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...
The reduction of acetylated or benzoylated glycosyl halides or selcnides with low concentrations of lribulylstannane leads to 2-dcoxy sugars. An important step in this radicalchain reaction is the cis-selective migration or an ester group. The broad applicability of this method is demonstrated by thc synthesis of mono-and dideoxy sugars, or PI-and Pdeoxy sugars, of dcoxypyranoses and deoxyiuranoses, of dcoxypentosyl-and -hexosyl carbohydrates.
The first disaccharides linked by a CH2 group can be synthesized by addition of carbohydrate radicals to “carbohydrate alkenes”. An example of these molecules, which are of interest because of their possible enzyme‐inhibitor properties, is the species 1.
E.s.r. spectra of C-2, -3, and -4 carbohydrate free radicals, generated regiospecifically by halogen abstraction from acylated deoxyhexopyranoses are recorded in benzene solution. Analysis of the hyperfine splittings reveals the retention of the 4C, chair conformation of the parent compounds in the radicals.In Part 2 ' the structures of various pyranosyl radicals with the radical centre at the anomeric (C-1) carbon atom have been examined by e m . spectroscopy. In non-aqueous media the pyranos-1-yl radicals exist as planar n-type radicals and their equilibrium conformations strongly depend on the type of carbohydrate. Thus, diastereoselectivities in radical C-C coupling reactions of carbohydrates have to be discussed in terms of carbohydrate conformations2 rather than in terms of 0configuration at the radical centre as had been suggested earlier. 3 Recently, it has been shown that diastereoselective C-C coupling reactions could also be performed at the C-2 position of the carbohydrate backbone, giving 2-deoxy sugars in reasonably good yield^.^ Similarly, radical-induced C-C bond formation is also possible at C-3 and -4 of the pyranosyl rings5The e.s.r. spectroscopic investigation of C-2, -3, and -4 deoxypyranosanyl radicals, which is presented here, should provide better insight into the dependence of diastereoselectivity on radical conformation. Structurally related radicals have been investigated in the reaction of unprotected sugar derivatives with O H radicals in aqueous s ~l u t i o n . ~. ~ Results and DiscussionThe deox ypyranosanyl radicals (Rl)-(R8) were regiospecifically generated in benzene solution from the corresponding bromoor iodo-compounds (1)-( 8) by reaction with trimethylstannyl radicals, generated photolytically from hexamethylditin (see Table ). The carbohydrates (lb), (2b), (3), and (6)-( 8) are new compounds that were synthesized by standard procedures. The e.s.r. spectra, recorded in the temperature range from -8 to
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