Tetracarbonylhydridoferrate Salts: NaHFe(CO)₄ and KHFe(CO)₄

Biographical Sketches

Jiří Pospíšil was born in Třebíč (Czech republic) in 1979. He completed his M.Sc. (2002) in chemistry at Masaryk University in Brno, Czech republic, under the supervision of Prof. M. Potáček. He is currently working towards his Ph.D. in organic chemistry under the supervision of Prof. I. E. Markó at the Université catholique de ­Louvain, Louvain-la-Neuve, Belgium. His research interests include the total synthesis of natural products and the development of new methodologies.

Introduction

The HFe(CO)₄ anion was first prepared by Feigl and Krumholtz in 1932 [1] in its sodium form and the structure was unambiguously established by X-ray diffraction ­analysis of an appropriate bis(triphenylphosphoranyl­idene)ammonium salt nearly 50 years later. [2] This monohydride iron complex is a source of hydride anion that behaves as an extremely soft reducing reagent. Therefore, this hydride complex is highly suitable for the reduction of, for example, unactivated olefins in the presence of a wide variety of other functional groups. Moreover, all the reactions mediated by NaHFe(CO)₄ or KHFe(CO)₄ ­proceed under extremely mild and essentially neutral ­conditions.

The second main property of NaHFe(CO)₄ or KHFe(CO)₄ is their high affinity for alkynes, with which they form various iron complexes. These complexes are not only ­interesting for their structure, but also for their chemical reactivity.

Preparation

The preparation of NaHFe(CO)₄ and KHFe(CO)₄ is easy and efficient, but the reagents cannot be stored for a long period of time, and they are light sensitive (formation of nonreactive dimer).

KHFe(CO)₄ is generally prepared in situ, prior to the reaction, from Fe(CO)₅ (1 equiv) and KOH (3 equiv) in MeOH, under an inert atmosphere. [3] THF solutions of NaHFe(CO)₄ can be prepared from sodium amalgam and Fe(CO)₅ in anhydrous THF.

NaHFe(CO)₄ or KHFe(CO)₄ can be also generated by careful acidification of Na₂Fe(CO)₄ or K₂Fe(CO)₄. [4]

Abstracts

(A) Brunet and co-workers showed that KHFe(CO)4 is a selective reducing agent for the reduction of electron deficient ketones as well as a highly selective reducing agent for the mono-reduction of the keto group in α-ketocarbonyl compounds. Ester, amide and ­nitrile groups, as well as other unactivated carbonyl functions, ­remain intact under the reaction conditions. [5]
(B) In the presence of NaHFe(CO)4 or KHFe(CO)4, azides or acyl azides can be converted into the corresponding primary amines or amides. [6]
(C) HFe(CO)4 salts are convenient and general hydride sources for reductive amination reactions. [7] This method can be applied to ­aromatic as well as aliphatic amines and aldehydes. When dialdehydes are employed, cyclic amines are obtained. [8]
(D) Selective reduction of α,β-unsaturated carbonyl compounds is another typical reaction of NaHFe(CO)4 or KHFe(CO)4. Under these conditions, a wide variety of functional groups such as ketones, esters, nitriles, amides, etc. are tolerated and the reaction can be performed in the presence of other olefins.3 One exception are α,β-unsaturated diketones that undergo deacylative reduction ­under the reaction conditions, affording saturated monoketones. [9]
(E) Umani-Ronchi and co-workers described a formal ‘α-alkylation’ of carbonyl compounds (aldol reaction/elimination/reduction) with aldehydes, in the presence of KHFe(CO)4. The yields depend upon the steric requirements of the aldehyde. [10]
(F) Shin and co-workers used KHFe(CO)4 to convert carboxylic acid chlorides into the corresponding esters under mild and neutral conditions. [11]
(G) The reagent, formed by combining in situ NaHFe(CO)4 and MeI, reacts with alkynes. After CuCl2 oxidation, cyclobutene­diones can be obtained in moderate yields. [4] [12] Similarly, the ­reagent generated in situ from NaHFe(CO)4/CH2Cl2 reacts with alkynes and, after CuCl2 oxidation, leads to α,β-unsaturated ­acids. [13] The reaction using NaHFe(CO)4/TMSCl system gives the corresponding α,β-unsaturated carboxylic acids at room temperature and the corresponding cyclobutenediones at 60 °C.

References

1 Feigl F. Krumholtz P. Monatsh. Chem.  1932,  59:  314 

2 Smith M. Bau R. J. Am. Chem. Soc.  1973,  95:  2388 

3 Boldrini GP. Umani-Ronchi A. Synthesis  1976,  596 

4 Periasamy M. Radhakrishnan U. Brunet J.-J. Chauvin R. El Zaizi AW. Chem. Commun.  1996,  1499 

5 Brunet J.-J. Chauvin R. Kindela F. Neibecker D. Tetrahedron Lett.  1994,  35:  8801 

6 Shim SC. Choi KN. Tetrahedron Lett.  1985,  26:  3277 

7a Boldrini GP. Panunzio M. Umani-Ronchi A. Synlett  1974,  733 

7b Nakagawa Y. Stevens RV. J. Org. Chem.  1988,  53:  1873 

8a Watanabe Y. Shim SC. Uchida H. Mitsudo T. Takegami Y. Tetrahedron  1979,  35:  1433 

8b Shim SC. Huh KT. Park WH. Tetrahedron  1986,  42:  259 

9a Yamashita M. Watanabe Y. Mitsudo T.-A. Takegami Y. Tetrahedron Lett.  1975,  31:  1867 

9b Noyori R. Umeda I. Ishigami T. J. Org. Chem.  1972,  1542 

10 Cainelli GF. Panuzio M. Umani-Rochi A. J. Chem. Soc., Perkin Trans. 1  1975,  1273 

11 Shim SC. Huh KT. Park WH. Synthesis  1987,  59 

12 Periasamy M. Rameshkumar U. Brunet J.-J. J. Org. Chem.  1998,  63:  4930 

13 Periasamy M. Radhakrishnan U. Rameshkumar C. Brunet J.-J. Tetrahedron Lett.  1997,  38:  1623