[4375-83-1]  · C6H18BN3  · Tris(dimethylamino)borane  · (MW 143.08)

(macrolactamization reagent;4 selective monoalkylation of tetraaza macrocycles;5 amide and enamine formation7)

Physical Data: mp -10 °C; bp 147-148 °C; d 0.817 g cm-3.

Solubility: sol most aprotic organic solvents; reacts readily with water and alcohols.

Form Supplied in: colorless liquid, commercially available.

Handling, Storage, and Precautions: air and moisture sensitive. Handle and store under nitrogen, use in a fume hood.

Tris(dimethylamino)borane undergoes facile transamination reactions with primary and secondary amines to give higher homologs of tris(alkylamino)boranes.2 Reaction with triamines, such as 3,3-diaminodipropylamine, affords unique heterocyclic aminoboranes (eq 1).3

An elegant application of this reaction has been found in the synthesis of macrocyclic spermidine alkaloids. The key step in the synthesis involves a B(NMe2)3-mediated macrolactamization reaction of triamino ester (1) (eq 2). The reaction is carried out with B(NMe2)3 in refluxing xylene and directly affords the macrolactam (3) in high yield.4 The reaction is believed to proceed via the intermediate (2), which allows for a boron-templated cyclization. The scope of this macrolactamization reaction is limited to triamino ester precursors. Attempted cyclization of the corresponding o-monoamino esters failed to give any lactam.

A related application of B(NMe2)3 has been utilized in the synthesis of monoalkylated derivatives of tetraaza macrocycles.5 Reaction of B(NMe2)3 with a tetraaza macrocycle affords a polycyclic aminoborane6 which incorporates three out of the four nitrogen atoms, thus allowing for selective monoalkylation of the single unprotected nitrogen (eq 3).

Amidation Reactions with B(NMe2)3 and Higher Homologs.

Tris(dialkylamino)boranes react readily with carboxylic acids to give amides.7 This reaction, however, has not been widely used in recent years. The reaction is typically carried out in benzene, in the absence of any catalyst, and one molar equivalent of the boron reagent is required for complete reaction.8 Similarly, reaction with 1,3-diketones gives enamino ketones (eq 4).7 When the 1,3-diketone is unsymmetrical, then the least-hindered ketone is attacked selectively. Reaction with ketones to give enamines is rather slow; however, in the presence of an acid catalyst, high yields are possible.7

1. For a general discussion on aminoboranes, see: Pelter, A.; Smith, K. In Comprehensive Organic Chemistry; Barton, D. H. R.; Ollis, W. D., Eds.; Pergamon: Oxford, 1979; Vol. 3, pp 925-932.
2. Steinberg, H.; Brotherton, R. J. Organoboron Chemistry; Interscience: New York, 1966; Vol. 2, pp 4-44.
3. Niedenzu, K.; Fritz, P.; Dawson, J. W. IC 1964, 3, 1077.
4. Yamamoto, H.; Maruoka, K. JACS 1981, 103, 6133.
5. Bernard, H.; Yaouanc, J. J.; Clement, J. C.; des Abbayes, H.; Handel, H. TL 1991, 32, 639.
6. For a related synthesis of polycyclic aminoboranes, see: Richman, J. E.; Yang, N.-C.; Andersen, L. L. JACS 1980, 102, 5790.
7. Nelson, P.; Pelter, A. JCS 1965, 5142.
8. For a review of the mechanism of the amidation reaction, see: Pelter, A.; Levitt, T. E. T 1970, 26, 1899.

Michael R. Michaelides

Abbott Laboratories, Abbott Park, IL, USA

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