Regioselective magnesiation of N-heterocyclic molecules: Securing insecure cyclic anions by a β-diketiminate-magnesium clamp

Article abstract of DOI:10.1039/c6cc09675a

Using a specially designed magnesium metallating manifold, combining kinetically activated TMP amide base with a sterically amplified β-diketiminate ligand, this study has established a new regioselective strategy for magnesiation of challenging N-heteroc

Full text of DOI:10.1039/c6cc09675a

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McLellan, A. Hernán-Gómez, B. Clegg, A. R. Kennedy, I. Stepek, M. Mertens and E. Hevia, Chem.
Commun., 2017, DOI: 10.1039/C6CC09675A.
Volume 52 Number
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January 2016 Pages 1–216
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Pleaseꢀd Co hꢀ ne omt ꢀ Ca do jmu smt ꢀmarginsꢀ
View Article Online
DOI: 10.1039/C6CC09675A
JournalꢀNameꢀ
COMMUNICATIONꢀ
ꢀ
RegioselectiveꢀmagnesiationꢀofꢀN-heterocyclicꢀmolecules:ꢀsecuringꢀ
insecureꢀcyclicꢀanionsꢀbyꢀaꢀβ-diketiminate-magnesiumꢀclampꢀꢀ
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
a
a
a
b
a
LaiaꢀDavin, ꢀRossꢀMcLellan, ꢀAlbertoꢀHernán-Gómez, ꢀWilliamꢀClegg, ꢀAlanꢀR.ꢀKennedy, ꢀMariaꢀ
a
a
a
Mertens, ꢀIainꢀStepek, ꢀandꢀEvaꢀHevia* ꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
www.rsc.org/ꢀ
Usingꢀ aꢀ speciallyꢀ designedꢀ magnesiumꢀ metallatingꢀ manifold,ꢀ Mg(TMP)2.2LiCl,ꢀtheꢀadditionalꢀpresenceꢀofꢀZnCl2ꢀisꢀcriticalꢀforꢀ
combiningꢀkineticallyꢀactivatedꢀTMPꢀamideꢀbaseꢀwithꢀaꢀstericallyꢀ theꢀ successꢀ ofꢀ theꢀ reaction,ꢀ asꢀ itꢀ facilitatesꢀ theꢀ inꢀ situꢀ
amplifiedꢀβ-diketiminateꢀligand,ꢀthisꢀstudyꢀhasꢀestablishedꢀaꢀnewꢀ formationꢀ ofꢀ heteroarylzincꢀ species,ꢀ whichꢀ areꢀ significantlyꢀ
regioselectiveꢀ strategyꢀ forꢀ magnesiationꢀ ofꢀ challengingꢀ N- moreꢀ stabilisedꢀ (dueꢀ toꢀ theꢀ lowerꢀ polarityꢀ ofꢀ theirꢀ metal-
heterocyclicꢀ molecules.ꢀ Theꢀ broadꢀ scopeꢀ ofꢀ theꢀ approachꢀ isꢀ carbonꢀ
bonds)ꢀ
thanꢀ
correspondingꢀ
magnesiatedꢀ
6ꢀ
illustratedꢀthroughꢀreactionsꢀofꢀpyrazine,ꢀtriazolesꢀandꢀsubstitutedꢀ intermediates. Theꢀ synergisticꢀ characterꢀ ofꢀ theseꢀ systemsꢀ
pyridinesꢀ byꢀ isolationꢀ andꢀ structuralꢀ elucidationꢀ ofꢀ theirꢀ thereforeꢀinvolvesꢀthreeꢀdistinctꢀmetals.ꢀꢀ
magnesiatedꢀintermediates.ꢀ
