Homogeneous Catalysis · 6602 –6605;D)R.E.Mulvey,F.Mongin, M

DOI:10.1002/chem.201801541 Full Paper

& HomogeneousCatalysis |Hot Paper| Donor-influenced Structure–Activity Correlations in Stoichiometric and Catalytic Reactions of Lithium Monoamido-Monohydrido- Dialkylaluminates Lara E. Lemmerz,[b] Ross McLellan,[a] NeilR.Judge,[a] Alan R. Kennedy,[a] Samantha A. Orr,[a] Marina Uzelac,[a] Eva Hevia,[a] Stuart D. Robertson,[a] Jun Okuda,*[b] and Robert E. Mulvey*[a]

Abstract: Aseries of heteroleptic monoamido-monohydrido- their performance in metallation of atriazole andphenylace- dialkylaluminate complexes of general formula tylene and addition across pyrazine. These resultslead to an

[iBu2AlTMPHLi·donor] were synthesized and characterisedin example of phenylacetylenehydroboration, which likely pro- solution and in the solid state. Applying these complexes in ceeds via deprotonation,rather than insertion as observed catalytic hydroboration reactions with representative alde- with the aldehydes and ketones. Collectively,the resultsem- hydes and ketones reveals that all are competent, however a phasise that reactivity is strongly influenced by both the definite donor substituent effect is discernible. The bifunc- mixed-metal constitution and mixed-ligand constitution of tional nature of the complexes is also probedbyassessing the new aluminates.

Introduction reagents are emerging as important reagents in (catalyst free) cross-coupling protocols[2] and in metallation.[3] In the latter

Well-defined main-groupmetal complexes are currently the case, we recently reported that heteroleptic iBu2AlTMP in subjectofburgeoning synthetic interest, in both stoichiometric tandem with LiTMP can metallate arange of sp2-and sp3-C H À and catalytic transformations.[1] This attention stems from the bonds, albeit the presence of the bulky TMP anion bound to realisation that, as chemists, we need to develop new sustain- lithium is crucial in the C Hbond activation.[3f–k] In fact it is À able solutionswithoutrecourse to scarce and toxic noble only in rare cases with relatively acidic hydrogen atoms that metals,whilst at the same time attempting to emulatetheir re- aluminium reagents in isolation have demonstrated utility in nownedreactivity.Furthermore, the plethoraofearth abun- deprotonative metallation of organic C Hsubstrates.[1a] In the À dant main group metals requires that we need to develop a catalytic arena vis-à-vishydroboration, the use of aluminium more fundamental understanding of the potentialand the complexes is gaining momentum.[4] Importantly,Roeskyetal. limits of mono- and bimetallic main group metal systems. In utilized a b-diketiminato stabilisedaluminium hydride complex this regard aluminium, the most abundant metal in the earth’s in hydroboration of alkynes andcarbonyl groups.[4a] Recently, crust, fits the requirement. Reports of aluminium complexes in the groups of Cowley and Thomas demonstrated that the important stoichiometric and catalytic processes are becoming commerically availableDIBAL(H)orEt3Al·DABCO are capable of increasingly common in the literature. For example aluminium catalysing the hydroboration of alkynes.[4c] At this point, most aluminium based catalysts have been neutral complexes, [a] Dr.R.McLellan, N. R. Judge, Dr.A.R.Kennedy,Dr. S. A. Orr,Dr. M. Uzelac, thoughrecent reports have implicated boratesasimportant Prof. Dr.E.Hevia, Dr.S.D.Robertson, Prof. Dr.R.E.Mulvey species in hydroboration.[5] WestCHEM, Department of Pure and Applied Chemistry University of Strathclyde Since our groups interest lie in the synergistically beneficial Glasgow G1 1XL (UK) interplay of two distinct metal centres in abimetallic “ate” E-mail:[email protected] complex, this prompted the question whether alkali-metal alu- [b] L. E. Lemmerz, Prof. Dr.J.Okuda minates would demonstrably impartexciting reactivity to hy- Institute of Inorganic Chemistry droboration chemistry.Therefore, in arecent communication RWTH Aachen University Landoltweg 1, 52056Aachen (Germany) we reported arange of Lewis donorsolvated lithium alumi- E-mail:[email protected] nates bearing two HMDS (1,1,1,3,3,3-hexamethyldisilazide) and Supporting information and the ORCID identification number(s) for the au- two hydride functionalities and established that lithium diami- thor(s) of this article can be found under: dodihydridoaluminates were able to function efficiently in hy- https://doi.org/10.1002/chem.201801541. droboration catalysis and metallation applications.[6] Very re- 2018 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. cently,following their aforementioned alkynehydroboration This is an open access article under the terms of the Creative Commons At- tributionLicense, whichpermits use, distributionand reproduction in any advances, Cowley and Thomas have been successful in hydro- medium, provided the original work is properly cited. boration of alkenesusing the commercial ates, LiAlH4 and

Chem. Eur.J.2018, 24,9940 –9948 9940 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper

