Hauser base

Last updated

Hauser bases, also called magnesium amide bases, are magnesium compounds used in organic chemistry as bases for metalation reactions. These compounds were first described by Charles R. Hauser in 1947. [1] Compared with organolithium reagents, the magnesium compounds have more covalent, and therefore less reactive, metal-ligand bonds. Consequently, they display a higher degree of functional group tolerance and a much greater chemoselectivity. [2] Generally, Hauser bases are used at room temperature while reactions with organolithium reagents are performed at low temperatures, commonly at −78 °C.

Contents

Structures

Hauser bases have the empirical formula R2NMgX (X = halide). The crystallize as dimers with halide bridges. Attached to Mg is amido (R2N) ligands derived from secondary amines 2,2,6,6-tetramethylpiperidine (TMP−) and HMDS −). [3] [4] [5]

Amido-bridged Hauser bases exist when the amido ligand is less bulky, such as Et2N− and Ph3P=N−. [4] [6] [7]

The structures of Hauser bases in solution have been investigated by diffusion-ordered NMR spectroscopy (DOSY). [8] These studies indicate that iPr2NMgCl is subject to the Schlenk equilibrium: [9]

iPr2NMgCl (A) ⇌ (iPr2N)2Mg (B) + MgCl2

This equilibrium is temperature-dependent: heteroleptic (A) are the main species at high temperatures and homoleptic (B) dominate at lower temperatures. Dimeric species with bridging chlorides and amides are also present in the THF solution. At low temperatures, adducts of MgCl2 are present in solution. [9]

Preparation and reactions

The Hauser bases are prepared by treating a secondary amine with a Grignard reagent:

R2NH + R′MgX → R2NMgX + R′H  X = Cl, Br, I

(:R2NH = diisopropylamine, TMP)

Like many organolithium reagents, Hauser bases are generally used for metalation reagents. iPr2NMgBr selectively magnesiate carboxamides. [10] iPr2NMgX (X = Cl, Br) effect the deprotonation thiophenes. [11] and phenylsulphonyl-substituted indoles. [12]

HauserBase Reaction1.png
HauserBase Reaction2 (cropped).png

Turbo-Hauser base

A major disadvantage of Hauser bases is their poor solubility in THF. In consequence, the metalation rates are slow and a large excess of base is required (e.g., 10 equiv.). This circumstance complicates the functionalization of the metaled intermediate with an electrophile. Improved solubility and reactivity can be achieved by adding stoichiometric amounts of LiCl to the Hauser base. These so-called Turbo-Hauser bases like e.g. TMPMgCl·LiCl and iPr2NMgCl·LiCl are commercially available. They show an enhanced kinetic basicity, regioselectivity and functional group tolerance. [13]

Related Research Articles

<span class="mw-page-title-main">Grignard reaction</span> Organometallic coupling reaction

The Grignard reaction is an organometallic chemical reaction in which, according to the classical definition, carbon alkyl, allyl, vinyl, or aryl magnesium halides are added to the carbonyl groups of either an aldehyde or ketone under anhydrous conditions. This reaction is important for the formation of carbon–carbon bonds.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

Metalation is a chemical reaction that forms a bond to a metal. This reaction usually refers to the replacement of a halogen atom in an organic molecule with a metal atom, resulting in an organometallic compound. In the laboratory, metalation is commonly used to activate organic molecules during the formation of C—X bonds, which are necessary for the synthesis of many organic molecules.

The Bouveault aldehyde synthesis is a one-pot substitution reaction that replaces an alkyl or aryl halide with a formyl group using a N,N-disubstituted formamide. For primary alkyl halides this produces the homologous aldehyde one carbon longer. For aryl halides this produces the corresponding carbaldehyde. The Bouveault aldehyde synthesis is an example of a formylation reaction, and is named for French scientist Louis Bouveault.

The Schlenk equilibrium, named after its discoverer Wilhelm Schlenk, is a chemical equilibrium taking place in solutions of Grignard reagents and Hauser bases

<span class="mw-page-title-main">Grignard reagent</span> Organometallic compounds used in organic synthesis

Grignard reagents or Grignard compounds are chemical compounds with the general formula R−Mg−X, where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride Cl−Mg−CH3 and phenylmagnesium bromide (C6H5)−Mg−Br. They are a subclass of the organomagnesium compounds.

The Kulinkovich reaction describes the organic synthesis of substituted cyclopropanols through reaction of esters with dialkyl­dialkoxy­titanium reagents, which are generated in situ from Grignard reagents containing a hydrogen in beta-position and titanium(IV) alkoxides such as titanium isopropoxide. This reaction was first reported by Oleg Kulinkovich and coworkers in 1989.

<span class="mw-page-title-main">Group 2 organometallic chemistry</span>

Group 2 organometallic chemistry refers to the organic derivativess of any group 2 element. It is a subtheme to main group organometallic chemistry. By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organometallic group 2 compounds are typically limited to academic interests.

Organomanganese chemistry is the chemistry of organometallic compounds containing a carbon to manganese chemical bond. In a 2009 review, Cahiez et al. argued that as manganese is cheap and benign, organomanganese compounds have potential as chemical reagents, although currently they are not widely used as such despite extensive research.