Recentlyꢀweꢀhaveꢀshownꢀthatꢀitꢀisꢀpossibleꢀtoꢀpromoteꢀdirectꢀ
Mg-Hꢀ exchangeꢀ reactionsꢀ ofꢀ 1,3-benzoazolesꢀ usingꢀ bespokeꢀ
Dipp
monomericꢀ amideꢀ [( Nacnac)Mg(TMP)]ꢀ (1)ꢀ ( Nacnac=ꢀ
Dipp
N-heterocyclicꢀ aromaticꢀ moleculesꢀ suchꢀ asꢀ diazines,ꢀ triazolesꢀ
andꢀ pyridinesꢀ areꢀ essentialꢀ fundamentalꢀ buildingꢀ blocksꢀ inꢀ
synthesis,ꢀpresentꢀinꢀaꢀmultitudeꢀofꢀpharmaceuticallyꢀrelevantꢀ
i
Ar*NC(Me)CHC(Me)NAr*;ꢀAr*ꢀ=ꢀ2,6- Pr2-C6H3)ꢀwhichꢀcombinesꢀ
aꢀ basicꢀ andꢀ kineticallyꢀ activatedꢀ TMPꢀ groupꢀ withꢀ aꢀ stericallyꢀ
7
amplifiedꢀinnocentꢀβ-diketiminateꢀligand. ꢀꢀ
1
andꢀ biologicallyꢀ activeꢀ molecules. ꢀ Deprotonativeꢀ metallationꢀ
constitutesꢀ oneꢀ ofꢀ theꢀ mostꢀ powerfulꢀ methodologiesꢀ forꢀ
installingꢀ theseꢀ ringꢀ systemsꢀ intoꢀ moreꢀ complexꢀ molecularꢀ
2
frameworks. ꢀ Lithiumꢀ basesꢀ suchꢀ asꢀ LiTMPꢀ (TMP=ꢀ 2,2,6,6-
Introducingꢀ aꢀ newꢀ syntheticꢀ applicationꢀ ofꢀ β-diketiminateꢀ
stabilisedꢀ magnesiumꢀ complexes,ꢀ hereꢀ weꢀ extendꢀ thisꢀ
approachꢀ intoꢀ aꢀ moreꢀ taxingꢀ territory.ꢀ Thusꢀ weꢀ describeꢀ theꢀ
successfulꢀ regioselectiveꢀ magnesiationꢀ ofꢀ challengingꢀ
syntheticallyꢀrelevantꢀN-heterocyclicꢀsubstrates,ꢀdisclosingꢀtheꢀ
abilityꢀofꢀtheseꢀmagnesiumꢀsystemsꢀtoꢀgenerate,ꢀstabiliseꢀandꢀ
entrapꢀsensitiveꢀanionsꢀofꢀtheꢀretainedꢀringꢀsystems.ꢀ
ꢀ
tetramethylpiperidide)ꢀareꢀtheꢀclassicalꢀreagentsꢀofꢀchoice,ꢀbutꢀ
theirꢀuseꢀimposesꢀsevereꢀlimitations.ꢀThisꢀincludesꢀtheꢀneedꢀforꢀ
o
extremelyꢀ lowꢀ temperaturesꢀ (-78 C),ꢀ largeꢀ excessesꢀ ofꢀ theꢀ
lithiumꢀbaseꢀandꢀinꢀsituꢀelectrophilicꢀinterceptionsꢀinꢀorderꢀtoꢀ
cutꢀ downꢀ sideꢀ reactions,ꢀ suchꢀ asꢀ fragmentationꢀ orꢀ additionꢀ
processes,ꢀ arisingꢀ fromꢀ theꢀ instabilityꢀ ofꢀ theꢀ involvedꢀ highlyꢀ
N
N
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Ar*
3
Ar*
Ar*
reactiveꢀ heteroarylꢀ lithiumꢀ intermediates. ꢀ Bimetallicꢀ
N
N
N
N
N
Mg
N
combinations,ꢀwhichꢀpairꢀLiꢀwithꢀaꢀsofterꢀmetalꢀsuchꢀasꢀZn,ꢀAlꢀ
orꢀ Gaꢀ inꢀ differentꢀ ligandꢀ setsꢀ haveꢀ shownꢀ greaterꢀ promise,ꢀ
improvingꢀtheꢀstabilitiesꢀofꢀtheꢀorganometallicꢀintermediates,ꢀ
allowingꢀtheseꢀsynergisticꢀmetallationsꢀtoꢀbeꢀperformedꢀunderꢀ
Mg
Mg
THF, rt
N
N
N
N
Ar*
Ph
1
Ar*
Ar*
N
N
2
N
Me
N
N
Ph
THF, rt
N
N
Ph
TH
N
F, rt
4
Ph
Ar*
N
milderꢀ conditions. ꢀ Contrastingly,ꢀ magnesiumꢀ reagentsꢀ haveꢀ
Me
N
Ar*
Ar* N