[7] sodium bis(2-methoxyethoxy)aluminum hydride. In our lithi- [iBu2AlTMP(H)Li]2.Similarsolvent-freestructures are knownin um diamidodihydridoaluminates cases the nature of the sup- the literatureand contain the common [Li2H2]central square porting Lewis donor (level of solvation, N-donor versusO- core depicted in 1.[14] donor) played an influential role in catalytic performance, Next, aseries of donor-solvated derivatives of 1 were ob- where easily displaced monodentate donors performed better tained in high yield by addition of the appropriate ligand (THF, than the polydentate donors,presumably because the chelat- PMDETA, diglyme or DABCO) into toluene solutions of 1,fol- ing effect proved to be deleterious, blockingthe active metal lowed by introductionofn-hexane and crystallisation at sites. Moreover,wealso observed similarphenomenainare- 30 8C(Scheme1). In each case, single-crystals suitable for X- À lated catalytic dehydrocoupling process using the metal hydride surrogate, dihydropyridine precatalyst, 1-Li-2-tBu-1,2-dihydropyridine.[8] The Okuda group also recentlyreported a series of alkali metal hydridotriphenylborates,[9] that showed the nature of the Lewis donor (including flexibility of coordination and consequential Lewisacidity of the metal) impacted hydroboration performance in an unpredictable manner.Thus, in main group (bi)metallic catalysis, even small changes in the nature of ancillary ligand(s) impart large differences in the ensuing reactivity.Given this, here we have examined for the first time structure–activity relationshipsofdonorsolvated heteroleptic dialkyl-monoamido-monohydrido complexes using TMP (2,2,6,6-tetramethylpiperidide) as the amide of choice. TMP is a superiorbase to its HMDS counterpart and is arguably the Scheme1.Synthesis of complexes 1–6,including postulated structure of powder 1 and molecular structuresofcrystalline 2–6.All hydrogen atoms most important utility amide having taken over from diisopro- other than hydrido typesbonded to aluminiumare omitted for clarity.Ther- [10] pylamide, on account of its widespread employment not mal ellipsoidsare drawn at 30%probability. only in LiTMP but in aseries of bimetallic formulations such as Knochel’s salt-supported magnesium and zinc reagents[11] and the organometallic ate type reagents introduced by Kondo/ ray diffractionstudies wereobtained and revealed aremark- Uchiyama/Wheatley,Mongin, and ourselves.[3,12] Thus, we able variance in the molecular arrangements. THF adduct 2 report here the synthesis of aseries of these new aluminates, and PMDETAadduct 3 in effect exhibit the same structure, investigate their solid state and solution structuresand com- wherein adistorted tetrahedral iBu2AlTMP(H) fragment is pare their reactivity in hydroboration of aldehydes andke- bonded, via the m-hydride, to aLi·donor fragment (donor= tones, and assess their ambi-utility in metallation and addition three THF in 2 and one PMDETA in 3). Both the Al Hand Li H À À reactions. distances are similar in each molecule [Al H1.61(3) in 2, À 1.66(4) in 3;Li H1.83(3) in 2,1.76(4) in 3], with those to À the group 13 metal systematically shorter by an average of Results and Discussion 0.16 .Changing the Lewis donor from PMDETA to diglyme gives 4,which adoptsamarkedlydifferent solid-state structure. Synthesis and characterisation The main differences between these tridentate chelating li- Cocomplexation between the commercially availablealumini- gands is achange from NtoOdonors, and importantly the um hydride DIBAL(H) and LiTMPinn-hexane at room tempera- latter only contains one terminal methyl substituent, reducing ture resulted in the immediate precipitation of awhite powder steric congestion when it ligatesametal atom. Charge-separat- 1 that so far has resisted recrystallisation thusruling out X-ray ed ion pair structure 4 is best described as alithium lithium-di- crystallographic authentication. However, NMR spectroscopic aluminate, since both cationic and anionic moieties contain studies have revealed its general constitution. 1Hand 13Cspec- lithium.The cationic moiety is adistorted octahedrallithium tra reveal the presence of two isobutyl groups andone TMP cation supported by two diglyme ligands. The anionic moiety 7 group and the hydride ligand.The Li spectrum displays a is comprised of two peripheral iBu2AlTMP(H)units in distorted sharp resonance at d= 0.4 ppm albeit the 27Al spectrum does tetrahedral environments, that bond to acentral lithiumvia m- not contain any identifiable resonance, acommon problem TMP and m-hydride ligands. Furthermore the lithium ion dis- [15] with quadrupolar aluminium centres in broadly unsymmetrical plays anear square planargeometry (t4 =0.04), albeit it is environments. The presence of an aluminium hydride was con- also likely to be further stabilised by electrostatic interactions firmed by performinga1H{27Al} experiment whichrevealed a from TMP methyl substituents [Li C range:2.946(7)– À Me sharpening of the broad resonance at 3.18 ppm, and suggest- 3.249(7) ]. This arrangementcan be described as an inverse ed formation of the expected cocomplex iBu2AlTMP(H)Li 1 in a Weiss motif. The Weiss motif is asurprisingly common struc- 75%isolated yield. Structural insight of the solution phase ture found in dialkali-metal “ate” complexes where the central, constitution of 1 was gainedbyperforming aDOSY experi- non-alkali-metal exists in atetrahedral arrangement, with re- [13] [16] ment in C6D6 solution, revealing the likely aggregation state spect to four bridging ligands. Here in contrast we have a of the solvent-free 1 as dimericwith the formula centralsquare planar alkali-metal that bridges through ligands

