Preparation and use of magnesium amides

The present application relates to magnesium amides, a method for the preparation of magnesium amides and the use of these amides.

The metalation of aromatics is one of the most useful transformations in organic synthesis since it allows the regioselective functionalization of various aryl derivatives.^[1] Traditionally, strong bases such as alkyl lithium (RLi) or lithium amides (R₂NLi) have been used to perform such deprotonations. However, these highly reactive bases display often undesirable side reactions due to the too high reactivity of the resulting aryl lithium compounds. Another serious limitation is the low stability of lithium amides in THF solutions at room temperature which requires an in situ generation of these reagents. Furthermore, the deprotonation of aromatics by lithium bases often requires very low temperatures (-78°C to -90°C) which complicates the scale-up of these reactions and the use of solvent mixtures such as THF/pentane may be needed.

Alternative methods have been developed using magnesium amides^[2] such as compounds 1-3 or amido zincates^[3] 4 (see scheme 1). The low solubility of the magnesium amides R₂NMgCl (1) could be improved by Eaton who developed the use of magnesium amides of type R₂NMgR' (2) and (R₂N)₂Mg (3). Nevertheless, for achieving high conversions it is usually necessary to use a large excess of the magnesium amides (2-12 equivalents), which complicates further quenching reactions with electrophiles (up to 10 equivalents of electrophile may have to be used). Similarly, the dialkyl amino zincate 4 requires the use of 3.5-4 equivalents of an electrophile in subsequent quenching reactions.

The use of these bases is thus either limited by their poor solubility, or they are not very efficient in view of the amounts of base and the amount of electrophile needed to perform the desired conversion.

It is an object of the present invention to provide an inexpensive magnesium base which is highly soluble. A further object of the present invention is to provide a magnesium base showing a high kinetic activity.

These objects are achieved by the features of the independent claims. Preferred embodiments are set forth in the dependent claims.

Recently, the inventors found that mixed magnesium and lithium amides of type R¹R²N-MgXzLiY (I) can be prepared by reacting an amine R¹R²NH with a Grignard reagent R³MgX in the presence of LiY or with R³MgX·zLiY in a solvent.

R¹, R² and R³ independently are selected from substituted or unsubstituted aryl or heteroaryl containing one or more heteroatoms, linear, branched or cyclic, substituted or unsubstituted alkyl, alkenyl, alkynyl, or derivatives thereof, and, for R¹ and R² only, the silyl derivatives thereof. One of R¹ and R² may be H; or R¹ and R² together can be part of a cyclic or polymeric structure.

X and Y independently are selected from the group consisting of F; Cl; Br; I; CN; SCN; NCO; HalO_n, wherein n = 3 or 4 and Hal is selected from Cl, Br and I; NO₃; BF₄; PF₆; H; a carboxylate of the general formula R^xCO₂; an alcoholate of the general formula OR^x; a thiolate of the general formula SR^x; R^xP(O)O₂; or SCOR^x; O_nSR^x, wherein n = 2 or 3; or NO_n, wherein n = 2 or 3; and a derivative thereof;

wherein R^x is a substituted or unsubstituted aryl or heteroaryl containing one or more heteroatoms; linear, branched or cyclic, substituted or unsubstituted alkyl, alkenyl, alkynyl, or derivatives thereof; or H. z>0.

Many common solvents can be used in the present invention. In principle, any solvent capable of dissolving the specific amine used and the Grignard reagent used as starting materials and the resulting products. In a preferred embodiment of the present invention, the solvent is selected from cyclic, linear or branched mono or polyethers, thioethers, amines, phosphines, and derivatives thereof containing one or more additional heteroatoms selected from O, N, S and P, preferably tetrahydrofuran (THF), 2-methyltetrahydrofuran, dibutyl ether, diethyl ether, tert-butylmethyl ether, dimethoxyethane, dioxanes, preferably 1,4-dioxane, triethylamine, ethyldiisopropylamine, dimethylsulfide, dibutylsulfide; cyclic amides, preferably N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N-butyl-2-pyrrolidone (NBP); cyclic, linear or branched alkanes and/or alkenes wherein one or more hydrogens are replaced by a halogen, preferably dichloromethane, 1,2-dichloroethane, CCl₄; urea derivatives, preferably N,N'-dimethylpropyleneurea (DMPU); aromatic, heteroaromatic or aliphatic hydrocarbons, preferably benzene, toluene, xylene, pyridine, pentane, cyclohexane, hexane, heptane; hexamethylphosphorus triamide (HMPA), CS₂; or combinations thereof.

The process of the invention is carried out by contacting an amine R¹R²NH with a Grignard reagent R³MgX in the presence of LiY or with R³MgX·zLiY in a solvent. The materials are contacted preferably at room temperature for the minimum time necessary to provide the desired yield. Temperatures between 0°C and 50°C are preferred, however, higher or lower reaction temperatures are also suitable.