Ar*
shownꢀlittleꢀpromise,ꢀasꢀtheirꢀlimitedꢀpolarityꢀleadsꢀtoꢀaꢀlimitedꢀ
reactivity,ꢀinsufficientꢀtoꢀproduceꢀmetallationꢀofꢀweaklyꢀacidicꢀ
C-Hꢀ bonds,ꢀ thoughꢀ someꢀ progressꢀ hasꢀ beenꢀ madeꢀ usingꢀ
N
N
N
N
N
N
Mg
Mg
Mg
N
Mg
N
N
Ar* N
N
N
N
Ar*
Ph
5
Ar*
Ar*
Me
N
N
kineticallyꢀ
Notwithstanding,ꢀevenꢀwhenꢀusingꢀtheꢀLiCl-poweredꢀMgꢀamideꢀ
activatedꢀ
alkali-metalꢀ
magnesiates. ꢀ
N
4
3
Ph
Schemeꢀ1ꢀRegioselectiveꢀalpha-magnesiationꢀofꢀN-heterocyclicꢀsubstratesꢀbyꢀ1.ꢀ
a. a
WestCHEM,ꢀDepartmentꢀofꢀPureꢀandꢀAppliedꢀChemistry,ꢀUniversityꢀofꢀ
b. Strathclyde,ꢀGlasgow,ꢀG1ꢀ1XL,ꢀUK;ꢀe-mail:ꢀeva.hevia@strath.ac.uk.ꢀ
SchoolꢀofꢀChemistry,ꢀNewcastleꢀUniversity,ꢀNewcastleꢀuponꢀTyne,ꢀUK,ꢀNE1ꢀ7RU,ꢀ
UKꢀꢀ
Electronicꢀ Supplementaryꢀ Informationꢀ (ESI)ꢀ available:ꢀ [Experimentalꢀ procedures,ꢀ
NMRꢀ spectraꢀ andꢀ crystallographicꢀ data.ꢀ CCDCꢀ 1519748-1519752].ꢀ Seeꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
Focussingꢀfirstꢀonꢀtheꢀclassicalꢀnakedꢀdiazine,ꢀpyrazine,ꢀwhoseꢀ
metallationꢀ isꢀ particularlyꢀ difficultꢀ dueꢀ toꢀ itsꢀ tendencyꢀ toꢀ
3
undergoꢀ competitiveꢀ nucleophilicꢀ addition, ꢀ weꢀ examinedꢀ itsꢀ
reactionꢀ withꢀ equimolarꢀ amountsꢀ ofꢀ 1ꢀ atꢀ roomꢀ temperatureꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ1ꢀꢀ
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COMMUNICATIONꢀ
JournalꢀNameꢀ
whichꢀ rapidlyꢀ producedꢀ aꢀ darkꢀ greenꢀ solution.ꢀ Coolingꢀ thisꢀ Interestingly,ꢀillustratingꢀtheꢀkineticꢀresilienceꢀofꢀMViegw-CAꢀrvticslꢀeMOngli-nNe ꢀ
DOI: 10.1039/C6CC09675A
ofꢀ bonds,ꢀ itꢀ shouldꢀ beꢀ notedꢀ that,ꢀ whenꢀ pyrazineꢀ wasꢀ treatedꢀ
solutionꢀ
Dipp
affordedꢀ
lightꢀ
brownꢀ
crystalsꢀ
{( Nacnac)Mg(C4H3N2)}2]ꢀ(2)ꢀ(isolatedꢀyieldꢀ51%,ꢀnoteꢀNMRꢀ withꢀ theꢀ relatedꢀ butylꢀ baseꢀ [( Nacnac)Mg(Bu)(THF)],ꢀ theꢀ
Dipp
[
analysisꢀ ofꢀ theꢀ reactionꢀ filtrateꢀ showedꢀ 2ꢀ isꢀ obtainedꢀ formationꢀofꢀ2ꢀisꢀnotꢀobserved,ꢀaffordingꢀinsteadꢀdonorꢀadductꢀ
Dipp
quantitatively)ꢀ resultingꢀ fromꢀ selectiveꢀ C2ꢀ metallationꢀ ofꢀ [( Nacnac)Mg(Bu)(C4H4N2)].ꢀThisꢀrunsꢀcounterꢀtoꢀtheꢀnormalꢀ
pyrazineꢀ (Schemeꢀ 1).ꢀ NMRꢀ experimentsꢀ ofꢀ 2ꢀ inꢀ C6D6ꢀ patternꢀ ofꢀ reactivityꢀ exhibitedꢀ byꢀ conventionalꢀ bases,ꢀ asꢀ forꢀ
10ꢀ
establishedꢀtheꢀregioselectivityꢀofꢀtheꢀreaction,ꢀasꢀindicatedꢀbyꢀ exampleꢀBuLi,ꢀwhichꢀisꢀaꢀmuchꢀstrongerꢀbaseꢀthanꢀLiTMP.
