A wide range of amides containing different functional groups were readily converted into their corresponding aldehydes by means of a simple protocol. The reaction with [Ti(OiPr)4] and Ph2SiH2 proceeds at room temperature via an enamine intermediate, which is then hydrolyzed to afford the aldehyde [Eq (a)].
While the chemistry of zirconocene derivatives has been well-developed over the past few years,2 a corresponding surge in the study of hafnocene compounds has not e n~u e d .~ As part of a study to probe the differences in structure and reactivity between complexes of these two elements, we now report the preparation and X-ray crystal structure determination of Cp2Hf(isobutylene).PMe3. This constitutes the first structurally characterized olefin complex of hafnocene and, additionally, is the first group 4 complex of a I,l-disubstituted ~l e f i n .~ Several years ago our groupS and that of Negishi6 reported that dibutylzirconocene (la) rapidly eliminated butene at 0 OC and, in the presence of excess trimethylphosphine, produced 2a in good yield (Scheme 1). We have recently found, in contrast, that thermolysis of the hafnocene analogue l b at 80 OC was necessary for the conversion of l b to 2b.6 The harsh conditions required for this transformation, presumably a reflection of the comparatively strong Hf-C bond, caused noticeable decomposition to occur during the formation of 2b. In order to decrease the temperature required to induce alkane elimination,' we prepared the unsymmetrical dialkylhafnocene 4, via chloro-fert-butylhafnocene (3) (Scheme 11). While the analogous zirconocene complex rearranges at low temperature to isobutylzirconocene c h l~r i d e ,~.~ complex 3 is moderately stable in the solid state and in solution. Addition of I equiv of n-BuLi to 3 produces 4 in good yield. To our surprise, 4 is an isolable, moderately stable solid. Compound 4 can be conveniently prepared in >90% yield on a large scale (-8 g) by the sequential addition of t-BuLi and n-BuLi to hafnocene dichloride at -78 OC, followed by warming to room temperature. When 4 is heated at 45 OC for 21 h in the presence of excess trimethylphosphine, isobutylene complex 5 is produced in 20-40% isolated yield. In contrast, attempts to form the zirconium analogue of 5 have produced only the knownIad dimeric Zr( I I I ) complex, I (pL-(~':qS-C5H,))(Cp)Zr(PMe3)J2.Recrystallization of 5 from hexane produced X-ray-quality crystals. The solid-state structure of 5 was determined by X-ray crystallography, and the ORTEP diagram, along with important bond lengths and angles, is shown in Figure I. The Cl-C2 bond distance of 1.46 ( I ) A is essentially the same as that seen in
The trimethylphosphine adducts of zirconocene and hafnocene silyl hydride complexes have been prepared and the X-ray crystal structure of the zirconocene complex has been determined. In addition, the mechanism of formation of these complexes and a study of the reactivity of the zirconocene complex are presented. The complex Cp2Zr(H)(SiPh3)(PMe3) (2) crystallizes in the monoclinic space group P2 Jc with o = 9.336
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