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9941 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper to two peripheralaluminium atoms.Asearch for this structural Complexes 2–6 were also characterisedinsolution by 1H, motif in the Cambridge StructuralDatabase (CSD) resulted in 13C, 7Li and 27Al NMR spectroscopy and revealed the expected zero entries, confirming the structural rarity of 4.Reaction of 1 resonances, albeit as with 1,the 27Al spectra were of little diag- with half an equivalent of DABCO affords 5 in which two sym- nostic value. 1Hand 13Cspectra confirmed the presence of 2 metry equivalent iBu2AlTMP(H)Li subunits are connected by iBu, 1TMP,and an aluminiumbound hydride ligand, as well as the bicyclic,binitrogen Lewis donor.Aswith 4,here the lithium the appropriate donor resonance(s).Interestingly the 1HNMR ion coordination sphere is completed by m-TMP and m-hydride spectrumsof5 and 6 are nearidentical, except for the chemi- ligands. Employing afull equivalent of DABCO affords the stoi- cal shift resonances of the DABCO CH2 protons. Moreover,in6 chiometric variant 6,which differs from 5 by aterminal DABCO the terminally bound DABCO ligand only contains one broad ligand,instead of the bridging mode observed in 5.In2–6,de- singlet insteadoftwo triplets (vide infra). The 7Li NMR spec- spite the diversity of donor ligands, the Al Hand Li Hdistan- trum of 4 displayed one noticeably broad resonance at d = À À ces do not display any systematic differences across the series 0.32 ppm, instead of the expectedtwo resonances in accord- À (Al Hrange 1.61(3)–1.69(3) ;LiHrange 1.76(4)–1.88(3) ). ance with the charged-separated molecules in the crystal. À À The structures of 2–6 and the related lithium diamidodihydri- Thus, avariable temperature 7Li NMR experiment was per- doaluminates we reported previously,[6] can be regarded as formed to investigate whether an exchange process is occur- 7 well-defined modifications of LiAlH4 containing arange of ring (Figure 2). Measuring a Li NMR spectrum of 4 in a either alkyl or amido groups in place of either two or three hydrogen atoms. Furthermore, these more substituted LiAlH4 modifications benefit from the synthetic advantages of enhanced solubility(soluble in hydrocarbon solvents), and easier to accurately weigh low loadings in catalytic applications. In the knowledge that alkali metal effects can profoundly influence structural morphology,reactivity and physicalproper- ties,[12,17] we next attempted to synthesise asodium variant by reactionofNaTMP with DIBAL(H) in n-hexane. After addition of asmall amountofTHF to the reaction mixture we were able to isolate afew crystals of (iBu2Alm-TMPm3-HNa)2·2THF, 7,that were found to be suitable for diffraction studies (Figure 1). 7 Crystallographic characterisationof7 revealed astructure Figure 2. Variable temperature Li NMRstudy of 4 in [D8]toluenesolution. markedlydifferent from the lithium congeners, highlighting the impact of replacing lithium with its larger congener.

Figure 1shows an arrangement wherein a(NaHNaH) near- [D8]toluene solution at 08Cresults in asignificant broadening planar kite shaped ring lies between two iBu2AlTMP in an compared with the room temperature collection. Further cool- asymmetricfashion. One sodium is supported by two m-TMP ing at temperatures down to 60 8Cresults in the appearance À and two m -hydride ligands(Na2 H1H 2.40(2) and Na2 H2H of two broad and one sharp resonance at d=1.68, 0.36 and 3 À À À 2.38(2) ). The second sodium is also bonded to the hydride li- 1.96 ppm, indicating the likelihood of an exchange process À gands [Na1 H1H 2.22(2) and Na1 H2H 2.19(2) ]and two occurring at room temperature. The identity of the three reso- À À solvating THFmolecules. Unfortunately,wewere unable to re- nances can be tentatively assigned. Those at d=1.68 and producibly prepare 7 in acceptable yields and as such its char- 1.96 can be assigned to astructure that resembles that in À acterisation remained restricted to this structural analysis. the solid state, with the latter sharper resonance correspond-

ing to the approximately octahedral Li·(diglyme)2 cation. The resonance at 0.36 ppm may be assigned to acontact ion pair À resembling PMDETAanalogue 3,whichexhibits asharp resonance at d=0.38 ppm. We probed this exchange processfur-

