n-Butylmanganese Chloride

n-BuMnCl

[88626-90-8]  · C4H9ClMn  · n-Butylmanganese Chloride  · (MW 147.51)

(butylmanganese halides are especially interesting for their chemoselectivity; acylation,1,2 carboxylation,3 1,2-addition to ketones and aldehydes,4 1,4-addition to conjugated unsaturated ketones,5 aldehydes,6 esters7 and amides as well as alkylation of alkyl bromides8 can be performed under mild conditions and in high yields)

Preparative Methods: 7b BuMnCl can be prepared in THF, at 0 °C, by reaction of n-Butyllithium or BuMgX with Manganese(II) Chloride or, better, with the soluble ate complex MnCl4Li2. The latter is easily obtained by mixing MnCl2 and Lithium Chloride (2 equiv) in anhydrous THF at rt until dissolution (about 1 h). BuMnBr can be obtained in ether or in THF, between -10 °C and 0 °C, from BuLi or BuMgX and MnBr4Li2 (prepared by mixing MnBr2 + 2 LiBr in ether or THF at rt). BuMnI is prepared by adding BuLi or BuMgX to MnI2 in ether or in THF between -10 °C and -20 °C. Organomanganese iodides are now of historical interest only since organomanganese chlorides in THF, or organomanganese bromides in ether, are both less expensive and more stable.

Commercial MnCl2 and MnBr2 must be dried by heating under vacuo before using (200 °C, 10-2 mmHg, 2 h). Anhydrous MnBr2 can be conveniently prepared (>95%) by reaction of Br2 with Mn (1.01 equiv, electrolytic grade) in anhydrous ethyl acetate. The reaction is exothermic and the temperature must be kept between 35 and 45 °C. Anhydrous MnI2 is obtained by adding Iodine slowly (exothermic) to a suspension of Mn (1.01 equiv, electrolytic grade) in ether at rt. The use of manganese halides containing some impurities such as Ni, Fe, and Cu salts can strongly decrease the stability of BuMnX (and more generally of all alkylmanganese halides).

Handling, Storage, and Precautions: butylmanganese halide solutions must be handled under an inert atmosphere in the absence of moisture. In THF, BuMnX are stable at rt. In ether, BuMnI is stable at -10 °C and BuMnBr at 10 °C. These reagents are best generated in situ and used rapidly.

Acylation.1

BuMnI1a,b and BuMnBr1f (prepared in ether) react under mild conditions with a vast array of acylating agents (RCOCl, (RCO)2O, RCO2CO2Et, etc.) to give the corresponding ketones in high yields (eqs 1 and 2); similarly, ClCO2Et leads to esters.1c BuMnCl prepared in THF is also acylated in high yield under mild conditions (eq 3).1d

In some cases the yield can be improved in the presence of copper salts (eq 4). A dramatic improvement is especially observed with s- or t-alkylmanganese chlorides (eq 5).

Interestingly, this acylation is highly chemoselective and numerous functional groups are tolerated;1b,d for example, halides, allylic halides, nitriles, esters, and even unprotected keto groups (eq 6).1b In addition, the reaction can be performed in the presence of various cosolvents which are not commonly used in organometallic chemistry, such as CH2Cl2 or EtOAc (eq 7).1e

The acylation of organomanganese reagents is a very general reaction and numerous functionalized or polyunsaturated ketones can be prepared in high yields.

Acylation: Catalytic Procedure.2

Butylmanganese compounds are also involved in the Mn-catalyzed acylation of BuMgCl (eq 8). This procedure is very economic and easy to carry out; in addition, the chemoselectivity is generally excellent (eq 9).

Carboxylation.3

Butylmanganese halides are easily carboxylated in ether or in THF to give the expected BuCO2H in good yields. Contrary to BuLi and BuMgX, butylmanganese halides smoothly react at room temperature (the exothermic effect is very limited) without side-reaction. Thus they do not add to the resulting BuCO2MnX. The related lithium or magnesium tributylmanganates can also be carboxylated under similar conditions (eq 10).

1,2-Addition.4

In ether, butylmanganese halides readily add to aldehydes and ketones at 0 °C, affording the corresponding alcohols in high yields (eqs 11 and 12).1g,4a However, they do not react with various less reactive carbonyl compounds such as amides and esters (except with formates).3 It should be noted that BuMnX can be used in the presence of various amides (NMP, DMF) or ester (EtOAc) as cosolvent.

In THF, butylmanganese halides are less reactive towards carbonyl compounds.4a Thus they also add to aldehydes at rt in high yields (eq 13) but with ketones the reaction is slower and results in lower yields than in ether (from BuCOBu, 70% instead of 92%).

