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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 hy- dride surrogate, dihydropyridine precatalyst, 1-Li-2-tBu-1,2-di- hydropyridine.[8] The Okuda group also recentlyreported a series of alkali metal hydridotriphenylborates,[9] that showed the nature of the Lewis donor (including flexibility of coordina- tion 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 en- suing reactivity.Given this, here we have examined for the first time structure–activity relationshipsofdonorsolvated hetero- leptic 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
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