ther via aDOSY NMR study of 4 at room temperature in C6D6. The estimated molecular weights in this instanceare 509 and 1] 615 gmolÀ (note the Lewis donor resonances diffuse sepa- rately from the remaining resonances), which are intermediate between thoseofeither the expected mass in the crystal 1 1 (846 gmolÀ )orastructure resembling 3 (424 gmolÀ ). In contrast, DOSYNMR studies of 3 indicate its greater robustness as no such exchange process occurs on the NMR timescale, 1 where an estimated molecular weight(463 gmolÀ )isinexcel- 1 lent agreement with the theoretical value (446 gmolÀ ). In summary,complexes 1–6 represent aseries of new mono- Figure 1. (top) Synthesisof7.(bottom) Molecular structureof7.All hydrogen atomsother than the hydrido types bonded to aluminium are omitted amido-monohydrido lithium aluminates, welldefined in the for clarity.Thermal ellipsoids are drawn at 30%probability. solid and/or solution states. Key distinctions are 1 is donor

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9942 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper free; 3 contains asolvating PMDETAligand in the solid state Table 1. Hydroboration of aldehydes and ketones using 1–6.[a] and retains this arrangement in solution (via DOSY studies); 4 adopts an entirely novel solid-state arrangementvide supra, and exhibits exchange of the diglyme ligands on the NMR timescale.

Substrate Catalyst Time Yield Hydroboration of aldehydes and ketones [h] [%]

[d] At the inception of these studies we sought to discover the 1 0.25 99 3[d] 0.25 99 answer to two key questions: (i)can these monoamido mono- 4[d] 0.25 99 hydrido lithium aluminates act as efficient catalysts in hydrobo- 1 0.5 99 ration applications;and (ii)does the structure, determined by 2 0.5 99 the donor ligand play arole in any such catalytic performance? 3 3 99 In this regardweinitiallyselected 1, 3,and 4,since 1 is donor- 4 0.5 99 free, while 3 and 4 contain broadly similardonor ligands, 5 0.5 98 6 0.5 98 except for the donor atom identity (N or O), and the slightly 2.5 reduced steric profile of diglyme whichgives rise to afluxional 1 97 1[b] 95 0.5 system in solution. Hydroboration catalysis using HBpinwas 2 93 0.5 selected to trial 1, 3 and 4,since it is an area currently attract- 3 99 65 ing increasing interestinthe main-group arena, andthe wider 3[c] 99 [b] 22 [18] 3 94 chemicalaudience. Importantly,simple aluminium reagents 0.5 4 90 [4c,7] 2 (AlEt3·DABCO, DIBAL(H),LiAlH4) have recently been discov- 5 96 0.75 ered as excellent catalysts in this regard, although occasionally 6 96 0.75 the presence of asecond metal has been overlooked when considering the elementary steps of the catalytic profile. Given 3 2.5 98 our groups’ longstanding association with bimetallic “ate” complexes,wepondered whether these systems might also shed further light on the reactionpathways and any significant 3 0.25 99 effect of the second metal. Results of catalytic hydroboration of aldehydes and ketonesare presented in Table 1. 1 0.5 99 Reaction of 1 (2.5 mol %based upon adimeric formula), 3 3 3 99 (5 mol%) or 4 (2.5 mol %) with benzaldehyde and pinacol 4 0.5 99 borane in C6D6 solution resulted in fast and quantitative hydro- 1 2 93 boration in 15 minutes at room temperature in every case. 3 3 99 Lowering the precatalyst loadings to 0.5 mol %of1 or 4 and 4 1.5 99 1mol%of3 resulted in no loss of catalytic performance. Im- 1 1.5 99 portantly these initial results demonstrate that 1, 3 and 4 are 3 3.5 98 all suitable candidates for aldehyde hydroboration. Expanding 4 3.5 97 the substrate scope to include dual-functional cinnamaldehyde [a] Reactionconditions (unlessstated otherwise): in C6D6 at roomtemper- demonstrates catalystselectivity with smooth and quantitative ature. NMRconversions determined with respectto10mol %ofhexame- hydroboration occurring only at the carbonyl functionality. 1 thylcyclotrisiloxaneasinternal standard.Catalyst loadingsare all 5mol % based upon Al/Lipresent in the solid-state structure. [b] In [D ]THF.[c] At and 4 achieve this transformation inside 30 min, whereas with 8 708C. [d] 1mol%catalyst loading 3 quantitative hydroboration occurs after 3h.This demonstrates that subtle structuraldifferences within thesesystems play arole in catalystperformance. Thisperformance is broadly in line with previous aluminium-based hydroboration catalysts. as such are readily accessible. Complexes 2, 5 and 6 all effi- Roesky et al. achieved quantitiative hydroboration of cinnamal- ciently catalyse hydroboration of cinnamaldehyde in 30 min. dehydewith his nacnacAlH2 complex in 6h with 1% loading, Encouraged by these findings we extended our substrates whereas we previously demonstrated 76 %conversion in 2h to include ketones, rationalising the increased intrinsic steric with (HMDS)2AlH(m-H)Li(THF)3.Completing our studies into al- bulk compared to that of aldehydes would magnify any ligand dehydehydroboration we screenedcomplexes 2, 5 and 6 with effects in these systems. Hydroboration of acetophenone with cinnamaldehyde as arepresentative example. 2 containsafour 1 (2.5 mol %) reaches 97 %conversion after 2.5 hatroom tem- coordinate lithium atom albeit bonded to three monodentate perature in C6D6.Using 3 as precatalyst,the reactionrequires THF ligands, which are more likely to desolvate during the re- 65 hfor quantitative hydroboration at room temperature, or action, compared to tridentate PMDETAin3,thus substrate 22 hat708C. Using 4 this reaction takes 2h for 90%conver- access to lithium may occur more readily. 5 and 6 both have sion, similartothe value using 1.Exploring the comparatively coordinatelyunsaturated three coordinate lithium atoms, and poor performance of 3 more thoroughly,2,2,2-trimethylaceto-