In another preferred embodiment, X and Y are independently or both Cl, Br or I, and preferably Cl.

In yet another preferred embodiment, the preparation is achieved by iPrMgCl·LiCl^[5]. This embodiment is particularly preferred since iPrMgCl·LiCl is commercially available.

Generally, any Grignard reagent can be used to prepare the mixed Mg/Li-amides in the presence of any lithium salt. It is nevertheless preferred to use a Grignard reagent the side or by-products of which can easily be removed from the reaction mixture. The presence of a lithium salt accelerates the exchange reaction compared to homoleptic reagents RMgX and R₂Mg without the use of a lithium salt.

According to a second aspect, the present invention is directed to the mixed Mg/Li amide R¹R²N-MgX·zLiY (I), wherein R¹, R², X, Y and z are defined as above. It is to be understood that the adduct of a solvent is also comprised by this formula.

A third aspect of the present invention is directed to a solution of the amide (I) in a solvent. The solvent can be any suitable solvent capable of dissolving the amide (I). Especially preferred solvents are the solvents listed above for the preparation of the amide (I).

In a fourth aspect, the present invention is related to the use of mixed Mg/Li amides (I). The amides of the present invention can be used to remove acidic protons. The deprotonated species can then subsequently be quenched with an electrophile. In principle it is possible to use all kinds of electrophiles that are, for example, cited in the following references, but are not limited thereto:

1. a) Handbook of Grignard reagents; edited by Gary S. Silverman and Philip E. Rakita (Chemical industries; v. 64).
2. b) Grignard reagents New Developments; edited by Herman G. Richey, Jr., 2000, John Wiley & Sons Ltd.
3. c) Methoden der Organischen Chemie, Houben-Weyl, Band XIII/2a, Metallorganische Verbindungen Be, Mg, Ca, Sr, Ba, Zn, Cd. 1973.
4. d) The chemistry of the metal-carbon bond, vol 4. edited by Frank R. Hartley. 1987, John Wiley & Sons.

The final aspect of the invention relates to the product of the reaction of an electrophile with a substrate which has been deprotonated with a reagent of the general formula R¹R²N-MgX·zLiY wherein R¹, R², X, Y and z are defined as above.

In all aspects of the invention, z is preferably in the range from 0.01 - 5, more preferably from 0.5 - 2, even more preferably from 0.9 to 1.2 and most preferred about 1. In the most preferred embodiment of the invention, z is used in an equimolar ratio compared to the amide.

The present invention is described in the following on the basis of specific examples. Especially, i-PrMgCl is used as Grignard reagent. However, it is to be understood that the present invention is not limited to such examples.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications and other references mentioned herein are incorporated by reference in their entirety.

As used herein, the terms "alkyl", "alkenyl" and "alkynyl" refer to linear, cyclic and branched, substituted and unsubstitued C₁-C₂₀ compounds. Preferred ranges for these compounds are C₁-C₁₀, preferably C₁-C₅ (lower alkyl) and C₂-C₁₀ and preferably C₂-C₅, respectively, for alkenyl and alkynyl. The term "cycloalkyl" generally refers to linear and branched, substituted and unsubstitued C₃-C₂₀ cycloalkanes. Here, preferred ranges are C₃-C₁₅, more preferably C₃-C₈.

When R¹ and R² can be part of a cyclic structure, it is to be understood that R¹ and R² together are a divalent saturated or unsaturated, linear or branched alkyl, alkenyl or alkynyl which forms in connection with the nitrogen atom of the amide (I) a cyclic amide. An example of such a cyclic amide is the amide of TMPH. Further, the residues R¹ and R² can be part of a polymeric structure. The nitrogen atom of the amide is the connected to a polymeric backbone which may even contain more than one nitrogen atom for the formation of an amide according to the invention.

The term "aryl" as used herein refers to substituted or unsubstituted C₄-C₂₄ aryl. By "heteroaryl", a substituted or unsubstituted C₃-C₂₄ heteroaryl, containing one or more heteroatoms as B, O, N, S, Se, P, is meant. Preferred ranges for both are C₄-C₁₅, more preferably C₄-C₁₀.

Mixed magnesium and lithium amides R₂NMgCl·LiCl (R = i-Pr or R₂N = 2,2,6,6-tetramethylpiperidyl) can be prepared by reacting i-PrMgCl·LiCl^[4,5] with diisopropylamine or 2,2,6,6-tetramethylpiperidine (TMPH), respectively, in THF (-20°C - 80°C, 0.1-48 h). The resulting Li/Mg-reagents 5a (R = i-Pr) and 5b (R₂N = 2,2,6,6-tetramethylpiperidyl) proved to have an excellent solubility in THF (0.6 M and 1.2 M, respectively) as well as an improved kinetic acidity and regioselectivity for the magnesation of various aromatics and heterocycles.

