Dicarbonyl(cyclohexadiene)(cyclopentadienyl)molybdenum Tetrafluoroborate

(BF4 salt)

[115252-55-6]  · C13H13BF4MoO2  · Dicarbonyl(cyclohexadiene)(cyclopentadienyl)molybdenum Tetrafluoroborate  · (MW 383.99) (PF6 salt)

[84117-10-2]  · C13H13F6MoO2P  · Dicarbonyl(cyclohexadiene)(cyclopentadienyl)molybdenum Hexafluorophosphate  · (MW 442.15) (cation)

[84117-09-9]  · C13H13MoO+2  · Dicarbonyl(cyclohexadiene)(cyclopentadienyl)molybdenum Tetrafluoroborate  · (MW 297.18)

(reagent for stereocontrolled multiple functionalization of cyclohexene rings1)

Physical Data: dec >300 °C.

Solubility: sol most polar organic solvents; very sol methylene chloride; insol diethyl ether and alkanes.

Form Supplied in: yellow crystals generally prepared by user from Mo(CO)6.

Analysis of Reagent Purity: carbonyl stretching bands at 2017 and 1962 cm-1 (CH2Cl2) are characteristic.

Preparative Methods: straightforwardly prepared in 97% yield by hydride abstraction from CpMo(CO)2(cyclohexenyl),2 which in turn is prepared following a procedure developed for the parent allyl complex by Hayter5 (>70% yield). This involves reflux of Mo(CO)6 in acetonitrile to produce Mo(CO)3(MeCN)3, addition of 3-bromo-1-cyclohexene, and subsequent treatment with lithium cyclopentadienide.

Handling, Storage, and Precautions: should be handled under nitrogen or argon. The crystals can be handled in air for short periods with no significant decomposition. Solutions of the compound are air sensitive, but can be handled with brief exposure to air or can be reduced in volume with a rotary evaporator. Use in a fume hood.

Introduction.

The use of this versatile reagent was originally developed by Faller and his co-workers2 and has been applied extensively in organic syntheses by Pearson3 and Liu.4

Hydride Abstraction From the Allyl Complex.

Hydride abstraction is one of several important processes that establish the routes available for multiple functionalization of the ring (eq 1).

Nucleophilic Addition to the Cationic Diene Complex.

The positive charge on the diene complex makes it susceptible to nucleophilic attack, which occurs on the side of the ring opposite to the metal. This yields a new substituted h3-cyclohexenyl complex, which can undergo a second hydride abstraction-nucleophilic addition process to yield a disubstituted h3-cyclohexenyl species (eq 2).2,3 Most soft nucleophiles, such as dimethyl malonate, cyanoborodeuteride, or enamines, are effective. Reactions with Grignard reagents provide access to the methyl and phenyl derivatives.

Deprotonation of the Cationic Diene Complex.

The positive charge on the complex renders the methylene protons adjacent to the bound double bonds acidic. Treatment with Triethylamine yields the h3-cyclohexadienyl complex (eq 3).6 Usually in this type of system both additions and abstractions of hydride occur on the side of the ring opposite to the metal.

Further Functionalization of the Ring.

The availability of the h3-cyclohexadienyl complexes allows further elaboration of the ring. For example, the h3-cyclohexadienyl complex undergoes a Boron Trifluoride-promoted ene reaction with aldehydes (-40 °C, toluene) (eq 4).4

An alternative to activation of the ring to nucleophilic attack is replacement of a carbonyl in the h3-cyclohexenyl complexes by NO+ (eq 5).7 Usually the addition occurs cis to the nitrosyl, although a number of factors can affect the stereochemistry of the addition.8

Removal of the Metal.

The functionalized cyclohexadiene can be decomplexed by oxidation of the cationic complex with Et3NO. Another alternative is to prepare the cyclohexene complex via NO+ addition to the h3-cyclohexenyl dicarbonyl and treat this with another nucleophile or cyanoborohydride to produce the h2-alkene complex. These h2-alkene complexes can be oxidized by Cerium(IV) Ammonium Nitrate (buffered with acetate)9 or air7 to yield the decomplexed cyclohexene. Pearson has also developed an iodolactonization methodology for decomplexation of the ring which is applicable in some cases (eq 6).9

Enantioselective Syntheses.

The sequential additions to the complexed cyclohexadienyl moiety should allow the development of a powerful strategy for asymmetric syntheses of substituted six-membered rings. Since the parent [CpMo(CO)2(cyclohexenyl)]+ complex has a plane of symmetry, attention has focused on obtaining nonracemic substituted derivatives. The h4-cyclohexadiene complex is essentially a meso structure since the bound alkenic carbons are all stereogenic centers. It follows that diastereoselectivity would be observed for one side or the other of a given face of the ring in nucleophilic attack by chiral nucleophiles. Moderate de's (78%) have been obtained on selective addition of (+)-t-butyldimethylsilyl sulfoximinyl ester enolates.10 The products then contain three new stereogenic centers, with the S(S)-enolate yielding primarily the (S,S,S)-product (eq 7).

Since the addition of chiral allylic acetates11,12 and presumably phosphonates13 to Mo(CO)3(MeCN)3 apparently occurs with retention of configuration, an alternative route into asymmetric cyclohexenols would be the use of enantiomerically pure cyclohexenols as starting materials. Although this has not yet been exploited extensively, Liebeskind12 recently reported stereocontrolled syntheses of a number of dihydropyrans and tetrahydropyrans using the [CpMo(CO)2(oxacyclohexadienyl)]+ system.


1. (a) Pearson, A. J. SL 1990, 10. (b) Pearson, A. J. In Advances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI: London, 1989; Vol. 1, p 1. (c) Blystone, S. L. CRV 1989, 89, 1663.
2. Faller, J. W.; Murray, H. H.; White, D. L.; Chao, K. H. OM 1983, 2, 400.
3. Pearson, A. J.; Kahn, M. N. I.; Clardy, J. C.; Cun-heng, H. JACS 1985, 107, 2748.
4. Wang, S.-H.; Cheng, Y.-C.; Lee, G.-H.; Peng, S.-M.; Liu, R.-S. OM 1993, 12, 3282.
5. Hayter, R. G. JOM 1968, 13, P1.
6. Pearson, A. J.; Mallik, S.; Mortezaei, R.; Perry, M. W. D.; Shively, R. J., Jr.; Youngs, W. J. JACS 1990, 112, 8034.
7. Faller, J. W.; Lambert, C. T 1985, 41, 5755.
8. Faller, J. W.; Chao, K. H.; Murray, H. H. OM 1984, 3, 1231.
9. Pearson, A. J.; Kahn, M. N. I. JACS 1984, 106, 1872.
10. Pearson, A. J.; Blystone, S. L.; Nar, H.; Pinkerton, A. A.; Roden, B. A.; Yoon, J. JACS 1989, 111, 134.
11. Faller, J. W.; Linebarrier, D. OM 1988, 7, 1670.
12. Rubio, A.; Liebeskind, L. S. JACS 1993, 115, 891.
13. McCallum, J. S.; Sterbenz, J. T.; Liebeskind, L. S. OM 1993, 12, 927.

John W. Faller

Yale University, New Haven, CT, USA



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