Chem. Eur.J.2018, 24,9940–9948 www.chemeurj.org 9943 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper phenoneand 2,2,2-trifluoroacetophenone were selected as lower the coordination number as requiredduring reaction. substrates. The former shows enhanced steric features with re- Therefore, one may anticipate that initial coordination of sub- spect to acetophenone, whereas the latter,approximately iso- strate to lithium, and thussubsequent insertion into the Al H À steric, is considerably more electron withdrawing. Hydrobora- bond would occur more readily in these cases. Importantly this tion of 2,2,2-trimethylacetophenonewith 3 (5 mol%) occurs in scenariovalidates the use andfuture exploration of bimetallic 2.5 hatroomtemperature, implying that ligand steric features systems. are not the only factor here in determining hydroboration effi- The generaltrend observedbetween 1, 3,and 4 in these re- ciency.2,2,2-trifluoroacetophenone is quantitativelyhydrobo- actions reveal that 1,the donor free complex is the best cata- rated in 15 min at room temperature. This fast reactivity may lyst and marginally outperforms 4. 3 in all cases is the least ef- be expected since the presenceofCF3 would significantly de- ficient catalyst in these processes. This reactivitycan be related plete the charge present at the ketonecarbon atom makingit to the solid and solutionphase structures. The structure of 3 more electrophilic andthus facilitatefaster nucleophilic hy- in solution,via DOSY NMR studies, resemblesthat in the crystal dride insertion. One possible contribution towards the slow re- structure. Thus, one expects the PMDETAgroup with its three activity of 3 with acetophenone is that the presence of a-hy- donor atoms to remaintightly bound to lithium during the drogen atoms may promote keto-enol tautomerism under the catalytic process. On the other hand, 4 can be considered a reactionconditions. In this scenario, one may envisage acoor- halfway-housebetween 1 and 3.Spectroscopicstudies indi- dination of the substrate to the lithium ion, resulting in asteri- cate that part of the time in solution the complex resembles cally congested 5-coordinate lithium ion, followed by tauto- that of the solid-statestructure, which is donor free, akin to 1, merisation and stabilisation of the enol form by H-bonding to (both diglyme ligandsare involved in bonding to the separat- one nitrogen atom of the PMDETAligand (Scheme 2). However, ed lithium cation). The remainder of the time the complex likely resembles 3,and thusingeneral, delivers reactivity intermediate between that of 1 and 3.Alower lithium coordination number also plays aroleincatalytic performance as indicated by reactivity of 2, 5 and 6 which hydroborate acetophenone faster than the other donor solvates studied.Once again,we turned to DOSY NMR studies to illuminate the solutionconstitution of these complexes to gain insightinto the catalytic 1 process. The estimated molecular weight of 2 is 486 gmolÀ ,in Scheme2.Keto–enoltautomerisation of acetophenone as apossible contri- 1 bution towards the slow catalyticperformance observedwith 3. good agreement with the theoretical value (506 gmolÀ ), sug- gesting that 2 largely remains intact in solution. However, care- ful inspection of the spectrum reveals that the THF resonances it is expected that the keto-form would predominate in solu- diffuse slightly faster than the remaining resonances of 2 (MW 1 tion, and we detect no spectroscopic evidence of its enol tau- 444 gmolÀ ), detailing that in solutionsome level of THF desol- tomer.Note no such tautomerisation and H-bond stabilisation vation is in operation, lowering the metal coordination is possible with the CF3 analogue.Asecond suggestion forthe number.With 5 and 6 the solutionconstitution is ambiguous. slow catalytic transformation of acetophenone with 3 is that a Bridging DABCO complex 5 has an estimated molecular weight 1 1 methyl hydrogen atom may be deprotonated by the TMP of 620 gmolÀ ,lower than the expected value of 691 gmolÀ . group. However,since this reactioneventually resultsinquan- Similarly terminally boundDABCO complex 6 has an estimated 1 titative hydroboration, any deprotonation must be under equi- molecular weight of 514 gmolÀ ,higher than the expected 1 librium. value of 402 gmolÀ .These intermediate values indicatethat Hydroboration catalysis of 1, 3 and 4 with benzophenone the solution constitutions may be in equilibria between the again reveals that 3 performsless efficiently(3h for quantita- two crystallographically observed extremes, and moreover,active formation) than either 1 or 4 (30 min). Using cyclohexa- count for the similarity in 1HNMR spectra of 5 and 6 (vide none as substrate reveals that 3 is again slowest (3 h);whereas supra).Ineach case the DABCO resonances diffuse with the 4 performsmarginally better than 1 (1.5 hversus 2h)for this same coefficient as the remaining complex resonances, ruling reaction. Finally,butan-2-one, an aliphatic ketone, is fastest out complete ligand desolvation,affording 1,onthe NMR ex- using 1 as acatalyst(1.5 h), while both 3 and 4 are complete periment timescale. Extending this argumentone step further, in 3.5 hatroom temperature. Butan-2-one contains two sets of we performed two representativereactions using bulk THF as a-hydrogenswhich may explain the slower reactivity with re- reactionsolvent. We selected 1 and 3 for these reactions and spect to 1.Hydroboration of acetophenone was also per- discoveredthat in each case quantitative hydroboration occurs formed using 2, 5 and 6.All three complexes demonstrate es- within 30 min. These fast reactions can be tentatively attribut- sentially the same reactivity (2,30min, 93%conversion, 5 and ed to, in the case of 1,breaking up of the dimeric aggregate 6,45min, quantitative). This can be rationalised by considering into amonomeric solvent separated species, and in the case of the structuraldistinctions of the complexes.Ineach case the 3 displacingthe PMDETA ligand resulting in asimilarmore DABCO ligand is monodentate, rendering the lithium atom 3- labile solution species, an arrangement that favours faster cata- coordinate, instead of the fully saturated 4-coordinate in 2–4, lytic transformation. Corroborating this, the 1HNMR spectrum albeit the high labilityofthe solvating THF in 2 is likelyto of 3 in [D8]THF solution reveals the presence of free PMDETA.

