Nachweis gehinderter Rotationen und Inversionen durch NMR‐Spektroskopie

Kessler, Horst

doi:10.1002/ange.19700820603

Cited by 442 publications

(74 citation statements)

References 171 publications

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“…At T = T c = 2 8C, k rot was found to be 67 s À1 , translating into k rot -(22 8C) % 2.7 10 2 s À1 using k rot (2 8C) = dnp/ ffiffi ffi 2 p (dn = difference in the chemical shifts of the two alkylidene signals obtained at T < T c ). [20] A similar result was obtained from k rot = p(dn) 2 /(2Dn), that is, k rot (22 8C) % 2.4 10 2 s À1 (Dn = difference in peak width at half peak height between the signal at T = T c and at T @ T c ). In view of the different factors that govern both the line width at half peak height and dn, a mean value of k rot at T = 22 8C of 2.6 10 2 s À1 appears to be a good approximation.…”

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confidence: 71%

Alternating Copolymerizations Using a Grubbs‐Type Initiator with an Unsymmetrical, Chiral N‐Heterocyclic Carbene Ligand

et al. 2008

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Reports on alternating copolymers prepared by metathesis copolymerization of two different monomers are relatively rare. [1][2][3][4][5][6][7] In 2005, Chen et al. reported on the mechanismbased design of a ROMP (ring-opening metathesis polymerization) catalyst for sequence-selective copolymerization. [8,9] Here, we report on an alternative system for the synthesis of alternating copolymers based on Grubbs-type initiators containing an unsymmetrical, chiral N-heterocyclic carbene (NHC) ligand.As part of our on ongoing research on ruthenium metathesis catalysts with saturated unsymmetrical NHC ligands [10,11] initiator 1 was prepared from [RuCl 2 (PCy 3 ) 2 -(CHPh)] and 1-mesityl-3-((1'R)-1-phenylethyl)-4,5-dihydroimidazolium tetrafluoroborate in 90 % yield. This initiator was used for the copolymerization of norbornene (NBE) and cyclooctene (COE) using a stoichiometry of 1/NBE/COE of 1:2000:100 000. Reactions kinetics recorded for this copolymerization in CH 2 Cl 2 revealed that NBE was consumed in less than 15 min. Terminating the copolymerization after 60 min in fact provided an almost perfectly alternating copolymer of NBE and COE. Unfortunately, the polymer was only partially soluble in standard solvents such as CHCl 3 . However, the soluble fraction showed a unimodal molecular weight distribution (PDI = 2.19), and M n = 40 400 g mol À1 was found. Longer reaction times led to the formation of substantial amounts of a poly(COE) homopolymer block attached to the alternating copolymer. This homopolymer block is easily identified by NMR spectroscopy.The 13 C NMR spectrum of the alternating copolymer (97 % alternating units, d = 128-136 ppm) is shown in Figure 1. Hardly any signals for poly(NBE) or poly(COE) can be detected. [2,12] The signals for the alternating copolymer are observed around d = 135.0 and 128.5 ppm. The first set of signals around d = 135.0 ppm is assigned to the C = C À CH 2 carbon atoms and consists of eight different signals at d = 135. 28, 135.26, 135.14, 135.00, 134.98, 134.95, 134.83, and 134.80 ppm, which we attribute, without specific assignment, to the ccc, tcc, ctc, cct, ttc, tct, ctt, and ttt triads. The fact that all eight signals display roughly the same intensity is indicative for a cis content of about 50 % and is in accordance with the 1 H NMR spectrum, which shows signals of the cis and trans double bonds in a 1:1 ratio at d = 5.36 and 5.28 ppm, respectively. Such a high cis content is rather unusual for polymers prepared by Grubbs-type initiators; most display a cis/trans ratio of roughly 25:75. The less well-resolved signals around 128.5 ppm are assigned to the C=CÀCH 2 carbon atoms and correspond to the same triads. DSC measurements of the alternating copolymer revealed a single glass transition at T g = À50.9 8C (Dc p = 0.32 J g À1 )), which is in between the glass transition temperatures of poly(COE) (À78 8C, 50 % trans) [13] and various poly(NBE)s (40-64 8C).[14]We next turned our attention to the initiation efficiency of 1. Second-generation Grubbs-type initiators having ...

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Alternating Copolymerizations Using a Grubbs‐Type Initiator with an Unsymmetrical, Chiral N‐Heterocyclic Carbene Ligand

et al. 2008

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“…[90] GC analysis were performed on CP-Sil 8 CB column (15 m, d i = 0.25 mm, Varian) with Perkin Elmer Clarus 500 GC AutoSystem. Electrochemistry: The standard electrochemical instrumentation consisted of an EG&G 273 A-2 potentiostat galvanostat.…”

Section: Methodsmentioning

confidence: 99%

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

Leuthäußer

Schmidts

Thiele

et al. 2008

Chemistry A European J

138

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The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.

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“…[9] Recently, we were also able to record the data for 10. For all three species the temperature of coalescence (T c ), the frequency difference (∆ν) and the estimated activation enthalpies [14] (∆G ϶ ) are listed in Table 1. A comparison with values derived from an AM1 calculation [15] shows a good agreement for 11 and 12 if a rotation of only one benzene ring is assumed.…”

Section: B) Spectroscopic Investigationsmentioning

confidence: 99%

π‐Prismands – Flexible Hosts for Metal Ions

Kunze¹,

Balalaie²,

Gleiter³

et al. 2006

Eur J Org Chem

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The π‐prismands dealt with in this paper are: 1,8‐diazabicyclo[6.6.6]eicosa‐4,11,17‐triyne (6), 1,8‐diazabicyclo[6.6.5]nonadeca‐4,11‐diyne (7), 1,8‐diazabicyclo[6.6.6]eicosa‐4,11‐diyne (8), 1,10‐diazabicyclo[8.6.6]docosa‐5,13,19‐triyne (9), 17‐(1,4)benzena‐1,8‐diazabicyclo[6.6.5]nonadecaphane‐4,11‐diyne (10), 11,16‐(1,4)dibenzena‐1,8‐diazabicyclo[6.5.5]octadecaphane‐4‐yne (11) and 4,10,15‐(1,4)tribenzena‐1,7‐diazabicyclo[5.5.5]heptadecaphane (12). The synthesis of 9 is reported. The molecular parameters obtained from X‐ray investigations on single crystals of 6–9 as well as of 10–12 are compared with each other. Also discussed are the structures of the monoprotonated forms of 6 and 12. The π‐prismands 6–12 form stable complexes with Ag+ and Cu+. The crystal structures of the metal complexes as well as their spectroscopic properties are reported. Both reveal that the metal center interacts with the nitrogen atoms and the π‐systems of the bridges.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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Nachweis gehinderter Rotationen und Inversionen durch NMR‐Spektroskopie

Abstract: Rotations-und Inversionsprozesse in organischen Molekiilen ~-

Cited by 442 publications

References 171 publications

Alternating Copolymerizations Using a Grubbs‐Type Initiator with an Unsymmetrical, Chiral N‐Heterocyclic Carbene Ligand

Alternating Copolymerizations Using a Grubbs‐Type Initiator with an Unsymmetrical, Chiral N‐Heterocyclic Carbene Ligand

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

π‐Prismands – Flexible Hosts for Metal Ions

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