Abstract
The lithium TMP-aluminate bases "LiTMP·Al(iBu) 3" 1 and "LiTMP·Al(TMP)(iBu) 2" 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as "aluminating" reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)·Li(μ-TMP)(μ-iBu)Al( iBu)2] 1·THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1·THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1·THF to complex equilibria involving five major species in LiTMP·THF, Al( iBu)3·THF, [{Li(THF)4} +{Al(TMP)(iBu)3}-] 1·(THF)4, [(THF)·Li(μ-TMP)(μ-OC4H 7)Al(iBu)2], 4, and (TMP)Al( iBu)2·THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP·THF and (TMP)Al( iBu)2·THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.
| Language | English |
|---|---|
| Pages | 3031-3045 |
| Number of pages | 15 |
| Journal | Chemical Science |
| Volume | 5 |
| Issue number | 8 |
| Early online date | 7 May 2014 |
| DOIs | |
| Publication status | Published - 1 Aug 2014 |
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Keywords
- aluminium–hydrogen exchange
- x-ray crystallography
- lithium-mediated alumination
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TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases : lithium-mediated alumination or lithiation-alkylaluminium-trapping reagents? / Armstrong, David R.; Crosbie, Elaine; Hevia, Eva; Mulvey, Robert E.; Ramsay, Donna L.; Robertson, Stuart D.
In: Chemical Science, Vol. 5, No. 8, 01.08.2014, p. 3031-3045.Research output: Contribution to journal › Article
TY - JOUR
T1 - TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases
T2 - Chemical Science
AU - Armstrong, David R.
AU - Crosbie, Elaine
AU - Hevia, Eva
AU - Mulvey, Robert E.
AU - Ramsay, Donna L.
AU - Robertson, Stuart D.
PY - 2014/8/1
Y1 - 2014/8/1
N2 - The lithium TMP-aluminate bases "LiTMP·Al(iBu) 3" 1 and "LiTMP·Al(TMP)(iBu) 2" 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as "aluminating" reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)·Li(μ-TMP)(μ-iBu)Al( iBu)2] 1·THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1·THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1·THF to complex equilibria involving five major species in LiTMP·THF, Al( iBu)3·THF, [{Li(THF)4} +{Al(TMP)(iBu)3}-] 1·(THF)4, [(THF)·Li(μ-TMP)(μ-OC4H 7)Al(iBu)2], 4, and (TMP)Al( iBu)2·THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP·THF and (TMP)Al( iBu)2·THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.
AB - The lithium TMP-aluminate bases "LiTMP·Al(iBu) 3" 1 and "LiTMP·Al(TMP)(iBu) 2" 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as "aluminating" reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)·Li(μ-TMP)(μ-iBu)Al( iBu)2] 1·THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1·THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1·THF to complex equilibria involving five major species in LiTMP·THF, Al( iBu)3·THF, [{Li(THF)4} +{Al(TMP)(iBu)3}-] 1·(THF)4, [(THF)·Li(μ-TMP)(μ-OC4H 7)Al(iBu)2], 4, and (TMP)Al( iBu)2·THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP·THF and (TMP)Al( iBu)2·THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.
KW - aluminium–hydrogen exchange
KW - x-ray crystallography
KW - lithium-mediated alumination
UR - http://www.scopus.com/inward/record.url?scp=84903713284&partnerID=8YFLogxK
UR - http://pubs.rsc.org/en/Journals/JournalIssues/SC
U2 - 10.1039/c4sc01108b
DO - 10.1039/c4sc01108b
M3 - Article
VL - 5
SP - 3031
EP - 3045
JO - Chemical Science
JF - Chemical Science
SN - 2041-6520
IS - 8
ER -