The activity of the amides (I) can be shown on the basis of the magnesiation of isoquinoline. Diisopropylamido magnesium chloride-lithium chloride 5a leads to the magnesiated isoquinoline 6 after 12 h reaction time at 25°C and by using 2 equivalents of the base. After iodolysis, the iodoisoquinoline 7a is isolated in 88% yield (Scheme 2). Even more active is the sterically more hindered and less aggregated 2,2,6,6-tetramethylpiperidino magnesium chloride-lithium chloride reagent 5b. It leads to a complete magnesiation within 2 h at 25°C. Remarkably, with this base only 1.1 equivalents are required to achieve a complete metalation. The resulting Grignard reagent 6 provides after iodolysis, the iodoisoquinoline 7a in 96% yield (Scheme 2 and Table 1).

After the magnesation of a reagent, it can be subjected to a transmetalation. After e.g. a transmetalation with CuCN·2LiCl (20 mol %), the addition of benzoyl chloride (1.2 equiv.) provides the ketone 7b in 86% yield (entry 2 of Table 1).

The presence of an excess of magnesium amides often hampers the performance of palladium-catalyzed cross-couplings. The inventors found that the Grignard reagents generated by deprotonation with 5b (1.1 equiv.) such as 6 are readily transmetalated to the corresponding zinc derivative (ZnCl₂ (1.1 equiv.), 0°C, 5 min.) and undergo a Negishi-cross-coupling reaction using Pd(dba)₂ (5 mol%), P(2-fur)₃ (7 mol%) with ethyl 4-iodobenzoate (1.2 equiv.; 50°C, 12 h) leading to the arylated quinoline (7c) in 82% yield. This behaviour is general and 3-bromoquinoline is metalated with 5b (1.1 equiv., -30°C, 0.5 h) leading to the 2-magnesiated quinoline 8 (entries 4 and 5 of Table 1). Thus, the quenching of 8 with I₂ and N,N-dimethylformamide (DMF) provides the two quinolines 9a and 9b in 96-93%.

Whereas the deprotonation of 2,6-dichloropyridine with i-Pr₂NMgCl·LiCl 5a and lithium diisopropylamide (LDA)^[8j] provides a 1:1 mixture of 3- and 4-magnesiated 2,6-dichloropyridine, the use of TMPMgCl·LiCl 5b furnishes only the 4-magnesiated pyridine 10. Its reaction with typical electrophiles (I₂, DMF and PhCHO) provides the expected products 11a-c in 84 - 93% yield (entries 6-8 of Table 1). Interestingly, metalation of 3,5-dibromopyridine with LDA proceeds selectively at 4-position^[6b] while in the case of (TMPMgCl·LiCl 5b 1.1 equiv, -20°C, 0.5h) regioselective metalation of 3,5-dibromopyridine is observed leading after the reaction with DMF to the pyridylaldehyde 13 in 95% yield (entry 9 of Table 1).

The magnesiation of heterocycles bearing more acidic protons^[7] such as thiazole, thiophene, furan, benzothiophene or benzothiazole proceeds smoothly between 0°C and 25°C leading to the organomagnesium derivatives 14a-c and 16a-b. After trapping with standard electrophiles, the expected products 15a-c and 17a-b are obtained in 81-98% yield (entries 10-14 of table 1).

The metalation of pyrimidine derivatives is a challenging problem due to the propensity of these heterocycles to add organometallic reagents.^[8] The inventors found that the inverse addition of the pyrimidine derivatives 18-20 to a THF solution of 5b (1.05 equiv.) at -55°C for approx. 5 min. provides the corresponding magnesiated derivatives 21-23 in 83-90% yields as indicated by iodolysis experiments leading to the iodinated pyrimidines 24-26 (scheme 3).

The mixed magnesium-lithium amide 5b is also well suited for the regioselective metalation of polyfunctional aromatic systems. Thus, the reaction of 2-phenylpyridine 27 in THF at 55°C with 5b (2.0 equiv.) for 24 h provides the Grignard reagent 28 showing a rare case where a phenyl ring is preferentially metalated compared to a pyridine ring. After iodolysis, the ortho-iodinated product 29 is obtained in 80% yield. Interestingly, the metalation of polyfunctional aromatics such as the bromodiester 30 also succeeds using only the stoichiometric amount of base 5b (1.1 equiv.) in THF (-30°C, 0.5 h) leading regioselectively to the arylmagnesium species (31) which after iodolysis furnishes the polyfunctional aromatic derivative 32 in 88% yield.

