2 styrene interjection into the Cu-H linkage at 3 leads to PPCuI alkyl 4 that couples with the benzylamine ester electrophile thereafter to generate amine product P and PPCuI benzoate 6. Various mechanistic piece of grounds fag be envisaged for this tra nsformation,9 hardly its detailed details remain largely unidentifiable and so far. Transmetalation of 6 with hydrosilane regenerates the catalytically competent PPCuI hydride for some other catalyst turnover. The performance of HA catalysis via the productive cycle can be compromised by the healthful known adroitness of the PPCuI hydride to cast down the amine electrophile. This may proceed by dint of various piece of lands to involve formation of both N-H (one plausible path via a PPCuIII benzoate amido hydride intermediate 7 is exemplified in Scheme 2) or O-H bonds to afford PPCuI benzoate 6 by consumption of a molar resembling of the amine electrophile.Precise knowledge of both the operative mechanism and of catalytic structure-performance relationships argon indispensable for the rational design of improved HA catalysts. In light(a) of the fact that precise details of mechanistic intricacies of CuH-mediated vinyl atomic number 18ne HA remain largely elusive thus fa r,10 a sophisticated computational communications protocol has been employ as an established and predictive means to study reaction mechanisms and to guide rational catalyst design. The present study scrutinises rival mechanistic tracts for HA of styrene (1a S) with O-benzoyl-N,N-dimethyl-hydroxylamine (1b A) by a catalytically competent dppbz-ligated CuI hydride labyrinthine in the presence of prototype trimethylsilane (1c H) as hydride source. No structural reduction (other than replacing O-benzoyl-N,N-diethyl-hydroxylamine and HSiPh3 used in experiment by 1b and 1c, respectively, solely for the social occasion of computational efficiency) of any kind has been imposed for any of the key species involved.The computational methodology employed (highly accurate DLPNO-CCSD(T) in conjunction with basis sets of def2-TZVP fibre and a sound treatment of bulk solvent effects) simulated accepted reaction conditions adequately and mechanistic analysis is based on Gibbs free- postal code profiles. This computational protocol can confidently be expected to reliably procedure the postcode landscape and this has allowed mechanistic conclusions with substantial predictive value to be drawn.As detailed herein, our comprehensive mechanistic interrogative sentence provides support that in force(p) HA catalysis involves irreversible hydrocupration with strict 2,1 regioselectivity to be followed by propagation of the branched tertiary amine product by interception of the thus create secondary PPCuI benzyl nucleophile with amine electrophile. The declareing pathway sees the initiative intramolecular SN2 fault of the benzoate leaving group and is followed by highly facile and strongly exergonic C-N bond-generating reductive elimination from a highly activated, interpose PPCuIII species. It leads to amine product and PPCuI benzoate, the last mentioned of which corresponds to the catalyst resting state. Its con magnetic declination back into the catalytically co mpetent PPCuI hydride is turnover limiting.Results and DiscussionThe aim of the present study is dickensfold. A number 1 part scrutinises thoroughly all the relevant elementary move of Scheme 2, with special attention devoted to the several mechanistic avenues that can be invoked regarding the interaction of PPCuI alkyl nucleophile with the amine electrophile and also the productivity-limiting reduction of the amine transfer agent. A second part explores the effect of electronic perturbations at the amine electrophile upon catalyst performance.PPCuH-mediated HA of styrene with amine electrophile 1b Catalyst initiation efficacious HA catalysis entails the sign renascence of PPCu(OtBu) 2 into the catalytically competent PPCuI hydride compound. The ability of hydrosilane 1c ( H) to affect transmetalation at 2, although being rather flimsy turnover limiting, will influence the performance of HA catalysis, since it determines the amount of catalytically competent PPCuI hydride spec ies available for catalyst turnover.Hydroxylamine ester 1b (displaying a subtle preference for 1-N over 1-O donor ligation) and THF (T) show a comparable adroitness to bind at copper in 2. However, the entropic costs linked with reactant connecter place the respective adducts 2A, 2T higher in free cleverness proportional to the disjointed fragments. This