theꢀ appearanceꢀ ofꢀ threeꢀ resonancesꢀ atꢀ 9.28,ꢀ 8.77ꢀ andꢀ 7.95ꢀ Movingꢀonꢀtoꢀfive-memberedꢀheterocyclicꢀtriazoles,ꢀwhichꢀareꢀ
1
ppmꢀ inꢀ theꢀ Hꢀ NMRꢀ spectrum,ꢀ consistentꢀ withꢀ theꢀ lackꢀ ofꢀ widelyꢀ usedꢀ inꢀ medicinalꢀ chemistry,ꢀ theirꢀ lithiationꢀ canꢀ beꢀ
symmetryꢀ inꢀ theꢀ pyrazinylꢀ fragment.ꢀ Determinedꢀ byꢀ X-rayꢀ accomplishedꢀbyꢀstrictꢀcontrolꢀofꢀtheꢀtemperature,ꢀfollowedꢀbyꢀ
11
crystallography,ꢀ theꢀ centrosymmetricꢀ dimericꢀ structureꢀ ofꢀ 2ꢀ electrophilicꢀ interception. ꢀ Contrastingly,ꢀ atꢀ roomꢀ
Figꢀ 1)ꢀ confirmedꢀ theꢀ C2-magnesiationꢀ ofꢀ theꢀ N-heterocycle,ꢀ temperatureꢀ theseꢀ intermediatesꢀ tendꢀ toꢀ undergoꢀ
(
withꢀ theꢀ metallatedꢀ pyrazinylꢀ ringsꢀ actingꢀ asꢀ asymmetricꢀ fragmentation,ꢀ eliminatingꢀ nitrogen,ꢀ affordingꢀ insteadꢀ N-
bridgesꢀ viaꢀ theirꢀ Nꢀ andꢀ C2ꢀ atomsꢀ (i.e.ꢀ N3ꢀ andꢀ C33ꢀ inꢀ Figꢀ 1a)ꢀ phenylalkynamideꢀ lithiumꢀ complexes,ꢀ asꢀ forꢀ exampleꢀ inꢀ theꢀ
Dip
betweenꢀtwoꢀ{( Nacnac)Mg}ꢀfragments.ꢀꢀ
12
caseꢀofꢀ1,4-diphenyl-1,2,3-triazole. ꢀTakingꢀthisꢀsubstrateꢀasꢀaꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
caseꢀstudy,ꢀweꢀreactedꢀitꢀatꢀroomꢀtemperatureꢀwithꢀoneꢀmolarꢀ
equivalentꢀofꢀ1ꢀinꢀTHF,ꢀwhichꢀresultedꢀinꢀtheꢀformationꢀofꢀ3,ꢀ
whereꢀ theꢀ triazoleꢀ hasꢀ beenꢀ magnesiatedꢀ atꢀ itsꢀ C5ꢀ positionꢀ
(isolatedꢀcrystallineꢀyield:ꢀ45%).ꢀThisꢀapproachꢀalsoꢀworksꢀwellꢀ
forꢀ 1-methyl-1,2,4-triazole,ꢀ aꢀ substrateꢀ furnishingꢀ 4ꢀ asꢀ aꢀ
crystallineꢀ solidꢀ inꢀ aꢀ 66%ꢀ yieldꢀ (Schemeꢀ 1).ꢀ Weꢀ previouslyꢀ
reportedꢀtheꢀmetallationꢀofꢀ1-methyl-1,2,4-triazoleꢀusingꢀate-
activatedꢀ baseꢀ [(PMDETA)2K2Mg(CH2SiMe3)4]ꢀ followedꢀ byꢀ
electrophilicꢀinterceptionꢀwithꢀI2;ꢀhowever,ꢀattemptsꢀtoꢀisolateꢀ
orꢀ characterizeꢀ theꢀ organometallicꢀ intermediateꢀ provedꢀ
13
unsuccessful. ꢀ Theꢀ molecularꢀ structuresꢀ ofꢀ 3ꢀ andꢀ 4ꢀ mimicꢀ
manyꢀ ofꢀ theꢀ featuresꢀ ofꢀ 2,ꢀ possessingꢀ dimericꢀ arrangementsꢀ
whereꢀ theꢀ triazolylꢀ anionsꢀ connectꢀ theꢀ Mgꢀ centresꢀ throughꢀ
theirꢀ C=Nꢀ junction,ꢀ givingꢀ riseꢀ toꢀ aꢀ six-memberedꢀ
ꢀ
Figꢀ1ꢀMolecularꢀstructureꢀofꢀ2.ꢀDisplacementꢀellipsoidsꢀareꢀdisplayedꢀatꢀtheꢀ35%ꢀ
probabilityꢀlevelꢀandꢀhydrogenꢀatomsꢀareꢀomittedꢀforꢀclarity.ꢀCarbonꢀatomsꢀofꢀ
Dipp
NacNacꢀframeworkꢀareꢀpicturedꢀasꢀcappedꢀsticksꢀwithꢀtranslucentꢀspace-fillingꢀ
vanꢀderꢀWaalsꢀsurfacesꢀforꢀaꢀprobeꢀofꢀ1.5ꢀÅꢀradius.ꢀ
{MgNCMgNC}ꢀring,ꢀwithꢀeachꢀMgꢀclampedꢀbyꢀaꢀβ-diketiminateꢀ