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9944 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper

Following from the hydroboration results we sought to discover some mechanistic evidence by performing aseries of stoichiometric reactions between complexes 1 or 3 with vari- ous substrate molecules. Unfortunately,wewere unable to crystallographically characterise such aspecies, despite repeated attempts. However,the different hydroborationresults, that is, the effect of the donor ligand leads us to surmise that hy- Scheme4.TMP basicity of 3 with 1-methyl-1,2,4-triazole, affording 8. droboration of aldehydes and ketones in C6D6 follows three basic elementary steps:(i) coordination of carbonyl substrate to lithium. Here, tentative evidence for the coordinationstep In agreement with the spectroscopic data (see Supporting originates from the slow performance of 3 with acetophenone, Information) the X-ray diffraction data revealed that the sub- (see Scheme2); (ii)insertion of carbonyl into aluminiumhy- strate hasbeen metallated at the 5-position, via amido basicity, dride bond,already primedfor addition;(iii)transelementation giving 8 in areasonable isolated yield (42%). Heterotrileptic 8 with HBpin to afford hydroborated product and regenerate an (Figure 3) crystallises with two independentmolecules in the active aluminium hydride catalyst. Scheme 3detailsthe pro- posed elementary steps in the reaction.

Figure 3. Molecular structures of the two crystallographically independent molecules in the asymmetricunit of 8. All hydrogen atoms other than those bonded to aluminium, and the triazolyl C Hare omitted for clarity.Thermal À ellipsoids are drawn at 30%probability.

asymmetricunit. In each case an iBu2AlH unit bonds to the 5- positionofthe triazole ring. Furthermore, the Al Hdistances À are the same within experimental error in each independent molecule 1.57(2) and 1.58(2) .Alithium·PMDETAmoiety is held in place by coordination from the triazolyl nitrogen atom of the aluminate moiety placed adjacent to the metallated carbon atom. Further,inone independent molecule (RHS Figure 3) the hydride ligand bridges to Li [Li H2.23(2) ], À while in the second (LHS Figure 3) the Li Hdistance is signifi- À cantly longer[Li H2.40(3) ]. Further inspection of the struc- À ture reveals akey factor for this drastic difference. In each case Scheme3.Representation of possible elementary steps using 3:coordina- the PMDETAligand coordinates to the lithium in adifferent tion of substrate;insertion into Al Hbondand s-bond metathesis with pin- À manner.Inone molecule (RHS) the methyl group attachedto acol borane. the central nitrogen atom lies approximately parallel to the Li À Hinteraction, whereas in the other (LHS) this same methyl group points in the opposite direction, afact we attributeto Metallation packing effects in the crystalline lattice. Complexes 1–6 all contain abasic TMP and anucleophilic hy- It is important to note that the aluminium reagent dride ligand and all are competent, in varying degrees, in cata- iBu2AlTMP on its own can also metallate 1-methyl-1,2,4-triazole. lytic hydroboration transformations. Well-defined PMDETA This reactionaffords (iBu2AlC3H4N3)2, 9 in 30%isolated yield, complex 3 was tested, as arepresentative complex in metalla- and is arare example of aneutral bis-alkylamidoaluminium tion reactions with substrates containing an acidic hydrogen compound acting as an amido base towards an aromatic C H À atom. Metallation of 1,2,4-triazoles in general can be problem- bond, albeit with hydrogen atoms in relatively acidic environ- atic and often requires low reaction temperatures, to prevent ments.[20] Though X-ray crystallographicdata for 9 were collect- unwelcome fragmentation reactions of metallated (commonly ed, these were of insufficientquality to justify publication (see lithiated) intermediate species.[19] Reaction of 3 with 1-methyl- Supporting Information). 1,2,4-triazoleatroom temperature for 1h in a n-hexane/tolu- Phenylacetylene was selected as acandidate metallation ene mixture, followed by cooling at 308Cresulted in forma- substrate because of its acidic CCH hydrogen and because of À tion of colourless crystalssuitable for X-ray diffraction studies precedence from Roesky’s research thatimplicated deprotona- (Scheme 4). tion as akey step in terminal alkyne hydroboration.[4b] In that