chess opening widens regularly for ever weaker donor molecules as clearly seen in catch 1 for silane (2H) adducted species. Furthermore, 2 exhibits no lust toward dimer formation, as all the efforts to localise a dimeric species failed. by and by the initial facile, but uphill association of trimethylsilane 1c at 2, transmetalation evolves by means of with(predicate) a metathesis-type transition-state (TS) structure (see infix S1 in the reinforcement Information), which decays thereafter into the PPCuI hydride complex 3 through facile freeing of Me3SiOtBu. Figure 2 reveals an affordable energising restraint (G = 22.4 kcal mol-1 relative to 2 + 1c) for conversion of 2 into the catalytically competent complex 3, which is driven by a thermodynamical force of substantial amount.PPCuI hydride compoundReactant (styrene S, hydroxylamine ester A, hydrosilane H), amine product (P) and THF (T) solvent molecules can swain in various ways at copper in the catalytically competent hydride compound (see Figure S2 in the supporting Information) to give machinate to a multitude of adducted species, all of which are expected to participate in mobile association/disassociation equilibria.11 Similar to what is found for 2, the copper centralize can accommodate totally a single molecule13 its moderate binding enthalpy, however, cannot compensate for the associated entropic costs, thereby rending the various adducted species to be higher in free energy than the respective separated fragments. Amines (A, P) and styrene (benefitting from coppers ability for backbonding) are found to associate preferably and thu s 3A, 3S display an energy gap (relative to separated fragments) that is more or less smaller than for 3T and 3H (Figure 3). On the other hand, 3 exhibits a pronounced propensity towards dimer formation with 3dim is favoured by 7.5 kcal mol-1 relative to 3 (Figure 3). then, the catalytically competent PPCuI hydride is predominantly present as dimer 3dim with relevant adducts 3S (productive cycle, Scheme 2) and 3A (non-productive cycle, Scheme 2) are well separated and higher in free energy by more than 12 kcal mol-1 (Figure 3).Styrene insertion into the Cu-H linkage Following the plausible catalytic scenario in Scheme 2, the productive cycle entails the first generation of PPCuI alkyl 4. Alternative regioisomeric pathways for migratory C=C bond insertion into the Cu-H -bond commencing from 3S have been examined. The possible participation of another reactant, amine product or solvent molecule has been probed explicitly, but neither encounter, product or TS structures featuring a s table coordination of a spectator molecule could be hardened.13 Common to both pathways for 1,2 and 2,1 insertion is the evolution of C-H bond formation through a four- pith planar TS structure describing metal-mediated migratory insertion of the styrene C=C linkage into the polar Cu-H bond, which occurs at standoffishnesss of 1.57-1.65 (see Figure S3 in the bread and butter Information) for the emerging C-H bond. Following the reaction path advance, TS structures decay into primary PPCuI alkyl 4b (1,2 insertion) and secondary PPCuI benzyl 4a (2,1 insertion), respectively.Effective delocalisation of electron density is known to markedly influence the stability of the polarised TS structure describing the interaction of an electron-rich Cu-H -bond with the styrene C=C linkage and also of 4, such(prenominal) that the regioselectivity of the hydrocupration is largely dictated on electronic grounds.14 The -electron-withdrawing arene functionality at the styrene carbon directly adja cent to the copper centre effects an effective depletion of electron density from the nonsubstituted olefinic CH2 centre in the TS structure and also assist through hyperconjugative interaction with the stability of 4a. Hence it electronically stabilises both the TS structure for 2,1 insertion and 4a when compared to the species involved in 1,2 insertion that are devoid of such an opportunity. The located TS and product species (see Figure S3 in the bread and butter Information) give no indication that the electronic predisposition towards 2,1 insertion is likely to be reversed imputable to approbative PPCu-arene interactions along the 1,2 pathway. Thus, 2,1 insertion is expected to prevail energetically on both kinetic and thermodynamic grounds.Indeed, Figure 4 reveals that migratory olefin insertion proceeds with complete 2,1 regioselectivity to afford secondary PPCuI benzyl 4a by overcoming a barrier of 21.6 kcal mol-1 (G relative