ligand.ꢀAsꢀdiscussedꢀforꢀ2,ꢀtheseꢀdualꢀsigmaꢀMg-C/dativeꢀMg-Nꢀ
bondingꢀ modesꢀ mustꢀ significantlyꢀ contributeꢀ toꢀ theꢀ
entrapmentꢀandꢀstabilizationꢀofꢀtheseꢀsensitiveꢀanions,ꢀwhichꢀ
Theꢀ roomꢀ temperature,ꢀ stoichiometricꢀ regioselectiveꢀ controlꢀ
ofꢀ thisꢀ reactionꢀ contrastsꢀ sharplyꢀ withꢀ previousꢀ lithiationꢀ
studiesꢀwhereꢀevenꢀusingꢀaꢀ4ꢀmolarꢀequivalentꢀexcessꢀofꢀLiTMPꢀ
despiteꢀtheirꢀsyntheticꢀrelevance,ꢀasꢀfarꢀasꢀweꢀareꢀaware,ꢀtheirꢀ
structuresꢀhaveꢀremainedꢀelusive.ꢀꢀ
o
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
atꢀ -75 C,ꢀ theꢀ relevantꢀ 2-substitutedꢀ derivativesꢀ areꢀ obtainedꢀ
onlyꢀinꢀmodestꢀyieldsꢀ(39-65%)ꢀmixedꢀinꢀsomeꢀcasesꢀwithꢀ2,5-
8
disubstitutedꢀ product. ꢀ Evenꢀ moreꢀ surprisingly,ꢀ highlightingꢀ
theꢀ inherentꢀ lackꢀ ofꢀ stabilityꢀ ofꢀ diazinylꢀ magnesiatedꢀ
intermediates,ꢀ Knochelꢀ hasꢀ shownꢀ thatꢀ whenꢀ relatedꢀ
quinoxalineꢀ isꢀ treatedꢀ withꢀ Mg(TMP)2.2LiClꢀ onlyꢀ tracesꢀ ofꢀ C2ꢀ
metallationꢀ areꢀ observedꢀ alongꢀ withꢀ theꢀ formationꢀ ofꢀ
6
homocoupledꢀproduct. ꢀInꢀ2,ꢀtheꢀemergingꢀinsecureꢀpyrazinylꢀ
anionꢀ canꢀ beꢀ trappedꢀ inꢀ itsꢀ crystallineꢀ formꢀ atꢀ roomꢀ
temperatureꢀ andꢀ securedꢀ byꢀ theꢀ low-coordinateꢀ
Dipp
{
(
Nacnac)Mg}ꢀ fragment,ꢀ whereꢀ theꢀ β-diketiminateꢀ ligandꢀ
ꢀ
ꢀ
actsꢀasꢀaꢀprotectiveꢀshelterꢀtowardsꢀtheꢀnewlyꢀgeneratedꢀMg-Cꢀ
bondsꢀ (Figꢀ 1).ꢀ Furthermore,ꢀ asꢀ revealedꢀ byꢀ theꢀ structuralꢀ
Figꢀ 2.ꢀ Molecularꢀ structureꢀ ofꢀ 3ꢀ (left)ꢀ andꢀ 4ꢀ (right)ꢀ withꢀ 50%ꢀ probabilityꢀ
i
studies,ꢀtheꢀuniqueꢀdimericꢀmotifꢀofꢀ2,ꢀwithꢀtheꢀα-nitrogenꢀofꢀ displacementꢀ ellipsoidsꢀ (inꢀ bothꢀ figuresꢀ hydrogenꢀ atomsꢀ andꢀ Prꢀ groupsꢀ haveꢀ
beenꢀomittedꢀforꢀclarity).ꢀ
theꢀ pyrazinylꢀ anionꢀ formingꢀ aꢀ stabilizingꢀ dativeꢀ Mg-Nꢀ
interactionꢀwithꢀaꢀMgꢀcentreꢀofꢀaꢀneighbouringꢀunit,ꢀfacilitatingꢀ
Whileꢀ2-4ꢀareꢀproductsꢀofꢀalpha-magnesiation,ꢀwithꢀtheꢀCꢀatomꢀ
furtherꢀstabilization,ꢀminimizingꢀelectronicꢀrepulsionꢀbetweenꢀ
experiencingꢀ theꢀ metallationꢀ adjacentꢀ toꢀ aꢀ Nꢀ atomꢀ whichꢀ inꢀ
theꢀNꢀloneꢀpairꢀandꢀtheꢀnegativeꢀchargeꢀofꢀtheꢀnewꢀcarbanion.ꢀ
turnꢀ engagesꢀ withꢀ aꢀ neighbouringꢀ unitꢀ viaꢀ N…Mgꢀ dativeꢀ
Chargeꢀdensityꢀinvestigationsꢀhaveꢀquantifiedꢀtheꢀcontributionꢀ
interactions,ꢀstudiesꢀonꢀC2-substitutedꢀpyridinesꢀrevealedꢀthatꢀ
ofꢀ relatedꢀ Mg-Nꢀ interactionsꢀ inꢀ theꢀ bondingꢀ presentꢀ inꢀ
9ꢀ
1ꢀ canꢀ promoteꢀ regioselectiveꢀ lateralꢀ metallationꢀ ofꢀ theseꢀ N-
[
Mg[(pz*)3C}2]ꢀ(pz*=3,5-dimethylpyrazolyl).
