The Scope of Radical CC‐Coupling by the “Tin Method”

Abstract

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...

Recent Advances in the Synthesis of C‐Oligosaccharides

“…The high α-selectivity of the reaction was in contrast with other reports. Thus radicals generated at the C-2, C-3 and C-4 centers all preferred to occupy equatorial positions [58]. When the anomeric bromide 291 was treated with tin hydride and AIBN, resulting anomeric radical, and reacted with lactone 292 in a Michael addition, it afforded 293.…”

Synthetic applications of radical C-C bond forming reactions

“…0rganomercuric hydrides react with alkyl radicals with rate coefficients larger than 10 7 (i mol-lsec "I) [8], whereas Bu3SnH is at least ten times slower [9]. Thus, the tin method often gives higher yields [10] than the mercury method, especially with less reactive alkenes. In successful syntheses using radical chains, the reactions between radicals and non-radicals must be faster than those between the radicals themselves.…”

“…Six-membered cyclic radicals are predominately attacked from the equatorial side, when the neighboring substituents are equatorial. For example, the carbohydrate 11 gives mainly the equatorially substituted product 12 with acrylonitrile [10].…”

Flow Giese reaction using cyanoborohydride as a radical mediator

“…Alkyl radicals are classified as nucleophilic radicals, and therefore they are able to add preferentially to alkenes possessing an electron-withdrawing substituent [6–7]. This type of reductive radical addition reaction, better known as the Giese reaction, was historically carried out most by using tributyltin hydride as the radical mediator [8–9]. Recently borane derivatives such as borohydride reagents [10–13] or NHC-boranes [14–18] can be used in simple radical C–C bond forming reactions or radical reduction as efficient substitutes for tin hydride reagents, whose toxicity is of great concern to organic chemists.…”