Alane complex with amines

Complexes with one and two amines are common. For example, bis-(trimethylamine) alane, A1H3(N(CH3)3)2, is a white crystalline solid with a low vapor pressure. Using ammonia as a model for trimethylamine, apply orbital interaction analysis to describe the bonding in the 1 1 and 1 2 complexes. Theoretical studies on the ammonia complexes of AIH3 have led to the conclusion that there is little dative bonding (as judged by the amount of charge transfer) (Marsh, C. M, B. Schaefer III, H. F., J. Phys. Chem., 1995, 99, 14309-14315). Comment on the theoretical results. 

At elevated temperatures 2 moles of dialkyl alane react with nitriles up to the amine stage. The dialkyl-dialkylamino alanes, R NR fCHaR"), resulting from azomethines form molecular compounds with strong donors. These are colored if the donor is an azomethine or an aromatic N heterocycle and may be used for the quantitative determination of A1H groups in organic aluminum compounds 191). Complex salts such as Li[(RO)jAlH] 37, 38) or Na iC s AlIR] 287) may be used in the preparation of aldehydes instead of diisobutylalane (cf. Section V,D,3b). 

High yields of amines have also been obtained by reduction of amides with an excess of magnesium aluminum hydride (yield 100%) [577], with lithium trimethoxyaluminohydride at 25° (yield 83%) [94] with sodium bis(2-methoxy-ethoxy)aluminum hydride at 80° (yield 84.5%) [544], with alane in tetra-hydrofuran at 0-25° (isolated yields 46-93%) [994, 1117], with sodium boro-hydride and triethoxyoxonium fluoroborates at room temperature (yields 81-94%) [1121], with sodium borohydride in the presence of acetic or trifluoroacetic acid on refluxing (yields 20-92.5%) [1118], with borane in tetrahydrofuran on refluxing (isolated yields 79-84%) [1119], with borane-dimethyl sulflde complex (5 mol) in tetrahydrofuran on refluxing (isolated yields 37-89%) [1064], and by electrolysis in dilute sulfuric acid at 5° using a lead cathode (yields 63-76%) [1120]. 

Subsequently, we examined several MFA derivatives as substrates for the Pt/C>2 reaction. When 32 was subjected to Pt/(>2 chemistry, three products (132-134) were isolated in poor yield [Fig. (36)]. In the case of 23, the Pt/C>2 reaction mixture was treated straightaway with m-CPBA without isolation of any intermediate products, giving the desired compound 10 (43% yield). Compound 10 can be converted to PHA (2) in three steps. Compound 132 was converted to PHA (2) by treatment with alane-dimethylethyl amine complex in THF (30% yield). 

In 1981 the first attempt for the asymmetric hydroalumination with a chiral alane reagent took place. The reaction of a chiral amine-trialkylaluminum complex, Al((-Bu)3-[(-)-DMMA] (172), with 1,1-dialkylethylenes (excess) catalyzed by Ni(Il)rMesal]2 (173) (2 mol%) without solvent, followed by exposure of the resulting adduct to the oxygen atmosphere gave the corresponding alcohols with low enantiopurity, for example, the reaction of 2-tm-butylpropene with 172 in the presence of 173 gave 144 with 27% ee (R) (Scheme 2.19) [98],... 

The most studied adducts are the trialkylamine alanes. Trimethylamine reacts to give both 1 1 and 2 1 adducts with AIH3, but the latter is stable only in the presence of an excess of amine. The monoamine has a tetrahedral structure (4) and is a white, volatile crystalline solid (mp 75 °C) that is readily hydrolyzed by water. Like the etherate, it slowly decomposes to (A1H3) . The bisamine has a trigonal bipyramidal stmcture with the amines in the axial positions (5). This was the first compound in which aluminum was demonstrated to adopt a five-coordinate stmcture. Tetrahydrofiuan also gives 1 1 and 1 2 complexes but diethyl ether only gives the 1 1 complex. 

Ruff et al. in a series of publications described the synthesis of amine complexes of aluminum hydride [32, 33]. Their study investigated the reaction of these materials with typical Lewis bases in order to define the conditions for the stability of aluminum hydride derivatives in which the aluminum atom exhibits a coordination number of five. They first described methods for making tertiary alkyl amine complexes of aluminum hydride utilizing lithium aluminum hydride and an amine hydrochloride. A finely ground lithium aluminum hydride was placed together with trimethylammonium chloride (ratio 1 2). They prepared other trialkylamine alanes and the N-dialkylaminoalanes, in a similar fashion. These adducts of alane were found to sublime readily at temperatures up to 40 °C except for the tri-n-propylamine alane, which sublimed very slowly and could also be recrystallized from hexane at — 80 °C. 

Chiral modification is not limited to boronate and aluminate complexes. Boranes or alanes are partially decomposed with protic substances such as chiral amines, alcohols or amino alcohols to form useful reagents for enantioselective reduction of carbonyl compounds. For example, reduction of acetophenone with borane modified with the amines (65) to (67) gives (5)-l-phenylethyl alcohol with 3.5-20%... 

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