Vanadium(II) Chloride

VCl2

[10580-52-6]  · Cl2V  · Vanadium(II) Chloride  · (MW 121.84)

(nitro group to carbonyl conversion; hydrodehalogenation of a-halo ketones; reductions; reductive cleavage of oximes; reductive hydrolysis of 2,4-DNP derivatives)

Alternate Names: vanadium dichloride, vanadium chloride.

Physical Data: mp 910 °C (subl); d 3.23 g cm-3.

Solubility: sol with decomposition in hot and cold water; sol alcohols, DMF, THF, ethers.

Form Supplied in: commercially available as a green deliquescent solid.

Vanadium(II) Chloride.

Many reagents based on chromium(II), titanium(II), and titanium(III) ions have found utility in organic synthesis because of their redox potentials (Table 1). Vanadium(II) chloride has found application in modern organic synthesis in part because its redox potential (+0.255 V) is greater than the SnII-SnIII couple (-0.15 V) although lower than CrII-CrIII (+0.41 V) or TiII-TiIII (+0.37 V).1 A variety of synthetic transformations have been documented.

Conversion of the Nitro Group to the Carbonyl Group.

When an aqueous solution of vanadium chloride is added to a primary or secondary aliphatic nitro compound dissolved in a mixture of water, hydrochloric acid, and DMF, a moderate to good yield (50-70% typically) of the corresponding ketone is isolated (eq 1).2 The pH-dependent procedure involves initial reduction and subsequent hydrolysis of the resulting carbonyl imine. Several alternative methods are known3 which will effect the same transformation.

Hydrodehalogenation of a-Halo Ketones.

When an aqueous solution of vanadium chloride is added to a-halo ketones1 in THF, a mildly exothermic reaction ensues which yields the halogen-free product after extractive workup (eq 2). Yields are generally quite high, ranging between 80 and 98%.

Reductions.

This reagent also reduces benzils to benzoins (THF-H2O, 80-90% yield),4 quinones to hydroquinones (90-95% yield),4 and aryl azides to the corresponding amines and N2 (70-95% yield) (eq 3).5

Reductive Cleavage of Oximes.

Aqueous solutions of VCl2 have been employed in the mildly exothermic deoximation of oximes to the corresponding carbonyl compounds in 75-90% yield (eq 4).6

Reductive Hydrolysis of 2,4-Dinitrophenylhydrazones.

The regeneration of carbonyl compounds from 2,4-dinitrophenylhydrazones is often problematic owing to the acid stability of the parent molecules. Vanadium chloride promotes the hydrolysis to the respective carbonyl moiety in 67-95% yield via initial reduction of the nitro group.7

Other Vanadium(II) Reagents.

Aryl and alkyl halides are reduced by a number of low-valent transition metals.8 The complex9 VCl2(py)4 reduces activated (e.g. Bn-Cl) but not unactivated halo compounds (e.g. vinyl halides). This reagent is selective towards the formation of coupled R-R products to the exclusion of R-H-type products. In contrast, CrII reduces10 Bn-Cl to various ratios of bibenzyl and toluene (dependent on the reaction conditions). Bis(cyclopentadienyl)vanadium11 (Cp2V) is also effective in these reactions. In an extension of earlier work,12 2,2,2-trichloroacetanilide has been selectively reduced to 2,2-dichloroacetanilide using VCl2(py)4. Other complexes13 of divalent vanadium having the general formula V(amine)4X2 are also known. The amine can be either aromatic (e.g. picoline) or aliphatic (e.g. ethylenediamine). The V(amine)4X2 chemistry remains largely unexplored.

Recently, a bimetallic VII species, [V2Cl3(THF)6]2[Zn2Cl6], prepared in situ from VIII, has been introduced14 to achieve the stereoselective cross coupling of two different alkanals under mild conditions. The intermolecular pinacol cross-coupling reaction has been modified15 to include chiral aldehydes which yield syn-diols in a 91:9 diastereoisomeric ratio (up to 84% ee). Reductive cyclization of d,ε-enals has also been demonstrated16 to proceed with excellent stereoselectivity (eq 5), in contrast to what is obtained with other reagents such as SmII.


1. Ho, T.-L.; Olah, G. A. S 1976, 807.
2. Kirchoff, R. TL 1976, 2533.
3. (a) McMurry, J.; Melton, J. JACS 1971, 93, 5309. (b) McMurry, J.; Melton, J.; Padgett, H. JOC 1974, 39, 259.
4. Ho, T.-L.; Olah, G. A. S 1976, 815.
5. Ho, T.-L.; Henninger, M.; Olah, G. A. S 1976, 815.
6. Olah, G. A.; Arvanaghi, M.; Surya, G. K. S 1980, 220.
7. Olah, G. A.; Chao, Y.-L.; Arvanaghi, M.; Surya Prakash, G. K. S 1981, 476.
8. Kustin, K. Prog. Inorg. Chem. 1969, 13, 107.
9. Cooper, T. A. JACS 1973, 95, 4158.
10. de Liefde Meijer, H. J.; Janssen, M. J.; van der Kerk, G. J. M. RTC 1961, 80, 831.
11. Eisch, J. J.; King, R. B. Organomet. Synth. 1965, 1, 65.
12. Cooper, T. A.; Sonneberg, F. M. JOC 1975, 40, 55.
13. Kamar, M. M.; Larkworthy, L. F.; Patel, K. C.; Philips, D. J.; Beech, G. AJC 1974, 27, 41.
14. Raw, A. S.; Pedersen, S. F. JOC 1991, 56, 830.
15. Annunziata, R.; Cinquini, M.; Cozzi, F.; Giaroni, P.; Benaglia, M. T 1991, 47, 5737.
16. Inokuchi, T.; Kawafuchi, H.; Torii, S. JOC 1991, 56, 4983.

Benoit Vanasse & Michael K. O'Brien

Rhône-Poulenc Rorer Pharmaceuticals, Collegeville, PA, USA



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