A solution of TMPMgCl·LiCl can easily be prepared in THF due to its excellent solubility and it is stable for more than 6 months at 25°C. The use of TMPMgCl·LiCl allows for the regioselective functionalization of various aromatics and heteroaromatics. It gives access to new magnesium species not readily available via a Br/Mg-exchange reactions or by previously reported metalation procedures.

The residues R¹ and R² are not limited to organic compounds. R¹ and R² may also be silylated compounds like trimethylsilyl. The preparation of the bis(trimethylsilyl) amide 33 can be achieved by reacting bis(trimethylsilyl)amine with i-PrMgCl·LiCl at room temperature (see Scheme 5). This base can efficiently be used to deprotonate ketones like e.g. cyclohexanone as can be seen from Scheme 5.

The Grignard reagents can also be used to prepare a polymeric base. 2,2,6,6-tetramethyl piperidine (TMPH) is a well known base. It can be used to prepare the corresponding mixed Mg/Li amide TMPMgCl·LiCl 5b. This monomeric base is very reactive but also very expensive. A corresponding polymeric base to TMPH is chimassorb 994, the structure of which is shown in Scheme 6.

Chimassorb 994 can be used to prepare the corresponding mixed Mg/Li amide by reacting chimassorb 994 with i-PrMgCl·LiCl at room temperature (see Scheme 5). This base is stable and soluble in THF before and after deprotonation. As being a polymeric base, it can be easily removed after completion of the reaction. Since chimassorb 994 is much cheaper than TMP, a corresponding base can be prepared at reduced costs. The polymeric base 34 shows slightly lower activity than monomeric TMPMgCl·LiCl but is nevertheless very effective in deprotonating compounds with acidic protons like isoquinoline. A corresponding example is shown in Scheme 7. The polymeric base can be used to deprotonate various substrates. For example, isoquinoline reacts at room temperature with the base 34 affording after quenching with iodine 1-iodoisoquinoline 7a.

The above given examples show that the new mixed Mg/Li-bases of the general type R¹R²NMgX·zLiY have a high kinetic activity due to the presence of a lithium salt which breaks oligomeric aggregates of magnesium amides.

As can be seen from the above given examples, the new mixed Mg/Li-bases are very effective in deprotonating organic compounds. The deprotonation can be achieved in different solvents and can preferably be conducted at temperatures between -90°C and 100°C. Further, due to the effective deprotonation reaction, the amides of the present invention preferably only require the use of 0.9 - 5 equivalents, more preferably 1 - 2 equivalents and most preferably 1.1 - 1.5 equivalents per proton to be deprotonated.

With this new type of base, which is highly soluble and the side products of which do not disturb the following reactions, many new products may be obtained, or known reactions pathways will be more efficient. A person skilled in the art will easily recognise the benefit of the new Mg/Li-base and will be able to use this base in a wide variety of chemical reactions.

In the following, examples are given to illustrate the present invention. However, these examples are given for illustrative purposes only and are not supposed to limit the scope of the invention which is determined by the claims below.

Experimental Section:

Preparation of the reagent TMPMgCl·LiCl (5b):

A dry and argon flushed 250 mL flask, equipped with a magnetic stirrer and a septum, was charged with freshly titrated i-PrMgCl·LiCl (100 mL, 1.2 M in THF, 120 mmol). 2,2,6,6-tetramethylpiperidine (TMPH) (19.8 g, 126 mmol, 1.05 equiv) was added dropwise at room temperature. The reaction mixture was stirred until gas evaluation was completed (ca. 24 h) at room temperature.

Preparation of 1-iodoisoquinoline (7a):

A dry and argon flushed 10 mL flask, equipped with a magnetic stirrer and a septum, was charged with TMPMgCl·LiCl (5 mL, 1.2 M in THF, 6.0 mmol). Isoquinoline (703 mg, 5.45 mmol) in THF (5 ml) was added dropwise at room temperature. During addition, the reaction mixture became red and the metalation was complete after 2 h (as checked by GC analysis of reaction aliquots quenched with a solution of I₂ in THF, the conversion was more than 98%). A solution of I₂ in THF (6 ml, 1 M in THF, 6.0 mmol) was slowly added at -20°C. The reaction mixture was quenched with sat. aqueous NH₄Cl solution (10 mL). The aqueous phase was extracted with ether (4 x 10 mL), dried with Na₂SO₄ and concentrated in vacuo. The crude residue was purified by filter column chromatography (CH₂Cl₂/pentane) yielding 1-iodoisoquinoline (7a; 1.33 mg, 96%) as slightly yellow crystals (mp = 74-76°C).

The products listed in table 1 below can be produced according to the preparation of 1-iodoisoquinoline (7a).

References and notes:

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