Reductive amination of various aldehydes and ketones has been performed effectively by pentacoordinate chloro-substituted tin hydride complex, Bu 2 SnClH-HMPA. The tin reagent worked particularly well for the case using weakly basic aromatic amines as starting substrates. Stoichiometric amounts of a substrate and a reducing agent were adequate for the reaction. The Sn-Cl bond in the complex plays an important role for both steps of imine formation and subsequent reduction. Highly chemoselective reduction of carbonyls could be achieved regardless of other functionalities such as halogen, carbon-carbon double bond and hydroxyl groups in the starting carbonyls and amines.

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Imine-selective reducing agent Bu 2 SnClH-HMPA (I) performed effective reductive amination of carbonyl compounds. Various functionalized amines could be prepared in a one-pot procedure. Noteworthy is that highly chemoselective reactions were achieved with the co-existence of other functionalities such as halogen, carbon-carbon double bond and hydroxyl groups.Reductive amination of aldehydes is one of the most convenient routes to various secondary amines, in which three components of carbonyls, amines and reductants are treated in one portion. 1 By this procedure, a wide range of functionalized amines can be prepared because intermediate imines, in particular unstable ones, do not have to be isolated. The choice of the reductant is very critical since the undesirable reduction of the carbonyls must be depressed to form intermediate imines. Sodium cyanoborohydride (NaBH 3 CN) has been used because of its wide applicability. 2 However, there are some drawbacks because of the requirement of excess amounts of amines and inapplication to weak bases such as aromatic amines. To solve these problems, modified borohydrides have been developed. 3 However, alternative metal hydride reagents have scarcely been focused so far. We have provided a set of organotin hydrides by the introduction of a halogen substituent or a ligand on the tin atom to achieve highly chemoselective reductions of multifunctionalized substrates. 4 The fivecoordinated tin hydride, Bu 2 SnClH-HMPA (I), 5 easily formed in situ from Bu 2 SnClH 6 and HMPA, has been revealed to reduce imines even in the presence of carbonyls. 7 This high imine-selectivity indicates that the reagent I would be a good candidate for the reductive amination. We present here the first use of tin hydrides for reductive amination of aldehydes and ketones under mild and neutral conditions (Scheme 1). The characteristic features are as follows. 1) Effective reactions proceeded in particular for the case using aromatic amines as starting substrates. 2) Stoichiometric amounts of substrates and tin reductant were adequate. 3) Co-existence of other functionalities was allowed in the starting carbonyls 1 and amines 2. Scheme 1Table 1 summarizes the reductive aminations of various carbonyl compounds by using Bu 2 SnClH-HMPA (I). The three reagents, the tin hydride I, carbonyl compound 1 and aniline 2a were successively added to THF solvent. Initially, we examined the reductive aminations of benzaldehyde 1a. The reaction with aniline 2a proceeded smoothly to give the corresponding secondary amines, 3aa in good yield (entry 1). 8 Benzyl alcohol derived from the reduction of starting material 1a was not produced under these conditions. Instead of 2a, the use of more weakly basic p-chloroaniline 2b also gave the corresponding secondary amine 3ab in good yield (entry 2). Next, other aromatic aldehydes were examined. In entry 3, the reducing system (I) did not affect the bromine functionality at all, which is generally reducible by trialkyltin hydrides. 9 In the reaction of 1c, the hydroxy grou...

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