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9945 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper report the basicity derivesfrom ahydride of a b-diketiminato stabilised AlH2 unit. Using 3 (as arepresentativeexample, and the PMDETA ligand to try and induce crystallisation of the formed product) we soughttodiscover whether deprotonation or addition would occur,and whether any deprotonation would derive from the more basic TMPunit or weaker hydride, Scheme6.Attempted hydroboration reactionsofdiphenylacetylene (LHS) and whether these complexes could then be implicated in sub- and phenylacetylene (RHS) using 1 as aprecatalyst. sequent catalytic hydroboration reactions. Reaction between 3 and phenylacetylene in aJ.Young’s NMR tube was carriedout at room temperature. 1HNMR monitoring revealed the appear- repeated, with 3 instead of 1 as catalyst. In this case hydrobo- ance of resonances corresponding to TMPH. Furthermore, the ration only occurs very slowly,affordingonly around 5% yield resonance corresponding to the acidic hydrogen of phenylace- after heating at 708Cfor 18 hours. tylene disappeared, confirming that 3 reacts, once again,asan This desirable outcometherefore suggeststhat well-defined amido base (Scheme 5). The reactionwas repeated in atolu- “ate” complexes can play an important role in hydroboration of avariety of unsaturated molecules. Scheme 7outlinesvari- ous potential reaction pathways in this process. Pathwaya,in Scheme7describes an insertion pathway,that is ahydroalumi- nation followed by a s-bond metathesis, analogous to that dis- cussed(vide supra) for aldehyde and ketone hydroboration. Moreover,arecent report from Cowley and Thomas discuss a

Scheme5.TMP basicity of 3 with phenylacetylene, affording 10. ene/n-hexane mixture at room temperature. After stirring the solution for 2h at room temperature 10 was isolated in 57% yield as an off-white solid. Spectroscopic characterisation of isolated 10 corresponds to amono(alkynyl)monohydrido lithium aluminate, iBu2AlHCCPhLi·PMDETA. Its structure closely resembles thatof3 via formal replacement of aTMP anion with aphenylalkynyl anion as was the case with the triazolyl anion in 8.Note similar structures are knowninthe literature, and Uhl reported dialkylaluminiumalkynides adopting the bridging motif shown in 10,moreover with less bulky alkyl groups,a crystallographically authenticated pi interaction between aluminium and the triple bond was identified. By extension to the presentsystem,such an interaction between the triple bond and the carbophilic aluminium centre may prime the complex for insertion.[21] Importantly, 10 is similar to an in silico modelled activecatalytic intermediate by Roesky et al.,(aneutral b-diketiminato Al-alkynyl hydride) from which the B Hbond of pinacol borane adds across the triple À bond, followed by addition of asecond equivalent of phenyl acetylene to regenerate the active component. Since Roesky’s neutralintermediate complexwas found to be an excellent catalystinalkyne hydroboration, the isolation of the bimetallic complex 7,bysimple deprotonation, afforded the opportunity to learn about the role of anionic aluminates in this context (note that in this report we have two iBu ligandsinstead of a bulky b-diketiminate).

In aJ.Young’s NMR tube 1 (2.5 mol %) was dissolvedinC6D6 to which phenylacetylenewas added, followed by HBPin and the reaction monitored by 1HNMR spectroscopy.After heating for 18 hat708Cclean formation of the anti-Markovnikov vinyl- boronate ester has occurredin76% yield, as referenced Scheme7.Alternative potential reaction pathways for lithium aluminate cat- against an internal standard (Scheme 6). The experiment was alysedhydroboration of phenylacetylene.

