[10241-05-1] · Cl5Mo · Molybdenum(V) Chloride · (MW 273.19)
(chlorinating agent for alkyl halides,5 alkenes,7 and alkynes;8 used as a Lewis acid and as a catalyst for CO2 insertion to oxiranes,9 for C-C coupling of alkenyllithium reagents,10 and for condensation of nitrobenzene with alcohols;11 easily reduced to MoIII species, useful as a reagent of deoxygenation;12,13 methylenemolybdenum reagents for chemoselective carbonyl alkenation are prepared from MoCl517,18)
Alternate Name: molybdenum pentachloride.
Physical Data: mp 204 ± 1 °C; bp 276.5 °C; d 2.936 g cm-3.
Solubility: sol H2O, where it undergoes hydrolysis; slightly sol TiCl4, diethyl ether.
Analysis of Reagent Purity: pure reagent (99.9%) is commercially available, packaged in ampules.
Preparative Methods: by the reaction of molybdenum(VI) oxide and carbon tetrachloride or CCl2=CClCCl3 as very dark-colored crystals.1-3 The industrial method is the direct chlorination of molybdenum metal. A convenient laboratory technique is the liquid-phase reaction system that uses refluxing thionyl chloride and MoO3.4
Handling, Storage, and Precautions: moisture sensitive; store under nitrogen. Use in a fume hood.
Alkyl iodides, bromides, and fluorides are converted to alkyl chlorides in fair to good yields by reaction with MoCl5 in CH2Cl2 at room temperature (eq 1). The reaction sequence is applicable to the conversion of secondary and tertiary halides (I, Br, or F). MoCl5 does not effect halogen interchange with primary alkyl bromides. The conversion of 1-fluorooctane to 1-chlorooctane is accompanied by rearrangement producing 2-chlorooctane (eq 2). A Lewis acid-assisted ionization of the carbon-halogen bond followed by conversion of the carbonium ion to chlorocarbon by the reaction with halometallo-ate complex is a likely mechanism for this transformation.
The reaction of alkyl chlorides with an excess of MoCl5 leads to vicinal chlorides in fair to excellent yields. For example, 2-chlorobutane is converted (eq 3) to (±)-2,3-dichlorobutane (58%) and meso-2,3-dichlorobutane (38%). The reaction sequence has been applied to the chlorination of alkyl halides containing either a secondary or tertiary vicinal carbon (eq 4).
Chlorination of alkenes with MoCl5 was first observed in the preparation of molybdenum(IV) chloride under irradiation, where upon hexachloroethane was produced from tetrachloroethylene (eq 5). Molybdenum(V) chloride reacts with vicinal disubstituted alkenes and internal alkynes to produce dichloroalkanes and -alkenes, respectively, in fair to good yields. In this case, irradiation is not necessary. Typically (eq 6), cyclohexene in CH2Cl2 was treated with MoCl5 to give cis-1,2-dichlorocyclohexane (68%), trans-1,2-dichlorocyclohexane (2%), cyclohexylcyclohexane (2%), and chlorocyclohexane (2%). Vicinal dichlorination may not proceed via the ionic or radical pathway characteristically observed in the reaction of alkenes with molecular chlorine. Chlorination of alkynes gives cis-addition (eq 7).
The reagent Molybdenum(V) Chloride-Triphenylphosphine (1:6) promotes the insertion of CO2 into methyloxirane at 20 °C under 1 atm (eq 8). This catalyst system gave best conversions and yields when using a 100- to 200-fold excess of oxirane relative to MoCl5.9
The conversion of trans-propenyllithium to 2,4-hexadiene is catalyzed by MoCl5 (eq 9). Comparable yields were obtained with MoCl5 and NiII reagents.10
Nitrobenzene reacts with ethanol and higher alcohols in the presence of this reagent to form 2-methyl- or 2,3-dialkylquinolines (eq 10). The best yields and conversions are obtained in the presence of about equimolar or 0.5:1.0 mixtures of Tetracarbonyl(di-m-chloro)dirhodium and MoCl5 at 180 °C. Molybdenum species, due to their Lewis acidity, catalyze mainly the condensation reaction.11
An MoIII species obtained by reaction of MoCl5 with Zinc in aqueous THF reduces sulfoxides to sulfides in 78-91% yields (eq 11) and aromatic N-oxides in 45-65% yield (eq 12). The reagent also reduces aromatic and heteroaromatic nitro compounds to amines in 30-70% yields (eq 13). See also Molybdenum(V) Chloride-Zinc.
Molybdenum(V) chloride is used as a Lewis acid for selective cleavage of carbon-oxygen bond of O,S-acetals and acetals in the reaction of trimethylstyrylsilanes, leading to the corresponding styryl adducts (eqs 14 and 15).
MoCl5 and substitution products of MoCl5, such as MoOCl3(THF)2 or (MeO)2MoCl3 (= MoCl5 + addition of THF or MeOH), are educts for a series of carbonyl methylenating reagents which are generated by reaction with Methyllithium16-18 or Trimethylaluminum.19 The reaction with MeLi is believed to occur according to eq 16 and yields carbonyl methylenating m-methylene complexes which are in contrast to the carbonyl alkenating reagents of Wittig, Horner-Emmons, Peterson, Schrock, and Tebbe Lewis-acids and are highly chemoselective.
A limiting factor is the high thermolability of the reagents. The most closely investigated reagents are the following.
MoCl5 + 2 MeLi.17
This reagent is inert towards the groups -COCl, -CO2R, and -CONR2, but methylenates aldehydes and ketones in good yields and discriminates well between aldehyde and keto groups in favor of the aldehyde group (e.g. eq 17). A corresponding reaction occurs with azomethines. The reaction with aromatic epoxides yields the corresponding alkenes, whereas in the case of aliphatic epoxides, chlorohydrins are produced.
MoOCl3(THF)2 + 2 MeLi.18
This reagent allows regioselective carbonyl methylenation due to high aldehyde vs. ketone selectivity and cheleselectivity (preference of a keto group in the a- or b-position to a hydroxy group before an isolated keto group, e.g. eq 18), and tolerates the groups -COCl, -CO2R, and -CONR2 as well as water or alcohols in the solvent. It allows highly selective monomethylenation of dialdehydes, diketones, and triketones (e.g. eq 19).16
Stirring NaCp with MoCl5 in THF at rt produces MoIVCp2Cl2; CpMgCl reacts with MoCl5 to give [Cp2MoIVCl]+ ions, which can be precipitated by Reinecke acid.20
Universität Münster, Germany
S. Torii & T. Inokuchi
Okayama University, Japan