Propynylmagnesium Bromide1

[32423-84-0]  · C3H3BrMg  · Propynylmagnesium Bromide  · (MW 143.27)

(powerful nucleophilic source of the propynyl unit for many electrophiles)

Solubility: sol THF, ether.

Form Supplied in: green viscous oil in ethereal solution; prepared in situ and used directly.

Preparative Method: prepared in situ by bubbling dried propyne through a solution of Ethylmagnesium Bromide in refluxing ether2 or THF3 at room temperature.

Handling, Storage, and Precautions: must be prepared and transferred under inert gas (Ar or N2) to exclude oxygen and moisture.

Additions to Carbonyl Compounds.

Propynylmagnesium bromide adds readily to aldehydes2 and ketones to give the corresponding secondary and tertiary alcohols. Addition of the optically active 2-chromanylacetaldehyde to propynylmagnesium bromide in ether gives a 2:1 mixture of the corresponding alcohols (eq 1).4 Intramolecular Diels-Alder/retro-Diels-Alder reactions of alkynones, prepared by propynylmagnesium bromide addition to aldehydes and Swern oxidation (see Dimethyl Sulfoxide-Oxalyl Chloride) of the resulting alcohols, give ring-fused thiophene derivatives (eq 2).5 Stereoselective addition to the exo face of a norbornenone is observed with only 4% of the minor isomer being formed (eq 3).6 Chelation-controlled addition to the b-alkoxy enone gives the (R) enantiomers of both geometric isomers with only traces of the diastereoisomers (eq 4), whereas addition of Propynyllithium to the corresponding aldehyde is less selective.3 Lithium Aluminum Hydride reduction of the propargylic alcohol, generated by addition of propynylmagnesium bromide to a-chlorocyclohexanone, is accompanied by migration to give the homoallylic alcohol (eq 5).7

Alkylations of Alkyl Halides.

a-Chlorination of cyclic sulfides with N-Chlorosuccinimide gives the corresponding chlorides, which react with propynylmagnesium bromide to give the propargylic sulfides. Sulfonium salt formation followed by treatment with 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) causes a ring expansion via [2,3]-sigmatropic rearrangement to the diene (eq 6).8 In a related transformation, propynylmagnesium bromide couples with a cationic glycine equivalent to give the a-alkynic amido ester free from allenic products (eq 7).9

Reactions with Other Electrophiles.

Enynes can be prepared by a double ring contraction sequence which involves reaction of propynylmagnesium bromide with cyclooctatetraene oxide to give the cycloheptatrienyl alcohol and subsequent dehydration with Thionyl Chloride (eq 8).10 Activation of Pyridine by reaction with Methyl Chloroformate facilitates addition of propynylmagnesium bromide to the resulting iminium salt (eq 9).11 Propynylmagnesium bromide reacts with the hindered aromatic chlorophosphine to give the alkynyl-1H-phosphine by direct substitution (eq 10), while the more basic propynyllithium gives the tautomeric 3H-phosphaallene.12

Related Reagents.

Ethynylmagnesium Bromide; Propynyllithium.

1. Brandsma, L. Preparative Acetylene Chemistry, 2nd ed.; Elsevier: Amsterdam, 1988.
2. Chan, K.-K.; Cohen, N.; De Noble, J. P.; Specian, A. C., Jr.; Saucy, G. JOC 1976, 41, 3497.
3. Jacobi, P. A.; Selnick, H. G. JOC 1990, 55, 202.
4. Chan, K.-K.; Specian, A. C., Jr.; Saucy, G. JOC 1978, 43, 3435.
5. Jacobi, P. A.; Egbertson, M.; Frechette, R. F.; Miao, C. K.; Weiss, K. T. T 1988, 44, 3327.
6. Fleming, I.; Terrett, N. K. TL 1984, 25, 5103.
7. Wender, P. A.; Holt, D. A.; Sieburth, S. M. JACS 1983, 105, 3348.
8. Sashida, H.; Tsuchiya, T. CPB 1986, 34, 3644.
9. Castelhano, A. L.; Horne, S.; Taylor, G. J.; Billedeau, R.; Krantz, A. T 1988, 44, 5451.
10. Hambrecht, J.; Straub, H.; Müller, E. CB 1974, 107, 2985.
11. Natsume, M.; Ogawa, M. H 1983, 20, 601.
12. Märkl, G.; Reitinger, S. TL 1988, 29, 463.

Nicholas Greeves

University of Liverpool, UK

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