Organosodium chemistry is the chemistry of organometallic compounds containing a carbon to sodium chemical bond. The application of organosodium compounds in chemistry is limited in part due to competition from organolithium compounds, which are commercially available and exhibit more convenient reactivity.

Reactions of organocopper reagents involve species containing copper-carbon bonds acting as nucleophiles in the presence of organic electrophiles. Organocopper reagents are now commonly used in organic synthesis as mild, selective nucleophiles for substitution and conjugate addition reactions.

<span class="mw-page-title-main">Metal bis(trimethylsilyl)amides</span>

Metal bis(trimethylsilyl)amides are coordination complexes composed of a cationic metal M with anionic bis(trimethylsilyl)amide ligands (the −N 2 monovalent anion, or −N 2 monovalent group, and are part of a broader category of metal amides.

Turbo-Hauser bases are amido magnesium halides that contain stoichiometric amounts of LiCl. These mixed Mg/Li amides of the type R2NMgCl⋅LiCl are used in organic chemistry as non-nucleophilic bases for metalation reactions of aromatic and heteroaromatic substrates. Compared to their LiCl free ancestors Turbo-Hauser bases show an enhanced kinetic basicity, excellent regioselectivity, high functional group tolerance and a better solubility.

<span class="mw-page-title-main">Vinyllithium</span> Chemical compound

Vinyllithium is an organolithium compound with the formula LiC2H3. A colorless or white solid, it is encountered mainly as a solution in tetrahydrofuran (THF). It is a reagent in synthesis of organic compounds, especially for vinylations.

<span class="mw-page-title-main">Isopropylmagnesium chloride</span> Chemical compound

Isopropylmagnesium chloride is an organometallic compound with the general formula (CH3)2HCMgCl. This highly flammable, colorless, and moisture sensitive material is the Grignard reagent derived from isopropyl chloride. It is commercially available, usually as a solution in tetrahydrofuran.

In organometallic chemistry, metal–halogen exchange is a fundamental reaction that converts an organic halide into an organometallic product. The reaction commonly involves the use of electropositive metals and organochlorides, bromides, and iodides. Particularly well-developed is the use of metal–halogen exchange for the preparation of organolithium compounds.

<span class="mw-page-title-main">(Trimethylsilyl)methyllithium</span> Chemical compound

(Trimethylsilyl)methyllithium is classified both as an organolithium compound and an organosilicon compound. It has the empirical formula LiCH2Si(CH3)3, often abbreviated LiCH2TMS. It crystallizes as the hexagonal prismatic hexamer [LiCH2TMS]6, akin to some polymorphs of methyllithium. Many adducts have been characterized including the diethyl ether complexed cubane [Li4(μ3-CH2TMS)4(Et2O)2] and [Li2(μ-CH2TMS)2(TMEDA)2].

A magnesium(I) dimer is a molecular compound containing a magnesium to magnesium bond (Mg-Mg), giving the metal an apparent +1 oxidation state. Alkaline earth metals are commonly found in the +2-oxidation state, such as magnesium. The M2+ are considered as redox-inert, meaning that the +2 state is significant. However, recent advancements in main group chemistry have yielded low-valent magnesium(I) dimers, also given as Mg(I), with the first compound being reported in 2007. They can be generally represented as LMg-MgL, with L being a monoanionic ligand. For example, β-diketiminate, commonly referred to as Nacnac, is a useful chelate regarding these complexes. By tuning the ligand, the thermodynamics of the complex change. For instance, the ability to add substituents onto Nacnac can contribute to the steric bulk, which can affect reactivity and stability. As their discovery has grown, so has their usefulness. They are employed in organic and inorganic reduction reactions. It is soluble in a hydrocarbon solvent, like toluene, stoichiometric, selective, and safe.

Organocalcium chemistry is the chemistry of compounds containing a calcium to carbon bond, or in broader definitions, organic compounds that contain calcium. Although discovered around the same time as the now commonly utilized organomagnesium compounds, organocalcium compounds were subject to greatly reduced interest due to drastic differences in stability. However, recent advances in stabilization of these highly reactive compounds has spurred increased interest in organocalcium compounds and allowed for multiple research directions to form. Because calcium metal is less reactive to organic reagents than magnesium and the organocalcium compounds are more reactive than organomagnesium compounds, synthesis of novel compounds still poses a significant challenge. Calcium also has access to empty d orbitals that the lighter alkaline earth metals cannot access, and the degree to which this affects bonding and reactivity has sparked a fundamental debate. Lastly, despite the inherent instability of most organocalcium complexes, the unique basicity and size of the calcium ion together with the highly polarized bonds formed has opened up applications for organocalcium compounds in organic transformations and catalytic cycles.