to 3dim), whereas the 1,2 pathway remains inac cessible due to higher kinetic demands (G = 4.1 kcal mol-1) and is also disfavoured thermodynamically (G = 4.4 kcal mol-1). It characterises hydrocupration via the energetically prevalent 2,1 pathway to be kinetically viable and irreversible.15Amine product generation upon interception of 4 by amine electrophileThe interception of PPCuI alkyl 4 with amine electrophile 1b gives rise to the generation of amine product P and releases PPCuI benzoate 6 (Scheme 2). Various mechanistic scenarios are conceivable for this transformation,9 but, unfortunately, virtually no precise details of the operative mechanism are available.10 This section intends to fill this gap by thoroughly examining several rival pathways. It includes the sectionalisation of the hydroxylamine ester N-O linkage via 1) SN2 switching of the benzoate leaving group 2) intramolecular SN2 displacement and 3) aerophilous addition across the N-O linkage. This affords transient PPCuIII intermediate 5, from which P and 6 ar e likely formed upon C-N bond-forming reductive elimination. The generation of the branched tertiary amine product Pa in a single step through nucleophilic attack of the Cu-C linkage at the plus N(amine) centre with concomitant N-O bond cleavage has been probed as a further plausible mechanistic avenue (dashed arrow in Scheme 2). Given that hydrocupration proceeds with strict 2,1 regioselectivity, the discussion will focus exclusively on pathways that commence from 4a. Notably, rival paths starting from 4b are found energetically non-competitive in every case studied. The encompassing account of all the studied pathways can be found in the Supporting Information.We start with examining N-O bond cleavage of 1b by nucleophilic PPCuI benzyl 4a. Figure 5 collates the free-energy profile of the most accessible pathway for the various mechanistic scenarios examined, whilst structural aspects of key species involved can be found in Figures S4-S9 (see the Supporting Information). The elec trophile 1b binds preferably via its N donor centre (1-N) at copper to furnish adducts with the unbound carboxylate oxygen pointing either towards (4a1A) or away (4a2A) from the metal, both of which are higher in free energy than the separated fragments. The located TS4a2A5a structure describes N-O bond cleavage that is redolent(p) of a SN2 displacement of the benzoate group, featuring distances of 1.72 and 2.02 for vanishing N-O and emerging Cu-N amido bonds (see Figure S5 in the Supporting Information). Progressing further along the reaction trajectory, the benzoate group binds eventually at copper to flip transient PPCuIII intermediate 5a. The intramolecular process version commencing from 4a1A evolves through a five-centre TS4a1A-5a that displays similar metrics regarding vanishing N-O and emerging Cu-N amido bonds, but crucially benefits from an already pre-established Cu-O(benzoate) contact (see Figure S7 in the Supporting Information). As it turns out, this contact likely renders 4a1A5a intramolecular SN2 displacement somewhat favourable kinetically over 4a2A5a with both pathways are indistinguishable on thermodynamic grounds. The TSOA4a2A-5a shown in Figure 5 (see also Figure S9 in the Supporting Information) has been located as energetically prevalent three-centre TS structure describing oxidative addition across the N-O linkage that occurs at distances of 2.43 and 1.89/2.60 for vanishing N-O and newly built Cu-N(amido)/Cu-O bonds, respectively. The condensed free-energy profiles in Figure 5 reveal that for cleavage of the electrophiles N-O linkage by PPCuI benzyl nucleophile the 4a1A5a intramolecular SN2 pathway (G = 19.8 kcal mol-1 relative to 4a+1b) prevails kinetically somewhat over 4a2A5a, with oxidative addition proceeding through TSOA4a2A-5a (G = 31.1 kcal mol-1 relative to 4a+1b) is found substantially more demanding kinetically and hence not accessible. The fine energy balance between the alternative SN2-type pathways is likely be influe nced by the diphosphine catalyst backbone.Figure 6 combines the dominant pathway for N-O bond cleavage at amine adduct 4aA with C-N bond-generating reductive elimination at transient PPCuIII 5a taking place thereafter. Given that benzyl and amido functionalities are already preferably arranged in 5a no major structural reorganisation is required prior to traversing TS5a6Pa, which occurs at a distance of 2.36 of the emerging C-N bond (see Figure S11 in the Supporting Information) and decays thereafter into the branched tertiary