Butylmanganese halides prepared from BuMgX are more reactive than those prepared from BuLi (electrophilic activation by MgX2) (eq 14). In general, organomanganese reagents are of use when performing chemoselective 1,2-additions to functionalized ketones or aldehydes (eq 15).

A recent report4b showed that in THF, BuMnO(CO)-t-Bu adds more stereoselectively than BuMnBr to 4-t-butylcyclohexanone (axial preference) (eq 16).

Conjugate Addition.5-7

BuMnCl readily adds to alkylidenemalonic esters to afford the conjugate addition products (eq 17).7b

With the less reactive Michael acceptors, such as a,b-ethylenic ketones, the conjugate addition reaction does not occur or gives low yields. However, in the presence of Copper(I) Chloride as catalyst, the 1,4-addition product is obtained in high yields under very mild conditions (eq 18).5 It is interesting to note that the conjugate addition reaction also takes place easily with the less reactive b-disubstituted enones (eq 19).

Butylmanganese compounds are also involved in the Mn-Cu-catalyzed conjugate addition of BuMgCl to enones (eq 20). The reaction has been successfully extended to a,b-ethylenic aldehydes (eq 21)6 and, in the presence of Me3SiCl, to a,b-ethylenic esters (eq 22)7a and amides.

Generally, the Cu-catalyzed conjugate addition of organomanganese chloride reagents (as well as the Mn-Cu-catalyzed conjugate addition of RMgCl) gives higher yields and is easier to carry out than the classical organocopper procedures.5

Alkylation.8

Butylmanganese halides react only with a few very reactive alkylating agents such as Allyl Bromide. In addition, even with this latter, the reaction is very slow and gives a moderate yield (~70%). However, BuMnCl prepared in THF can be easily alkylated at rt in the presence of both copper salt (CuCl) as catalyst and NMP as cosolvent. Under these conditions the alkylated products are generally obtained in excellent yields even from the less reactive alkyl bromides (eq 23).

This procedure is as simple to carry out as the classical Cu-catalyzed Grignard reaction but the yields are very often better and the chemoselectivity is clearly superior. For example, in the case of eq 24, BuMgCl alone gives only a 45% yield.

Related Reactions.

All the reactions described above for butylmanganese halides can be performed with n-, s-, or t-alkyl- as well as with alkenyl-, aryl-, and allylmanganese halides. Some examples are: acylation (eqs 25-27),1d,2 one-pot acylation-alkylation (preparation of alcohols) (eq 28),1i conjugate addition (eqs 29-31),5 1,2-addition (eq 32),4 and alkylation (eqs 33 and 34).8

Alkynylmanganese halides also react easily with various acylating agents to give the a,b-alkynyl ketones in excellent yields, and they add to carbon dioxide as well as to ketones or aldehydes (eqs 35-37).1d,f,4 However, they cannot undergo the conjugate addition to a,b-unsaturated carbonyl compounds (except with the very reactive alkylidenemalonic esters)7b and they do not react with alkyl bromides under the conditions described above.


1. (a) Cahiez, G.; Bernard, D.; Normant, J.-F. S 1977, 130. (b) Friour, G.; Cahiez, G.; Normant, J. F. S 1984, 37; S 1985, 50. (c) Cahiez, G.; Normant, J. F. BSF(2) 1977, 570. (d) Cahiez, G.; Laboue, B. TL 1989, 30, 7369. (e) Cahiez, G. TL 1981, 22, 1239. (f) Cahiez, G.; Laboue, B. TL 1989, 30, 3545. (g) Cahiez, G.; Rivas-Enterrios, J.; Granger-Veyron, H. TL 1986, 27, 4441. (h) Cahiez, G.; Chavant, P.; Métais, E. TL 1992, 33, 5245. (i) Cahiez, G.; Rivas-Enterrios, J.; Clery, P. TL 1988, 29, 3659.
2. Cahiez, G.; Laboue, B. TL 1992, 33, 4439.
3. Friour, G.; Cahiez, G.; Alexakis, A.; Normant, J. F. BSF(2) 1979, 515.
4. (a) Cahiez, G.; Figadère, B. TL 1986, 27, 4445. (b) Reetz, M. T.; Haning, H.; Stanchev, S. T 1992, 33, 6963.
5. Cahiez, G.; Alami, M. TL 1986, 27, 569; TL 1989, 30, 3541.
6. Cahiez, G.; Alami, M. TL 1989, 30, 7365.
7. (a) Cahiez, G.; Alami, M. TL 1990, 31, 7423. (b) Cahiez, G.; Alami, M. T 1989, 45, 4163.
8. Cahiez, G.; Marquais, S. SL 1993, 45.

Gerard Cahiez

Université Pierre & Marie Curie, Paris, France



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