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9946 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper similar mechanism for the DIBAL(H) catalysed hydroboration of alkynes. Since deprotonation was observed in stoichiometric reactions between phenylacetylene and either 1 or 3,wera- tionalise that this is akey step in any hydroboration catalysis of phenylacetylene, using the complexes herein. Interestingly, attempts to corroboratethis observation proved unsuccessful. Reactions between 10,orthe PMDETA-free variant, generated in situ, with HBpin both afforded 1Hand 11BNMR spectrathat were broadly uninterpretable, and indicative of decomposition of the reaction mixtures in stoichiometricregimes. Asecond Scheme8.Reactionbetween 3 and pyrazine giving Al Haddition prod- À proposal (Pathway b) proceeds in broad agreementtothat of uct 11.

Roesky using aneutralNacNacAlH2 catalyst. Here, lithium aluminate 1,performsaninitial deprotonation, then hydroboration occurs, followed by adeprotonation of asecond phenyl- Conclusions acetylene molecule, inducing catalytic turnover. Next, we decided to attemptthe hydroboration of an inter- This contribution has demonstrated the synthesis, spectroscop- nal alkyne.Reasoning that since di-phenylacetylene did not ic andstructural characterisation of anew family of donor-sol- contain any hydrogen atom “primed” for deprotonation, that vated heteroleptic dialkyl-monoamido-monohydrido com- any hydroboration activity would, by necessity,occur via an in- plexes. The presence andnature of the Lewis donor imparts insertion of the alkyneinto the aluminium hydride bond, and teresting structuralcharacteristics that influence the catalytic furthershed light on the likely reaction pathway.Thus, diphe- activity in hydroboration reactions of aldehydes and ketones nylacetylene wasadded to aJ.Young’s NMR tube in C6D6,fol- with pinacolborane.Polydentate donors (PMDETA) that remain lowed by 1 (2.5 mol%) and HBpin. After heating the solution boundtothe lithium atom in solution slow down hydrobora- at 708Cfor 18 hthe 1HNMR spectrum remainedunchanged tion. On the other hand, the related tridentate ligand diglyme, revealing that the addition of an Al Hacross diphenylacety- displays exchange on the NMR timescale, and performs similar- À lene is not favoured, and gives credence to the reaction mech- ly to the donor free species. We suggest this is, at least in part anism depictedpathway binScheme 7, although based on due to the coordination saturation of the lithium ion. Using the resultsofstoichiometric reactions of 10 with HBpin,we the Lewis donor ligand DABCO leads to areduction of the sol- cannotcompletely discount pathway a. vation at lithium and leads to faster hydroboration in arepre- sentativehydroborationofaketone.Inthese catalytic transformations, insertion of the polar carbonyl group into the Al H À Addition reactions bond is mooted as akey step. Heterolepticdialkyl-monoamido-monohydrido complexes are also revealed to be capable of Despite the fact that addition of the Al Hbond did not freely hydroborating phenylacetylene, however this activity is sug- À occur across diphenylacetylene in the attempted hydrobora- gestedtoproceed via deprotonation of the substrate. The bi- tion catalysis, we wanted to test whether addition reactions functional activity of the complexes is also demonstratedstoi- were aviable utility of theselithiumaluminate complexes, chiometrically in metallation of substrates containing an acidic thus we elected to react 3 with pyrazine. Both metallation and hydrogen atom, and in an addition reactionwith pyrazine. addition across pyrazine has previously been observed de- pending on the specificreagent(s) employed,[22] prompting a consideration of whether the basicity of 3,arising from the Experimental Section TMP ligand,orthe nucleophilicity of the Al Hfunctionality À would win out. Atoluene/n-hexane solution of 3 was stirred Full experimental characterisation and synthetic procedures are de- with one equivalent of pyrazineatroom temperature for 2h scribed in the supporting information. resultinginformationofapale brown oil. 1HNMR spectra of the oil revealed clean formation of asingle product, displaying four resonancesbetween d=7.3–4.0 ppm, in a1:1:1:2 ratio Acknowledgements consistentwith hydride addition at the a-carbon atom (Scheme 8). Importantly this result indicates that as well as The authors thank the EPSRC (grant award EP/N011384/1, DTP being competent hydroboration catalysts, these complexes studentship to S.A.O.,and vacation bursarytoN.R.J.) and demonstrate great versatility as either metallating agentsfor GeorgeFraser (scholarship to S.A.O.) for their generous spon- acidic hydrogen atoms, or as HÀ sourcesinaddition reactions sorship of this research. REM thanks the Alexander von Hum- across pyrazine. Furthermore, it detailsthe fine balance in boldt foundation foraHumboldtResearch Award. L. E. L. is these systems on the ensuing reactivity,which is highly sub- gratefultothe Deutscher AkademischerAustauschdienst strate dependent. (DAAD) for aresearch fellowship. The dataset underlying this researchcan be located at https://doi.org/10.15129/5fc161a9- 08a7-4bd5-839a-164fa5660576.

Chem. Eur.J.2018, 24,9940 –9948 www.chemeurj.org 9947 2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper

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