Organomagnesium chemistry, a subfield of organometallic compounds, refers to the study of magnesium compounds that contains Mg-C bonds. Magnesium is the second element in group 2 (alkaline earth metals), and the ionic radius of Mg2+ is 86 pm, which is larger than Be2+ (59 pm) and smaller than the heavier alkaline earth metal dications (Ca2+ 114 pm, Sr2+ 132 pm, Ba2+ 149 pm), in accordance with periodic trends. Magnesium is less covalent compared to beryllium, and the radius is not large enough for accommodating large number of ligands compared to calcium, strontium and barium. Thus, organomagnesium compounds exhibit unique structure and reactivity in group 2.

References

  1. ↑ Hauser C. R., Walker H (1947). "Condensation of Certain Esters by Means of Diethylaminomagnesium Bromide". J. Am. Chem. Soc. 69 (2): 295. doi:10.1021/ja01194a040.
  2. ↑ Li–Yuan Bao, R.; Zhao, R.; Shi, L. (2015). "Progress and developments in the turbo Grignard reagent i-PrMgCl·LiCl: a ten-year journey". Chem. Commun. 51 (32): 6884–6900. doi:10.1039/C4CC10194D. PMID   25714498.
  3. ↑ e.g.; Seven, Ö.; Bolte, M.; Lerner, H.-W. (2013). "Di-μ-bromido-bis[(diethyl ether-κO)(2,4,6-trimethylphenyl)magnesium]: the mesityl Grignard reagent" (PDF). Acta Crystallogr. E . 69 (7): m424. doi:10.1107/S1600536813017108. PMC   3772445 . PMID   24046588.
  4. 1 2 García–Álvarez, P.; Graham, D. V.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; O'Hara, C. T.; Weatherstone, S. (2008). "Unmasking Representative Structures of TMP-Active Hauser and Turbo-Hauser Bases". Angew. Chem. Int. Ed. 47 (42): 8079–8081. doi: 10.1002/anie.200802618 . PMID   18677732.
  5. ↑ Yang, K.-C.; Chang, C.-C.; Huang, J.-Y.; Lin, C.-C.; Lee, G.-H.; Wang, Y.; Chiang, M. Y. (2002). "Synthesis, characterization and crystal structures of alkyl-, alkynyl-, alkoxo- and halo-magnesium amides" (PDF). J. Organomet. Chem. 648 (1–2): 176–187. doi:10.1016/S0022-328X(01)01468-1.
  6. ↑ Batsanov, A. S.; Bolton, P. D.; Copley, R. C. B.; Davidson, M. G.; Howard, J. A. K.; Lustig, C.; Price, R. D. (1998). "The metallation of imino(triphenyl)phosphorane by ethylmagnesium chloride: The synthesis, isolation and X-ray structure of [Ph3P=NMgCl·O=P(NMe2)3]2". J. Organomet. Chem. 550 (1–2): 445–448. doi:10.1016/S0022-328X(97)00550-0.
  7. ↑ Armstrong D. R.; García–Álvarez, P.; Kennedy, A. R.; Mulvey, R. E.; Parkinson, J. A. (2010). "Diisopropylamide and TMP Turbo-Grignard Reagents: A Structural Rationale for their Contrasting Reactivities". Angew. Chem. Int. Ed. 49 (18): 3185–3188. doi: 10.1002/anie.201000539 . PMID   20352641.
  8. ↑ Neufeld, R.; Stalke, D. (2015). "Accurate Molecular Weight Determination of Small Molecules via DOSY-NMR by Using External Calibration Curves with Normalized Diffusion Coefficients". Chem. Sci. 6 (6): 3354–3364. doi:10.1039/C5SC00670H. PMC   5656982 . PMID   29142693. Open Access logo PLoS transparent.svg
  9. 1 2 Neufeld, R.; Teuteberg, T. L.; Herbst-Irmer, R.; Mata, R. A.; Stalke, D. (2016). "Solution Structures of Hauser Base iPr2NMgCl and Turbo-Hauser Base iPr2NMgCl·LiCl in THF and the Influence of LiCl on the Schlenk-Equilibrium". J. Am. Chem. Soc. 138 (14): 4796–4806. doi:10.1021/jacs.6b00345. PMID   27011251.
  10. ↑ Eaton, P. E.; Lee, C. H.; Xiong, Y. (1989). "Magnesium amide bases and amido-Grignards. 1. Ortho magnesiation". J. Am. Chem. Soc. 138 (20): 8016–8018. doi:10.1021/ja00202a054.
  11. ↑ Shilai, M.; Kondo, Y.; Sakamoto, T. (2001). "Selective metallation of thiophene and thiazole rings with magnesium amide base". J. Chem. Soc. Perkin Trans. 1 (4): 442–444. doi:10.1039/B007376H.
  12. ↑ Kondo, Y.; Yoshida, A.; Sakamoto, T. (1996). "Magnesiation of indoles with magnesium amide bases". J. Chem. Soc. Perkin Trans. 1 (19): 2331–2332. doi:10.1039/P19960002331.
  13. ↑ Li-Yuan Bao, Robert; Zhao, Rong; Shi, Lei (2015). "Progress and Developments in the turbo Grignard Reagent i-PrMgCl·LiCl: A Ten-Year Journey". Chemical Communications. 51 (32): 6884–6900. doi:10.1039/c4cc10194d. PMID   25714498.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.