[110615-13-9] · C4H10Cl3NbO2 · Trichloro(1,2-dimethoxyethane)niobium · (MW 289.40)
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.
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.
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
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
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.
Carrie A. Roskamp & Eric J. Roskamp
Northwestern University, Evanston, IL, USA