heterocyclicꢀ substratesꢀ (Schemeꢀ 2).ꢀ Thusꢀ reactionsꢀ withꢀ 2-
2
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
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ꢀCOMMUNICATIONꢀ
picolineꢀ andꢀ 2-(2,4-difluorophenyl)pyridineꢀ (ppf),ꢀ underꢀ theꢀ atomꢀinꢀtheꢀfluorinatedꢀringꢀ[i.e.ꢀC34ꢀinꢀFigꢀ3]ꢀisꢀnowꢀfilledꢀbyꢀaꢀ
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Dip
DOI: 10.1039/C6CC09675A
sameꢀconditionsꢀpreviouslyꢀdescribedꢀforꢀtheꢀsynthesisꢀofꢀ2-4,ꢀ {( Nacnac)Mg}ꢀfragment,ꢀtheꢀMgꢀofꢀwhichꢀisꢀalsoꢀsolvatedꢀbyꢀ
ledꢀtoꢀtheꢀisolationꢀofꢀ5ꢀandꢀ6ꢀinꢀ67ꢀandꢀ68%ꢀcrystallineꢀyield,ꢀ aꢀTHFꢀmolecule.ꢀInꢀthisꢀcase,ꢀtheꢀremoteꢀlocationꢀofꢀtheꢀpyridylꢀ
1
respectively.ꢀ Hꢀ NMRꢀ monitoringꢀ ofꢀ theseꢀ reactionsꢀ inꢀ Nꢀ precludesꢀ theꢀ formationꢀ ofꢀ dimers.ꢀ Theꢀ regioselectivityꢀ
deuteratedꢀ THFꢀ demonstratedꢀ thatꢀ formationꢀ ofꢀ 5ꢀ (resultingꢀ observedꢀisꢀconsistentꢀwithꢀmetallationꢀatꢀtheꢀmostꢀacidicꢀsiteꢀ
fromꢀtheꢀmagnesiationꢀofꢀtheꢀmethylꢀgroupꢀinꢀ2-picoline)ꢀandꢀ6ꢀ ofꢀ theꢀ molecule,ꢀ leavingꢀ theꢀ pyridineꢀ ringꢀ untouched.ꢀꢀ
(whereꢀ ppfꢀ isꢀ metallatedꢀ atꢀ theꢀ C3ꢀ positionꢀ ofꢀ theꢀ Althoughꢀ thisꢀ regioselectivityꢀ hasꢀ beenꢀ previouslyꢀ describedꢀ
fluoroaromaticꢀring),ꢀoccursꢀquantitatively,ꢀwithoutꢀobservingꢀ forꢀ organolithiumꢀ basesꢀ (whichꢀ haveꢀ toꢀ beꢀ employedꢀ atꢀ lowꢀ
otherꢀregioisomersꢀinꢀsolution.ꢀꢀ
ꢀ
temperaturesꢀ toꢀ avoidꢀ LiFꢀ eliminationꢀ andꢀ benzyneꢀ
19
formation), ꢀ6ꢀconstitutesꢀtheꢀfirstꢀexampleꢀwhereꢀthisꢀanionꢀ
hasꢀbeenꢀisolatedꢀandꢀstructurallyꢀcharacterized.ꢀThisꢀreactivityꢀ
contrastsꢀwithꢀthoseꢀreportedꢀforꢀtransitionꢀmetalsꢀsuchꢀasꢀIrꢀ
whereꢀppfꢀundergoesꢀcyclometallation,ꢀremovingꢀtheꢀHꢀatꢀtheꢀ
Ar*
Mg
*Ar
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Ar*
N
N
N
R= CH
i)
3
N
N
N
Mg
C
H
C
H
(
Mg
N
2
2
N
N
20
Ar*
Ar*
*Ar
C5ꢀ positionꢀ ofꢀ theꢀ fluorinatedꢀ ringꢀ inꢀ ppf. ꢀ Theseꢀ findingsꢀ
showꢀtheꢀpotentialꢀofꢀ1ꢀtoꢀactꢀasꢀaꢀregioselectiveꢀbaseꢀnotꢀonlyꢀ
5
1
Ar*
+
F
THF, rt
R
R=
forꢀN-heterocyclicꢀsubstratesꢀbutꢀalsoꢀforꢀsubstitutedꢀbenzeneꢀ
andꢀrelatedꢀringꢀsystems.ꢀꢀ
N
N
THF
F
F
Mg
N
