ever, the preferred ground-state structure of uncharged metal complexes can be determined experimentally much more easily than for the carbenium ion systems.We obtained the C-substituted (methy1eneamido)zirconocene chlorides 5a and 5b as moisture-sensitive, yellow solids in high yields by hydrozirconation of benzonitrile and acetonitrile, re~pectively [~]. In addition to the typical absorptions of the bent metallocene, an intense -C=N stretching frequency is observed in the IR spectra of 5a and 5b (in toluene; v=1678 and 1700 cm-I, resp.). In Schiff s bases with +C-N=CHR groups this band appears at slightly lower wave numbers[41. The 'H-and I3C-NMR spectral data of the -N=CHR moiety in the complex are very similar to those of many comparable organic systems[41: 'H, [I3c] ([Ddbenzene), 5a: 6=9.0 [167.3 (d, J = 168 Hz)]; 5b: 6=8.3 [167.5 (d, J=168 Hz)]. It is noteworthy that a chemical differentiation of the cyclopentadienyl ligands on the zirconocene moiety, which would be expected for a metalla-ally1 structure 2', cannot be detected by NMR spectroscopy in 5a, even at low tempera-X-ray structure analysis of 5a ( Fig. 1 ; crystals from toluene) confirms the chemical equivalence of the Cp ligands. The Cp(centr.),Zr,Cp'(centr.) plane is orientated orthogonally to the C(arom.),Cl,Hl plane. The most striking feature of the molecular structure of 5a is the almost linear Zr-Nl-Cl-moiety (170.5(5)"). The C=N bond distance (1.259(7) A) is somewhat shorter than that of normal C=N (sp') double-bond systems. In contrast, the metal-nitrogen linkage is extremely short: at 2.013(5) A in 5a it is, to the best of our knowledge, the shortest Zr-N bond in a molecular zirconium compound known to dater5]. Despite this pronounced Zr-N1 interaction, the Nl-Zr-Cl angle (101.1(2)') and the Zr-C1 bond length (2.497(2) A) are clearly within the normal order of magni-tude16]. (6=5.8 [110.7]). I W Fig. 1. Molecular structure of 5s in the crystal. Space group P2Jc; a =7.456(6), b = 12.655(7), c = 16.305(8) A, j3=94.09(6)", V = 1534.6 A', Z=4, 1695 observed reflextions, R =0.034, R,=0.041. Details of the crystal structure investigation may be obtained from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen by quoting the depository number CSD 50559, the names of the authors, and the journal citation.5a can therefore be considered as an organometallic compound with heteroallene structure of type 1' with sphybridized nitrogen. In contrast to the trivalent carbenium ion center in 1, the incorporation of the isolobal Cp'ZrC1 moiety with tetravalent zirconium could principally lead to stereoisomeric heteroallene systems. However, in the reaction of the nitriles 4 with 3 apparently only one isomer is formed in each case. In 5a, C1 and H1 are arranged cis to Angew. Chem. Int. Ed. Engl. 23 (1984) No. I 0 Verlag Chemie GmbH, the linear Zr-N1 -C1 moiety ; hydrozirconation usually proceeds with cis-stereochemistry['].r 7 3 5 a , R = Ph; b, R = MeThe similarity between the structural chemistry of the positively charged o...
Das vielfältig nutzbare Tributylzinnhydrid bewirkt als Katalysator (in Gegenwart von NaBH4) die radikalische Kettenverlängerung von Alkyliodiden mit Acrylonitril oder Alkenen, die z. B. Keton‐, Aldehyd‐, Ester‐ oder Phenylsubstituenten enthalten. Als Reagens ermöglicht Tributylzinnhydrid eine bequeme Synthese C‐verzweigter Desoxyzucker. Dazu wird ein Zucker wie 1, Y = OH, in ein Xanthogenat oder Thiourethan umgewandelt, das z. B. mit Acrylonitril zu 2, R = CH2CH2CN, reagiert.
The successful exploitation of radicals in syntheses necessitates a detailed knowledge of the stereoselectivity of their reactions. With the vinyl radical 1 as example it can be shown how this is achieved by variation of the radical trapping agents (H donors; c‐C6H11HgH, Bu3SnH, c‐C6H12). Below the isoselective temperature the stereoselectivity increases with decreasing rate of formation of the products. In H‐abstractions the less stable (Z)‐alkenes are formed.
Die Anwendung der Titelreaktion auf die Olefine (I) und die Alkyliodide (II) führt in einer Radikalkettenreaktion zu Produkten des Typs (III).
084ChemInform Abstract Using vinyl radical (II) formed by addition of cyclohexyl radicals to phenylacetylene as an example it is demonstrated that variation of the H-donor as well as of the reaction temp. (-20 to 260 rc C) affects the (Z)/(E) relation of products (I) and (III), which may even be reversed. About 60-80 rc C there is an isoselective temp. at which all three H-donors studied possess the same stereoselectivity. The results obtained are interpreted with changes in activation parameters which are tabulated.
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