[59863-13-7]  · C12H36N2Si4Sn  · Bis[N,N-bis(trimethylsilyl)amino]tin(II)  · (MW 439.46)

(stannylene precursor; reduction of azides to primary amines; insertion with alkyl and aryl halides; enamine synthesis; conversion of esters and carboxylic acids to secondary and tertiary amides)

Alternate Name: tin(II) bis[bis(trimethylsilyl)amide].

Physical Data: mp 37-38 °C, 1H NMR(C6D6) d 0.30; 119Sn NMR(C6D6) d 770.

Solubility: sol hexane, benzene, Et2O, THF, acetonitrile.

Preparative Methods: a 500 mL round bottomed flask equipped with a nitrogen inlet was charged with 65 mL of dioxane and 40.1 g of anhydrous Tin(II) Chloride (0.21 mol). This suspension was allowed to stir for 30 min at rt, and was then treated with 71.0 g of Lithium Hexamethyldisilazide (0.43 mol) in several small portions. During the addition of LiN(TMS)2 the reaction mixture became warm and turned dark red-orange in color. A Vigreux column and vacuum distillation apparutus were attached to the flask, and it was heated at 35 °C for 45 min. The apparatus was then placed under vacuum (0.5 mmHg) and the bath warmed to 85 °C. The forerun, mainly dioxane, was collected with the help of a dry ice acetone bath. Bis[N,N-bis(trimethylsilyl)amino]tin(II) distilled as a bright red oil between 105-115 °C (80.4 g, 86%); this material solidified upon standing at rt.2

Handling, Storage, and Precautions: is nonpyrophoric but should be handled under a nitrogen atmosphere, and stored in a refrigerator. Since the solid is very soluble in hexane, it is convenient to prepare a hexane solution which may then be transferred via conventional syringe techniques. It readily hydrolyzes to tin(II) oxide, which is of reputed low toxicity. Use in a fume hood.

Stannylene Precursor.

The two silazane ligands on Sn[N(TMS)2]2 undergo facile metathesis reactions with amines, alcohols, amides, carboxylic acids, amine hydrochloride salts, and related compounds.1 For example, addition of 2 equiv of piperidine to Sn[N(TMS)2]2 gives bis(piperidinyl)tin(II) (1) as a bridging dimer (eq 1). Subsequent addition of 1 equiv of t-BuOH to (1) yields tin(II) alkoxyamide (2), also as a dimer. Primary amines react with Sn[N(TMS)2]2 to give tin(II) amines with a cubic structure.1

Reduction/Insertion Chemistry.

Sn[N(TMS)2]2 has been used for the reduction of azides to primary amines.3 It undergoes insertion reactions with both alkyl and aryl halides, especially bromides and iodides (eq 2).4

Enamine Synthesis.

Sn[N(TMS)2]2 converts a-unsubstituted aldehydes to trans-N,N-bis(trimethylsilyl)enamines in 22-40% isolated yield (eq 3).5 It does not react with more hindered a-substituted aldehydes, nor with ketones, esters, nitriles, acetals, terminal alkynes, alkenes, epoxides, or ethers. Addition of a secondary alkylamine to Sn[N(TMS)2]2 prior to addition of either an aldehyde or a ketone results in formation of a N,N-dialkylenamine (eq 4).6

Esters to Tertiary Amides.

Esters can be converted to tertiary amides using reagents derived in situ from Sn[N(TMS)2]2 (see Dimethylaluminum Amide). For example, addition of 1 equiv of piperidine to Sn[N(TMS)2]2 generates an unsymmetrical tin(II) amine, which converts methyl 2-phenylacetate to an amide in 74% yield (eq 5).7 Similarly, addition of both piperidine and N-t-butylacetamide to Sn[N(TMS)2]2 generates a new tin(II) amine via metathesis of both silazane ligands; this reagent converts methyl 2-phenylacetate to its piperidine amide in 93% yield.

An intramolecular variant of this methodology can be used for the preparation of b-lactams from b-amino esters. Nonsterically encumbered b-amino esters are readily cyclized to b-lactams simply with Sn[N(TMS)2]2 (eq 6).8 Substrates with bulky groups on nitrogen or at the b-position of the amino ester are converted to b-lactams in high yield using unsymmetrical tin(II) amines.

Finally, the ester to tertiary amide transformation can also be effected using the amine hydrochloride salts of volatile amines. Addition of Sn[N(TMS)2]2 to dimethylamine hydrochloride generates a tin(II) amido chloride, which efficiently converts a-unsubstituted esters to amides (eq 7).9

Esters to Secondary Amides.

Tin(II) amines with a cubic structure are formed in the reaction of Sn[N(TMS)2]2 with primary amines. These tetrameric tin(II) amines are less reactive towards esters than the dimeric tin(II) amines formed from Sn[N(TMS)2]2 and secondary amines. This lack of reactivity can be circumvented via two procedures, one of which is substrate based and the other, reagent based.10 In the substrate based approach, a glycol ester reacts with Sn[N(TMS)2]2 and a primary amine to give an intermediate tin(II) alkoxyamine. Intramolecular delivery of the amino group to the ester provides the secondary amide in high yield (eq 8). In the second approach, simple methyl esters can be converted to secondary amides using a reagent derived from Sn[N(TMS)2]2, N,N-dimethylethanolamine, and a primary amine (eq 9). Incorporation of N,N-dimethylethanolamine as the auxillary ligand appears to retard formation of cubic tetramers and help promote transfer of the primary amino group.

Carboxylic Acids to Secondary or Tertiary Amides.

Carboxylic acids that are less sterically encumbered than pivalic acid can be converted to amides using Sn[N(TMS)2]2.11 For example, reaction of 2-(t-butyldimethylsiloxy)-2-phenylacetic acid with Sn[N(TMS)2]2 and either a primary or secondary amine generates a tin(II) carboxyamine (eq 10). This intermediate is smoothly converted to an amide without racemization of the a-chiral center when heated at reflux for 1 h or stirred at rt overnight.

Related Reagents.

Dimethylaluminum Amide; Hexamethyldisilazane; Lithium Hexamethyldisilazide; Piperidine; Tin(II) Chloride; Titanium(IV) Chloride.

1. Veith, M. CRV 1990, 90, 3. Neumann, W.P. CRV 1991, 91, 311.
2. Burnell-Curty, C.; Roskamp, E. J. unpublished results. Schaeffer, C. D., Jr.; Myers, L. K.; Coley, S. M.; Otter, J. C.; Yoder, C. H. J. Chem. Educ. 1990, 67, 347.
3. Khmaruk, A. M.; Pinchuk, A. M. ZOR 1983, 19, 883.
4. Gynane, M. J. S.; Lappert, M. F.; Miles, S. J.; Power, P. P. CC 1978, 192.
5. Burnell-Curty, C.; Roskamp, E. J. JOC 1992, 57, 5063.
6. Burnell-Curty, C.; Roskamp, E. J. SL 1993, 131.
7. Wang, W.-B.; Roskamp, E. J. JOC 1992, 57, 6101.
8. Wang, W.-B.; Roskamp, E. J. JACS 1994, 116, 9417.
9. Smith, L. A.; Wang, W.-B.; Roskamp, E. J. SL 1993, 850.
10. Wang, W.-B.; Restituyo, J. A.; Roskamp, E. J. TL 1993, 34, 7217.
11. Burnell-Curty, C.; Roskamp, E. J. TL 1993, 34, 5193.

Carrie A. Roskamp & Eric J. Roskamp

Northwestern University, Evanston, IL, USA

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