Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolisable ketones: structural characterisation of [{ButC(=CH2)OMgBr.HMPA}2] and [MgBr2.(HMPA)2]

J.F. Allan, W. Clegg, K.W. Henderson, L. Horsburgh, A.R. Kennedy

Research output: Contribution to journalArticle

25 Citations (Scopus)

Abstract

The Hauser base reagents Pri2NMgCl 1 and Pri2NMgBr 2 react with a variety of enolisable ketones to yield magnesium enolates. Attempts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solvated dihalide salts (MgX2 · Sx, where X=Cl or Br and S=THF, TMEDA or HMPA). Using diethyl ether as solvent media and one molar equivalent of HMPA, the halomagnesium enolate compounds [{ButC(=CH2)OMgBr · HMPA}2] 3 and [Me2CHC(=CMe2)OMgBr · HMPA] 5 were isolated and identified. Both 3 and 5 precipitate as mixtures with the dihalide salt [MgBr2 · (HMPA)2] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A molecular-orbital theoretical study (HF/6-31G*) was conducted to determine the relative bridging abilities of several model anions. The enolate anion H(CH2=)CO− was determined to be a favoured bridge in preference to the halides F−, Cl− and Br−, which is consistent with the X-ray evidence. The amido anions Me2N−, (H3Si)2N− and (Me3Si)2N− are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)2NMgCl · (Et2O)}2] 8 which bridges through the chloride atoms. The influence of solvent may be critical in determining which anion bridges. Solvent also plays a decisive role in the dismutation reaction of Hauser bases or halomagnesium enolates into their homoleptic components, similar to the Schlenk equilibrium for Grignard reagents.
LanguageEnglish
Pages173-179
Number of pages6
JournalJournal of Organometallic Chemistry
Volume559
Issue number1-2
DOIs
Publication statusPublished - 29 May 1998

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Hempa
Ketones
ketones
Anions
Negative ions
anions
reagents
Chlorides
Salts
chlorides
X-Rays
salts
X rays
diethyl ether
Molecular orbitals
Carbon Monoxide
Dimers
Ether
Magnesium
halides

Keywords

  • Magnesium enolate
  • MO calculations
  • Ab initio
  • Crystal structure
  • Magnesium halide
  • Solvent effects
  • Bridging ability
  • Equilibria

Cite this

@article{975bdcc926524c51b3994968a8aa35ce,
title = "Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolisable ketones: structural characterisation of [{ButC(=CH2)OMgBr.HMPA}2] and [MgBr2.(HMPA)2]",
abstract = "The Hauser base reagents Pri2NMgCl 1 and Pri2NMgBr 2 react with a variety of enolisable ketones to yield magnesium enolates. Attempts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solvated dihalide salts (MgX2 · Sx, where X=Cl or Br and S=THF, TMEDA or HMPA). Using diethyl ether as solvent media and one molar equivalent of HMPA, the halomagnesium enolate compounds [{ButC(=CH2)OMgBr · HMPA}2] 3 and [Me2CHC(=CMe2)OMgBr · HMPA] 5 were isolated and identified. Both 3 and 5 precipitate as mixtures with the dihalide salt [MgBr2 · (HMPA)2] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A molecular-orbital theoretical study (HF/6-31G*) was conducted to determine the relative bridging abilities of several model anions. The enolate anion H(CH2=)CO− was determined to be a favoured bridge in preference to the halides F−, Cl− and Br−, which is consistent with the X-ray evidence. The amido anions Me2N−, (H3Si)2N− and (Me3Si)2N− are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)2NMgCl · (Et2O)}2] 8 which bridges through the chloride atoms. The influence of solvent may be critical in determining which anion bridges. Solvent also plays a decisive role in the dismutation reaction of Hauser bases or halomagnesium enolates into their homoleptic components, similar to the Schlenk equilibrium for Grignard reagents.",
keywords = "Magnesium enolate, MO calculations, Ab initio, Crystal structure, Magnesium halide, Solvent effects, Bridging ability, Equilibria",
author = "J.F. Allan and W. Clegg and K.W. Henderson and L. Horsburgh and A.R. Kennedy",
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Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolisable ketones: structural characterisation of [{ButC(=CH2)OMgBr.HMPA}2] and [MgBr2.(HMPA)2]. / Allan, J.F.; Clegg, W.; Henderson, K.W.; Horsburgh, L.; Kennedy, A.R.

