SYNTHESIS AND MOLECULAR-STRUCTURES OF NOVEL ALPHA-AMINO ORGANOALUMINUM ESTER ENOLATES - KEY INTERMEDIATES IN THE SELECTIVE FORMATION OF TRANS-3-AMINO-2-AZETIDINONES

This paper describes the synthesis, characterization, and synthetic application of new alpha-amino dialkylaluminum ester enolates. With simple imines, these in situ prepared aluminum enolates afford trans-3-amino-2-azetidinones 5 and 6 with good de's (72-92%) in excellent yields when an (small) excess of dialkylaluminum chloride is present. Transmetalation of LiO(RO)C=C(H)NR1R2 (2, R = R1 = R2 = alkyl) with (R3)2AlCl (R3 = alkyl) affords pure (R3)2AlO(RO)C=C(H)NR1R2 (3) in quantitative yields. Upon addition of a second equivalent of (R3)2AlC1, stable adducts (R3)2AlO[(RO)C = C(H)NR1R2] --> Al(R3)2Cl (3') are formed. In both types of enolates, the ester enolate anions are N,O-chelate bound to the metal centers through Al-O and dative Al-N bonds. Consequently, the configuration of these enolates is Z. A crystal structure determination of 3b (R = R1 = R2 = R3 = Me) shows a dimeric associate, having a planar skeleton, consisting of two symmetry-related AIOCCN units (Al-O = 1.853 (3) angstrom, Al-N = 2.260 (3) angstrom), that are connected via Al-O-Al bridges (Al-O(dative) = 2.055 (3) angstrom). The aluminum centers are five-coordinated with a slightly distorted trigonal bipyramidal geometry. Enolate 3b crystallizes as colorless crystals in the space group P2(1)/n with a = 8.095 (1) angstrom, b = 13.747 (1) angstrom, c = 9.641 (2) angstrom, beta = 104.41 (1) degrees, V = 1039.1 (3) angstrom3, Z =2, and T = 295 K. Full-matrix least-squares refinement on F converged at R = 0.056 and R(W) = 0.057. A crystal structure determination of 3b' (R = R1 = R2 = R3 = Me) shows a dinuclear monomeric adduct, consisting of a planar AlOCCN unit (Al-O = 1.870 (2) angstrom, Al-N = 2.039 (2) angstrom) to which a Me2AlCl molecule is coordinated through a (dative) Al-O bond of 1.895 (2) angstrom. The aluminum centers are four-coordinated with a distorted tetrahedral geometry. The chlorine atom is located in a semibridging position, but the long Al-Cl distance of 3.260 (1) angstrom is indicative for a nonbonding interaction. Enolate 3b' crystallizes as pale yellow crystals in the space group P2(1)/n with a = 9.456 (1) angstrom, b = 14.145 (1) angstrom, c = 12.208 (1) angstrom, beta = 103.78 (1) degrees, V = 1585.9 (2) angstrom3, Z = 4, and T = 295 K. Full-matrix least-squares refinement on F converged at R = 0.051 and R(W) = 0.043. The structures of enolates 3 and 3' in solution as well as the reactions of these enolates with imines 4 is discussed. It is shown that both dimeric enolates 3 as well as monomeric adducts 3' may act as the reactive intermediates in the one-pot synthesis of 3-amino-2-azetidinones 5 and 6, which is characterized by the reactions of in situ prepared alpha-amino aluminum ester enolates with imines. Furthermore, on the basis of the structural features of 3b and 3b' the trans stereoselectivity, observed in these reactions, is explained in terms of two highly ordered transition states, constructed from Z-aluminum enolates and E-imines.