Manganese(II) Chloride


[7773-01-5]  · Cl2Mn  · Manganese(II) Chloride  · (MW 125.84) (MnCl2.4H2O)

[13446-34-9]  · Cl2H8MnO4  · Manganese(II) Chloride  · (MW 197.92)

(starting material for preparation of organomanganese chloride reagents;1 in combination with RLi or RMgX catalyzes reduction of vinyl and aryl halides,2 stereoselective coupling of vinyl iodides,3 and alkylation of enol triflates;4 also used for Mn-Cu-catalyzed conjugate addition of RMgX to enones5 and for Mn-catalyzed acylation of RMgX by acyl chlorides;6 in combination with NaBH4 is diastereoselective reducing agent;7 starting material for the preparation of Mn enolates regio- and stereoselectively8)

Physical Data: anhydrous material: mp 650 °C; bp 1190 °C; d 2.98 g cm-3; hygroscopic. Tetrahydrate: mp 58 °C, d 2.01 g cm-3.

Solubility: sol H2O and some polar organic solvents (DMF, NMP, DMPU, etc.); insol hydrocarbons, ether, THF. MnCl2 can be dissolved in THF as its ate complex MnCl4Li2. The latter is easily obtained by mixing LiCl (2 equiv) and anhydrous MnCl2 in THF at rt until dissolution (about 1 h). MnCl4Li2 is generally used in place of MnCl2; it reacts similarly but more rapidly owing to its solubility.1

Form Supplied in: pink grayish powder; widely available. Drying: for organometallic uses, it is recommended that the commercial anhydrous MnCl2 be dried since the industrial anhydrous material contains 0.1-1% H2O. The following procedure is convenient: heating at 200 °C under vacuo (10-2 mmHg) for 2 h.

Handling, Storage, and Precautions: anhydrous MnCl2 is a very hygroscopic material. It must be handled very quickly and stored in a tightly closed bottle.

Preparation of Organomanganese Reagents.1

RMnCl are easily prepared by adding RLi or RMgX to a solution of MnCl4Li2 (see above) in THF (eq 1):

According to the ratio RLi:MnCl2 or RMgX:MnCl2, several other types of organomanganese reagents can be prepared: the symmetrical organomanganeses R2Mn and the organomanganates R3MnLi and R4MnLi2 or R3MnMgX and R4Mn(MgX)2.

In most cases, the transmetalation reaction can be performed between 0 and 20 °C (-20 °C to -10 °C when R = s- or t-alkyl) and the Li-Mn or Mg-Mn exchange occurs quantitatively and almost instantaneously. RMnCl react with carboxylic acid chlorides to give ketones and with ketones or aldehydes to lead to the corresponding alcohols. In the presence of copper, they also add to conjugated unsaturated ketones, aldehydes, esters, and amides to afford the 1,4-addition products. All these reactions have a large scope and give high yields under mild conditions. It is important to note that RMnCl react chemoselectively and are especially useful in the preparation of multifunctionalized molecules (see n-Butylmanganese Chloride).

Reduction of Vinyl and Aryl Halides.2

In the presence of MnCl2, in THF, i-PrMgCl reduces vinyl and aryl halides in good yields (eqs 2 and 3). Some functional groups such as ethers or amines are tolerated.

Stereospecific Symmetrical Coupling of Vinyl Iodides.3

In ether, in the presence of MnCl2, MeLi or n-BuLi react with vinyl iodides to give stereospecifically the corresponding conjugated dienes in excellent yields (eq 4).

Alkylation of Enol Triflates.4

Organomagnesium compounds react with enol triflates, in the presence of MnCl4Li2, to give the substituted alkenes (eq 5). However, good yields are obtained only with aryl, benzyl, or allyl Grignard reagents.

MnCl2 as Precursor of Mn0.

MnCl2 is reduced by Lithium Aluminum Hydride in THF at 0 °C (eq 6) to give a very fine black suspension of Mn0 (the exact nature of this precipitate has not been established). The subsequent addition of allyl bromide leads to an intermediate species which reacts with aldehydes or reactive ketones to give the corresponding homoallylic alcohols (eq 7).9

From a practical point of view, it is interesting to note that such a Mn-mediated Barbier reaction can be more efficiently performed by using commercial manganese metal,10 a very cheap material. Thus, upon reaction of Mn0 with allylic halides in EtOAc at 50 °C in the presence of aldehydes or ketones, the 1,2-addition product is obtained in high yields. With functionalized aldehydes or ketones, the chemoselectivity is excellent (eq 8).