In: Journal of Organometallic Chemistry, Vol. 559, No. 1-2, 29.05.1998, p. 173-179.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Solvent effects and molecular rearrangements during the reaction of Hauser bases with enolisable ketones: structural characterisation of [{ButC(=CH2)OMgBr.HMPA}2] and [MgBr2.(HMPA)2]

AU - Allan, J.F.

AU - Clegg, W.

AU - Henderson, K.W.

AU - Horsburgh, L.

AU - Kennedy, A.R.

PY - 1998/5/29

Y1 - 1998/5/29

N2 - The Hauser base reagents Pri2NMgCl 1 and Pri2NMgBr 2 react with a variety of enolisable ketones to yield magnesium enolates. Attempts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solvated dihalide salts (MgX2 · Sx, where X=Cl or Br and S=THF, TMEDA or HMPA). Using diethyl ether as solvent media and one molar equivalent of HMPA, the halomagnesium enolate compounds [{ButC(=CH2)OMgBr · HMPA}2] 3 and [Me2CHC(=CMe2)OMgBr · HMPA] 5 were isolated and identified. Both 3 and 5 precipitate as mixtures with the dihalide salt [MgBr2 · (HMPA)2] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A molecular-orbital theoretical study (HF/6-31G*) was conducted to determine the relative bridging abilities of several model anions. The enolate anion H(CH2=)CO− was determined to be a favoured bridge in preference to the halides F−, Cl− and Br−, which is consistent with the X-ray evidence. The amido anions Me2N−, (H3Si)2N− and (Me3Si)2N− are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)2NMgCl · (Et2O)}2] 8 which bridges through the chloride atoms. The influence of solvent may be critical in determining which anion bridges. Solvent also plays a decisive role in the dismutation reaction of Hauser bases or halomagnesium enolates into their homoleptic components, similar to the Schlenk equilibrium for Grignard reagents.

AB - The Hauser base reagents Pri2NMgCl 1 and Pri2NMgBr 2 react with a variety of enolisable ketones to yield magnesium enolates. Attempts at isolation of these enolates when THF was present in the solvent media was unsuccessful, with the exclusive precipitation of the solvated dihalide salts (MgX2 · Sx, where X=Cl or Br and S=THF, TMEDA or HMPA). Using diethyl ether as solvent media and one molar equivalent of HMPA, the halomagnesium enolate compounds [{ButC(=CH2)OMgBr · HMPA}2] 3 and [Me2CHC(=CMe2)OMgBr · HMPA] 5 were isolated and identified. Both 3 and 5 precipitate as mixtures with the dihalide salt [MgBr2 · (HMPA)2] 4. X-ray crystallographic studies reveal 3 to be dimeric utilizing enolate bridges, whereas 4 is a simple monomer. A molecular-orbital theoretical study (HF/6-31G*) was conducted to determine the relative bridging abilities of several model anions. The enolate anion H(CH2=)CO− was determined to be a favoured bridge in preference to the halides F−, Cl− and Br−, which is consistent with the X-ray evidence. The amido anions Me2N−, (H3Si)2N− and (Me3Si)2N− are also calculated to be favoured over the chloride anion in three-coordinate dimer systems. This is contrary to the known structure of [{(Me3Si)2NMgCl · (Et2O)}2] 8 which bridges through the chloride atoms. The influence of solvent may be critical in determining which anion bridges. Solvent also plays a decisive role in the dismutation reaction of Hauser bases or halomagnesium enolates into their homoleptic components, similar to the Schlenk equilibrium for Grignard reagents.

KW - Magnesium enolate

KW - MO calculations

KW - Ab initio

KW - Crystal structure

KW - Magnesium halide

KW - Solvent effects

KW - Bridging ability

KW - Equilibria

UR - http://dx.doi.org/10.1016/S0022-328X(98)00429-X

U2 - 10.1016/S0022-328X(98)00429-X

DO - 10.1016/S0022-328X(98)00429-X

M3 - Article

VL - 559

SP - 173

EP - 179

JO - Journal of Organometallic Chemistry

T2 - Journal of Organometallic Chemistry

JF - Journal of Organometallic Chemistry

SN - 0022-328X

IS - 1-2

ER -