amine product that is initially bound to PPCuI benzoate (6Pa), but is readily released thereafter. The reductive elimination is found highly facile (G = 5.2 kcal mol-1 relative to 5a) and driven by a remarkably strong thermodynamic force (Figure 6). Of the twain consecutive stairs converting PPCuI benzyl 4a into amine product Pa and PPCuI benzoate 6 through interception with electrophile 1b, the first intramolecular SN2 displacement of the benzoate leaving group determines the boilers suit kinetic demands (G = 19.8 kcal mol-1 relative to 4a+1b) with Pa and 6 are so generated from transient, highly reactive PPCuIII intermediate 5 upon rapid and strongly downhill reductive elimination.Nucleophilic attack of the Cu-C linkage at the positive N centre of the amine electrophile with concomitant N-O cleavage, thereby affording 6a + Pa in a single step, describes an alternative mechanistic scenario. Despite all our efforts, a precise TS structure associated to this pathway could not be located, but examination by means of a state-of-the-art reaction-path-optimisation (chain-of-state see the Computational Methodology) method provided a reasonably approximate TS structure. The multicentre TS4a1A-6Pa describes concerted N-O bond cleavage (2.30 ) together with C-N (2.54 )/Cu-O(2.37 ) bond formation, all occurring in the immediate vicinity of the copper centre (see Figure S12 in the Supporting Information). A substantial barrier of nigh 30.7 kc al mol-1 (G relative to 4a+1b) has to be overcome (Figure 7), which renders the concerted 4a1A6Pa pathway non-accessible kinetically in the presence of the viable two-step conversion shown in Figure 6.16PPCuI benzoate compound In light of the strong thermodynamic force associated with generating the C-N bond, the PPCuI benzoate may become, among others, a candidate for the catalyst resting state. Hence, the aptitude of 6 to accommodate additional reactant, amine product and THF solvent molecules has been probed in order to clarify its precise identity. In accordance with findings for 2 and 3, a single molecule only can bind at copper at the expense of one of the two Cu-O(carboxylate) linkages, but adduct formation is disfavoured in terms of free energy. Hence the PPCuI benzoate is predominantly present as non-adducted form 6 featuring a 2-O ligated benzoate functionality (Figure 8).Regeneration of PPCuI hydride from PPCuI benzoate Transmetalation of 6 with trimethylsilane 1c regener ates the catalytically competent PPCuI hydride 3 for another catalyst turnover, thereby closing the cycle for productive HA catalysis. Two scenarios have been analysed that are distinguished by which of the carboxylate oxygens at silane adducted 6H participate in Si-O bond formation. The transfer of silyl onto the oxygen directly bound to Cu evolves through a four-centre metathesis-type TS6H-3OS1 and leads eventually to 3 upon facile liberation of Me3SiOBz. On the other hand, a six-centre TS6H-3OS2 is traversed along an alternative pathway representing silyl transfer onto the unbound carboxylate oxygen (Figure 9 and Figure S13 in the Supporting Information). The enhanced stability of six-centre TS6H-3OS2 versus four-centre TS6H-3OS1 discriminates among the two pathways, which are driven by a thermodynamic force (G = 0.5 kcal mol-1 relative to 6 + 1c) of identical magnitude. The assessed barrier of 26.2 kcal mol-1 (G relative to 6 + 1c) for the most accessible pathway characterises 6 + 1c3 + Me3SiOBz as a kinetically demanding, but viable transformation that is moderately uphill thermodynamically (Figure 9).Reduction of the benzylamine ester by PPCuI hydrideThe well known tendency of the catalytically competent PPCuI hydride to reduce the amine electrophile under N-H bond formation, hence giving rise to PPCuI benzoate, or alternatively via O-H bond formation to afford a PPCuI amido can severely compromise the catalyst performance. To this end, several conceivable pathways (some of which are sketched in Scheme 2) have been studied. Whilst focusing on thermodynamic aspects amine reduction with N-H bond formation via 3 + 1b6 + HNMe2 (G = -71.9 kcal mol-1) appears to be strongly favoured over O-H bond generating 3 + 1bPPCuI(NMe2) + benzoic acid (G = -51.3 kcal mol-1). The cleavage of the N-O linkage of 1b at amine adduct 3A does preferably proceed through SN2 displacement of the benzoate leaving group with the intramolecular pathway proved to be somewhat favourabl e energetically (see Figure S14 in the Supporting Information), whilst oxidative addition of electrophile 