(ii)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
N
F
Ar*
6
Schemeꢀ2.ꢀLateralꢀmagnesiationꢀofꢀ2-substitutedꢀpyridinesꢀbyꢀ1ꢀ
Synchrotronꢀ X-rayꢀ crystallographicꢀ studiesꢀ confirmedꢀ theꢀ
lateralꢀdeprotonationꢀofꢀtheseꢀheterocyclicꢀsubstratesꢀ(Figꢀ3).ꢀ
Picolineꢀ derivativeꢀ 5ꢀ displaysꢀ aꢀ dimericꢀ motif,ꢀ reminiscentꢀ ofꢀ
thoseꢀ describedꢀ aboveꢀ forꢀ 2-4,ꢀ whereꢀ twoꢀ Mgꢀ centresꢀ areꢀ
connectedꢀ byꢀ twoꢀ newlyꢀ formedꢀ 2-picolylꢀ anions,ꢀ usingꢀ theirꢀ Figꢀ 3ꢀ Molecularꢀ structureꢀ ofꢀ 5ꢀ (left)ꢀ andꢀ 6ꢀ (right)ꢀ withꢀ 50%ꢀ probabilityꢀ
i
displacementꢀ ellipsoidsꢀ (inꢀ bothꢀ figuresꢀ hydrogenꢀ atomsꢀ andꢀ Prꢀ groupsꢀ haveꢀ
CH2ꢀ andꢀ Nꢀ groups,ꢀ generatingꢀ anꢀ internalꢀ eight-memberedꢀ beenꢀomittedꢀforꢀclarity).ꢀ
{MgCCNMgCCN}ꢀ ringꢀ whichꢀ adoptsꢀ aꢀ pseudo-boatꢀ
conformation.ꢀInꢀthisꢀcase,ꢀbothꢀpicolylꢀanionsꢀpointꢀupwardsꢀ Initialꢀreactivityꢀstudiesꢀofꢀ6ꢀwithꢀiodineꢀandꢀdeuteratedꢀwaterꢀ
inꢀ aꢀ splayedꢀ openꢀ arrangementꢀ [dihedralꢀ angleꢀ betweenꢀ theꢀ demonstratedꢀthatꢀtheꢀnewꢀC-Mgꢀbondsꢀinꢀtheseꢀsystemsꢀareꢀ
o
pyridineꢀ ringsꢀ isꢀ 73.38(7) ],ꢀ ratherꢀ thanꢀ lyingꢀ approximatelyꢀ accessibleꢀtoꢀelectrophiles,ꢀaffordingꢀproductsꢀ7ꢀandꢀ8ꢀinꢀyieldsꢀ
coplanarꢀasꢀseenꢀinꢀ2-4.ꢀThisꢀalternativeꢀgeometryꢀappearsꢀtoꢀ ofꢀ65ꢀandꢀ72%ꢀrespectivelyꢀ(seeꢀESIꢀforꢀfullꢀdetails),ꢀasꢀaꢀresultꢀ
beꢀ imposedꢀ byꢀ theꢀ picolylꢀ anionꢀ bondingꢀ mode,ꢀ withꢀ theꢀ Nꢀ ofꢀtheꢀincorporationꢀofꢀiodineꢀorꢀdeuteriumꢀatꢀtheꢀC3ꢀpositionꢀ
atomꢀboundꢀtoꢀoneꢀMgꢀwhileꢀtheꢀCH2ꢀbindsꢀtoꢀtheꢀotherꢀMg.ꢀ ofꢀtheꢀphenylꢀringꢀinꢀppfꢀꢀ
Previousꢀstructuralꢀandꢀtheoreticalꢀstudiesꢀhaveꢀuncoveredꢀtheꢀ Inꢀ summary,ꢀ aꢀ newꢀ improvedꢀ strategyꢀ forꢀ theꢀ regioselectiveꢀ
diversityꢀ ofꢀ feasibleꢀ electronicꢀ situationsꢀ inꢀ 2-picolylꢀ anionsꢀ metallationꢀ ofꢀ challengingꢀ N-heterocycilcꢀ moleculesꢀ isꢀ
resultingꢀ fromꢀ theꢀ delocalisationꢀ ofꢀ theꢀ negativeꢀ chargeꢀ intoꢀ described.ꢀ Designedꢀ forꢀ enhancedꢀ reactivityꢀ throughꢀ aꢀ singleꢀ
theꢀ ringꢀ fromꢀ theꢀ carbanion,ꢀ givingꢀ riseꢀ toꢀ differentꢀ bondingꢀ Mg-TMPꢀ bondꢀ andꢀ forꢀ exceptionalꢀ selectivityꢀ throughꢀ aꢀ β-
14
