[110615-13-9]  · C4H10Cl3NbO2  · Trichloro(1,2-dimethoxyethane)niobium  · (MW 289.40)

(synthesis of 1,2-amino alcohols,2 pyrroles,3 1,2-diols,5 cis-alkenes, and naphthols7)

Physical Data: mp 230 °C (dec).

Solubility: sol THF.

Preparative Methods: can be prepared according to the literature procedure,1 or purchased from commercial sources. The best results are obtained with freshly prepared material.

Handling, Storage, and Precautions: all reactions and manipulations should be performed under nitrogen, either in a dry box or using standard Schlenk techniques. NbCl3(DME) is stable at room temperature when stored under an inert atmosphere.

Synthesis of 1,2-Amino Alcohols.2

Addition of an imine to NbCl3(DME) results in formation of a metallaaziridine, which is functionally the equivalent of a C,N dianion (eq 1). Subsequent addition of aldehydes or ketones to this intermediate, followed by hydrolysis, gives 1,2-amino alcohols. The yield of this reaction ranges from 33-97%, with syn-amino alcohols being obtained as the major diastereomer (Table 1). A wide variety of aldehydes are tolerated; however, nonenolizable imines generally give superior results.

Synthesis of Pyrroles.3

Pyrroles may be directly obtained from the reaction of a,b-unsaturated imines with NbCl3(DME), followed by addition of either a primary ester or N,N-Dimethylformamide (Table 2). The niobium intermediate behaves like a homoenolate equivalent, condensing with the ester or formamide at the b-position. Following carbon-carbon bond formation, cyclization and dehydration occur to give the pyrrole. Aldehydes and ketones react with a,b-unsaturated imines and NbCl3(DME) to give 2-aminotetrahydrofurans, which hydrolyze to lactols when chromatographed on silica gel.4

Synthesis of 1,2-Diols.

Aldehydes can be converted to 1,2-diols in the presence of NbCl3(DME), as shown for the coupling of benzaldehyde to (±)-stilbenediol in 78% yield (eq 2).5 This reaction must be conducted at -10 °C in order to avoid acetalization of the product or reduction of the 1,2-diol to an alkene. Mixtures of products were obtained when unsymmetrical cross couplings of aldehydes were attempted.6

Synthesis of cis-Alkenes and Naphthols.7

NbCl3(DME) reacts with internal alkynes to form alkyne complexes, which can be hydrolyzed with aqueous hydroxide to give cis-alkenes exclusively (eq 3). In contrast, reaction of NbCl3(DME) with terminal alkynes yields cyclotrimerization products.

Niobium alkyne complexes are functionally equivalent to 1,2-dianions of cis-alkenes, and further reaction of these intermediates with benzene-1,2-dicarbaldehyde yields 2,3-disubstituted 1-naphthols (Table 3). The regioselectivity of these reactions appears to be influenced predominantly by steric factors. Alkynes substituted with t-Bu, TMS, or TBDMS groups, for example, give naphthols with the bulky group at the 3-position.

1. Pedersen, S. F.; Hartung, Jr., J. B.; Roskamp, E. J.; Dragovich, P. D. Inorg. Synth. 1992, 29, 119.
2. Roskamp, E. J.; Pedersen, S. F. JACS 1987, 109, 6551.
3. Roskamp, E. J.; Dragovich, P. S.; Hartung, Jr., J. B.; Pedersen, S. F. JOC 1989, 54, 4736.
4. Roskamp, E. J.; Pedersen, S. F. Unpublished results.
5. Szymoniak, J.; Besancon, J.; Moïse, C. T 1992, 48, 3867.
6. Roskamp, E. J.; Pedersen, S. F. Unpublished results.
7. (a) Hartung, Jr., J. B.; Pedersen, S. F. JACS 1989, 111, 5468. (b) Hartung, Jr., J. B.; Pedersen, S. F. OM 1990, 9, 1414.

Carrie A. Roskamp & Eric J. Roskamp

Northwestern University, Evanston, IL, USA

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