1b across the N-O linkage is found substantially more demanding kinetically. all in all these aspects closely parallel the findings regarding the preferable avenue that leads to generate PPCuIII 5a (see above). Likewise, N-H bond-forming reductive elimination at PPCuIII amido hydride intermediate 7 is highly facile kinetically and strongly downhill as 5a6a + Pa is. Unfortunately, all the efforts to localise the associated TS structure have not been successful, but the examination of the reaction path thoroughly by means of a chain-of-state method intemperately indicates that reductive amine elimination at 7 has an only marginal barrier, if at all, to overcome, hence it proceeds almost instantaneously. Overall, a smooth, kinetically affordable pathway for undesired 3 + 1b6 + HNMe2 conversion has been located that comprises a first intramolecular SN2 cleavage of the N-O bond (G = 26.3 kcal mol-1 relative to 3dim, Figure 10), to be followed by highly rapid reductive amine elimination from an interpose and highly reactive PPCuIII intermediate 7, thus reflecting the well-documented aptitude of 3 to engage in performance-limiting reduction of the electrophilic amination reagent.6, 7Further attempts devoted upon localisation principle a proper TS structure for the concerted attack of the nucleophilic Cu-H linkage at the N centre of the amine electrophile to be accompanied with N-O bond folie has not been successful. However, a reasonably approximate TS (see Figure S15 in the Supporting Information) is seen to be above TS31A-7 by another 19.8 kcal mol-1, such that the concerted pathway can confidently be discarded as energetically viable alternative to the operative two-step process.Proposed catalytic cycleThe mechanistic express based upon the above thoroughly conducted examination of relevant elementary steps is presented in Scheme 3. An energetically smoo th and downhill sequence of steps transforms the starting material into the catalytically competent PPCuI hydride 3, which is in a mobile equilibrium11b with its thermodynamically prevalent dimer 3dim. Migratory insertion of the styrene C=C linkage into the Cu-H bond is kinetically facile (G = 21.6 kcal mol-1 relative to 3dim+1a), thermodynamically downhill and proceeds with strict 2,1 regioselectivity. Hence irreversible hydrocupration is regioselectivity ascertain and occurs prior to the turnover-limiting step. The interception of the thus generated secondary PPCuI benzyl 4a with amine electrophile 1b produces the branched tertiary (Markovnikov) amine product Pa and PPCuI benzoate 6. This transformation favours a two-step process comprising the first intramolecular SN2 displacement of the benzoate leaving group (G = 19.8 kcal mol-1 relative to 3dim+1a+1b) to furnish transient, highly reactive PPCuIII intermediate 5 from which Pa and 6 are generated upon highly rapid and strongly downhill reductive elimination. It renders 6 to be the most stable species of the cycle for productive HA catalysis and it thus corresponds to the catalyst resting state. The regeneration of catalytically competent 3 from 6 through transmetalation with hydrosilane 1c is found most demanding, but affordable, kinetically (G = 26.2 kcal mol-1 relative to 6+1c) and hence is turnover limiting its assessed barrier is compatible with reported catalyst performance data.6 It is worth mentioning that a similar picture regarding hydrocupration and transmetalation steps has emerged from a recent experimental study by the Buchwald group on a DTBM-SEGPHOS-based CuI catalyst.7dA stepwise pathway closely related to 4aA6 + Pa is preferably traversed for the non-productive reduction of amine electrophile by 3. An intramolecular SN2 N-O bond disruption (G = 26.3 kcal mol-1 relative to 3dim+1b) precedes highly facile and strongly downhill reductive amine elimination from intervening, highly reactive P PCuIII intermediate 5a. The assessed small kinetic gap (G = 0.1 kcal mol-1) between discriminative TS structures for productive and non-productive reaction branches reflects adequately the observed close competition between the two processes, which can lead to compromised catalyst performance for improper chosen amine electrophiles. Furthermore, it provides further confidence into the substantial predictive ability of the herein employed high-level computational methodology.17Effect of the amine electrophile upon catalyst performance A second part of this study explores the effect of the amine electrophile upon catalyst performance. To this end, the energy profile for two