modesꢀ toꢀ metal. ꢀ Forꢀ exampleꢀ 2-picolyllithiumꢀ complexesꢀ diketiminato-shieldedꢀ Mgꢀ centre,ꢀ monomericꢀ baseꢀ 1ꢀ allowsꢀ
formꢀ dimericꢀ structuresꢀ whereꢀ theꢀ anionsꢀ actꢀ asꢀ aza-allylꢀ theꢀtrappingꢀandꢀstructuralꢀelucidationꢀofꢀtheꢀnewlyꢀgeneratedꢀ
15
ligandsꢀ(simultaneouslyꢀformingꢀLi-CꢀandꢀLi-Nꢀbonds) ꢀwhereasꢀ N-heterocyclicꢀ anions.ꢀ Theseꢀ findingsꢀ provideꢀ theꢀ firstꢀ
lithiumꢀ derivativesꢀ ofꢀ Cα-substitutedꢀ picolinesꢀ haveꢀ shownꢀ structuralꢀinsightꢀintoꢀtheirꢀconstitution,ꢀwhichꢀcorrelatesꢀwellꢀ
16
bothꢀ aza-allylꢀ andꢀ enamidoꢀ bondingꢀ modes, ꢀ withꢀ theꢀ latterꢀ withꢀtheirꢀunprecedentedꢀstabilityꢀatꢀroomꢀtemperature.ꢀꢀ
usingꢀexclusivelyꢀitsꢀNꢀatomꢀtoꢀbondꢀtoꢀLi.ꢀInterestingly,ꢀaꢀcloseꢀ
ꢀ
inspectionꢀ ofꢀ theꢀ geometricalꢀ parametersꢀ ofꢀ 5ꢀ revealedꢀ thatꢀ WeꢀareꢀgratefulꢀtoꢀtheꢀERCꢀ(Stg-2011ꢀMixMetAppsꢀtoꢀEH)ꢀforꢀ
picolylꢀ anionsꢀ canꢀ beꢀ bestꢀ describedꢀ asꢀ carbanionicꢀ ligands,ꢀ theirꢀ generousꢀ sponsorshipꢀ ofꢀ thisꢀ researchꢀ andꢀ toꢀ Diamondꢀ
althoughꢀsomeꢀdegreeꢀofꢀdelocalisationꢀofꢀtheꢀnegativeꢀchargeꢀ LightꢀSourceꢀforꢀaccessꢀtoꢀbeamlineꢀI19.ꢀ
overꢀtheꢀNꢀatomꢀisꢀalsoꢀevidentꢀasꢀindicatedꢀbyꢀtheꢀrelativelyꢀ
shortꢀ Mg-Nꢀ bondꢀ distancesꢀ [meanꢀ valueꢀ 2.1226ꢀ Å],ꢀ whenꢀ
comparedꢀ toꢀ thatꢀ foundꢀ byꢀ Hillꢀ forꢀ theꢀ coordinationꢀ adductꢀ
Notesꢀandꢀreferencesꢀ
Dip
17
Nacnac)Mg(Bu)(2-picoline)]ꢀ[Mg-N,ꢀ2.216ꢀ(4)ꢀÅ]. ꢀ
[
(
1
ꢀ(a)ꢀ A.ꢀ F.ꢀ Pozharskii,ꢀ A.ꢀ Soldatenkovꢀ andꢀ A.ꢀ R.ꢀ Katritzky,ꢀ
Heterocyclesꢀ inꢀ Lifeꢀ andꢀ Society:ꢀ Anꢀ Introductionꢀ toꢀ
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severalꢀ applicationsꢀ inꢀ materialsꢀ scienceꢀ asꢀ aꢀ precursorꢀ forꢀ
18
organicꢀlightꢀemittingꢀdiodesꢀ(OLEDs), ꢀaffordsꢀmonomericꢀ6.ꢀ
Inꢀ 6ꢀtheꢀ positionꢀ previouslyꢀ occupiedꢀ byꢀ aꢀ Hꢀ atomꢀ atꢀ theꢀ C3ꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ3ꢀ
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Pleaseꢀd Co hꢀ ne omt ꢀ Ca do jmu smt ꢀmarginsꢀ
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COMMUNICATIONꢀ
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ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
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