Organometallics 1996,14, 5316-5324
5316
Mechanism of the Chelation Controlled Addition of CH3TiCl3 to a-AlkoxyCarbonyl Compounds. A Theoretical Study1 Volker Jonas,+>$ Gernot Frenking,*>tand Manfred T. ReetzP Fachbereich Chemie, Philipps- Universitat Marburg, Hans-Meerwein-Strasse, 0-35032 Marburg, Germany, and Max-Planck-Institut f i r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 0-45470 Miilheim an der Ruhr, Germany Received May 9, 1995@ The mechanism of the chelation controlled addition of CH3TiCl3 to a-alkoxy aldehydes has been studied using quantum chemical ab initio methods. The geometries of the CH3Tic13 complexes of a-alkoxy aldehydes have been optimized at the MP2 level of theory employing effective core potentials with valence double-l; basis sets at titanium and polarized double-l; all electron basis sets at the ligands. IGLO-calculations of the 13C chemical shifts have been carried out at the MP2 optimized geometries with large all electron basis sets. The calculations indicate that the rearrangement between the isomeric octahedral CH3TiCl3-complexes occurs via a dissociation-association mechanism with pentacoordinated intermediates. The barrier for the methyl shift from titanium to the carbonyl group of the complexed aldehyde is calculated to be ca. 19 kcaVmol at the MP2 level of theory. The calculations also give a possible explanation for the different mechanism of the analogous reaction of CH3TiCl3 with a-alkoxy ketones. 1. Introduction
Scheme 1 CH,TiC13
Octahedral chelate complexes of Tic14 and CH3TiC13 are important species in stereoselective reactions, because the formation of chelates as intermediates may strongly influence the stereoselectivity of carbonyl addition reactions.2 For example, CH3TiCl3 reacts with chiral a-alkoxy carbonyl compounds 1 and 4 with high diastereoselectivity to form the chelation controlled Octaproducts 3a and 6a, respectively (Scheme hedral chelate complexes 2 and 5 as intermediates have been suggested to account for the observed diastereoselectivity, which is opposite t o what is predicted by the Felkin-Anh model.4 Later, these Cram-type chelates were observed directly by lH and 13C NMR spectroscop^.^,^ Since the ligands are not symmetrical, four diastereomeric octahedral chelates 2a-d (and 5a-d, respectively; Figure 1) are possible, but only two are observed. It was proposed that the two observed complexes have the methyl groups at titanium trans to the chelating donor ligand.5
f\,
R=Et
1
R=H
4
2 5
l h 2 t 3
’Philipps-Universitat Marburg.
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* Present address: Bayer, AG, MD-IM-FA, Ql8, D-51368 Leverku-
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Max-Planck-Institut fur Kohlenforschung, Miilheim. Abstract published in Advance ACS Abstracts, October 15, 1995. (1)Theoretical Studies of Organometallic Compounds. 16. Part 15: Stegmann, R.; Frenking, G. Organometallics 1995, 14, 5308. (2) (a) Reetz, M. T. Angew. Chem. 1984, 96, 542. Angew. Chem., Int. Ed. EngZ. 1984,23,556. (b) Reetz, M. T. Organotitanium Reagents in Organic Synthesis; Springer: Berlin, 1986. (3)Reetz, M. T.; Hiillmann, M. J . Chem. SOC.,Chem. Commun. 1986, 1600. (4) (a) Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968, 2201, 2205. (b) Anh, N. T.; Eisenstein, 0. NOUIJ. J . Chim. 1977,1, 61. ( c ) Anh, N. T. Top. Curr. Chem. 1980, 68, 145. (5) (a) Reetz, M. T.; Hiillmann, M.; Seitz, T. Angew. Chem. 1987, 99, 478. Angew. Chem., Int. Ed. Engl. 1987, 26, 477. (b) Seitz, T.; Dissertation, Marburg, 1987. ( c ) Hiillmann, M.; Dissertation, Marburg, 1986. (6) Reetz, M. T.; Raguse, B.; Seitz, T. Tetrahedron 1993, 49, 8561. @
R = Et R=H R Et R=H
3a 6a
> 99 % > 92 %
3b 6b