In THF, the reaction takes place at 60 °C and only in the presence of Zinc Chloride. Even allylic chlorides can be used (eq 9).

A Mn-mediated Reformatsky reaction can also be effected in EtOAc in the presence of ZnCl2. The alcoholates resulting from the addition of the intermediate ester Mn-enolates to aldehydes or ketones can be trapped by Ac2O to give good yields of b-acetoxy esters (eq 10).

Mn-Cu-Catalyzed Conjugate Addition of Organomagnesium Reagents to Enones.5

RMgX add to enones in the presence of both Mn and Cu chlorides to afford good yields of 1,4-addition products. This procedure is generally superior to the classical Cu-catalyzed conjugate addition of RMgX as well as to the reaction of organocuprates (eq 11).

Mn-Catalyzed Acylation of Organomagnesium Reagents by Acyl Chlorides.6

In THF, in the presence of MnCl4Li2, RMgX react with acyl chlorides under mild conditions to give the corresponding ketones in high yield. This procedure is very interesting from both practical and economical points of view. The chemoselectivity is excellent (eq 12).6

Diastereoselective Reduction of Ketones.7

In the presence of MnCl2, the reduction of a-methyl b-keto esters or amides by Sodium Borohydride leads to the corresponding b-hydroxy esters or amides with an excellent stereoselectivity (eq 13).

Regioselective and Stereoselective Preparation of Manganese Enolates: Applications in Organic Synthesis.8

Mn enolates are easily prepared from Na, Mg, or Li enolates by transmetalation with MnCl2 (or MnBr2). The stereo- and the regiochemistry of the starting enolate is maintained during the reaction.

It is important to note that Mn enolates can also be quantitatively obtained regio- and stereoselectively (>95% of the less hindered (Z) isomers) by enolization of ketones with Mn amides prepared from MnCl2. This procedure very often gives better yields and the (Z) stereoselectivity is clearly superior.

Mn enolates are regioselectively alkylated in good yields; interestingly, the formation of polyalkylated products is never observed (eq 14). They react regio- and stereospecifically with Me3SiCl or carboxylic acid anhydrides to give respectively silyl enol ethers and enol esters (95-99% Z) in high yields (eq 15). Finally, they add to a,b-ethylenic carbonyl compounds (Michael addition) as well as to aldehydes and reactive ketones (diastereoselective aldolization condensation). In all cases, the reactions give excellent yields and occur regioselectively.

Related Reagents.

n-Butylmanganese Chloride.

1. Cahiez, G.; Alami, M. T 1989, 45, 4163.
2. Cahiez, G.; Bernard, D.; Normant, J. F. JOM 1976, 113, 107.
3. Cahiez, G.; Bernard, D.; Normant, J. F. JOM 1976, 113, 99.
4. Fugami, K.; Oshima, K.; Utimoto, K. CL 1987, 2203.
5. Cahiez, G.; Alami, M. TL 1989, 30, 3541.
6. Cahiez, G.; Laboue, B. TL 1992, 33, 4439.
7. Fujii, H.; Oshima, K.; Utimoto, K. TL 1991, 32, 6147.
8. (a) Cahiez, G. Fr. Patent Appl. 88/15 806, 1988; Eur. Patent Appl. 373 993, 1990 (CA 1991, 114, 61 550y). (b) Cahiez, G.; Cléry, P.; Laffitte, J. A. Fr. Patent Appl. 90/16 413, 1990 and 91/11 814, 1991; PCT Int. Appl. 93/06 071, 1993 (CA 1993, 118, 169 340b and 1993, 119, 116 519f).
9. Hiyama, T.; Obayashi, M.; Nakamura, A. OM 1982, 1, 1249.
10. Cahiez, G.; Chavant, P. TL 1989, 30, 7373.

Gérard Cahiez

Université Pierre & Marie Curie, Paris, France

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