Ligands hindered rotation

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

Best overall selectivity for all M /Mf couples is shown by compound 30, [2.2.2], However a given selectivity may be increased by structural modifications. For instance the K+/Rb+ selectivity increases markedly from 30 to 36. Indeed the introduction of the benzo ring is expected to decrease the cavity size (shorter O. .. O distance) and to increase ligand rigidity (rotation about the central C—C bond in the benzo bridge is frozen). Both effects hinder the cavity dilatation required for Rb+ inclusion on the other hand, they also favour the smaller Na+ cation with respect to K+, thus diminishing the K+/Na+ selectivity of 36 as compared with 30. 

Quinazolines take part in the same types of reactions as pyrimidines, but because of their additional benzene ring, the products of these reactions may have the added feature of hindered rotation. An example of this is the synthesis of 2-phenyl-Quinazolinap by Guiry and co-workers <99TA2797>. Suzuki coupling of 4-chloro-2-phenylquinazoline (115) with boronic acids 116 led to 117 (R = OMe). These intermediates were parlayed into phosphinamines 117 (R = PPh2) and then subjected to chiral resolution to produce new chiral phosphinamine ligands for asymmetric catalysis. 

The dimer (OEP)Sc—O—Sc(OEP) has been examined by optically detected magnetic resonance with the conclusion that the rings are approximately coplanar.27 The NMR spectra of the dimer and the monomer show that the methylene protons are anisochronous, this probably not being caused by hindered rotation but by the inherent asymmetry of the ligands.28... 

Whereas trisilylamine or W-methyldisilylamine do not react with CS2, AOV-dimcthylsilylamine reacts with CS2 to give silyl-jV.jV-dimethyldithio-carbamate (208). Evidence for hindered rotation around the S2C—N bond in the Me2Dtc ligand was found in the PMR spectrum of the H3Si(Me2Dtc) complex. Thus just below room temperature the broad methyl resonance of this compound splits into two peaks of equal intensity. 

The biscarbene complexes were found (142) from their H-NMR spectra to be mixtures of geometrical isomers, due to hindered rotation about the C—N or C—O bonds, and the isomers have been separated by fractional crystallization in the case of [Au C(0Et)NHC6H4Me 2]+C10. Treatment of these species with triphenylphosphine gave mixed ligand complexes and formamidines [Eq. (39)]. They also underwent oxidative addition of iodine to yield the first gold(III) carbene complexes. 

In metal complexes, good correlation exists between the size of the silyl ligand and the metal—Si bond distance. For example, the Mo—Si bond distances are 2.538(2), 2.560(1) and 2.604(1) A for (Cp)2Mo(H)(SiMe2H), (MeCp)2Mo(H)(SiMe3) and (MeCp)2Mo(H) Si(Bu-Zj2n. respectively (cf. entry 63 in Table l)76. Complexes containing a bulky silyl ligand [i.e. (z-Bu)i1 ISi exhibit hindered rotation around the metal—Si bond in solution as determined by variable temperature 111 NMR spectroscopy. Moreover, the steric bulk... 

The unsymmetrical diene ligands in 85b and 85c may form two dia-stereomers with the less substituted double bond in the cis or trans configuration with respect to the Mo—C a bond. For 85b, only the isomer with the less substituted double bond cis to this bond is found. Compound 85c forms both isomers, but the cis isomer is distinctly preferred. In contrast with the fluxional >j2-diene complexes 84a-84c, compounds 85a-85c do not show temperature-dependent H-NMR spectra for the //4-diene ligands. However, this does not disprove a hindered rotation of the f/4-diene ligands when one of the two conformers (o or u) is energetically more favored than the other. 

By D-NMR spectroscopy, hindered rotation of the acetyl substituents in 92 (AG208 = 40.3 0.2 kJ/mol) and in 93 (AG208 = 43.1 0.2 kJ/mol) has been detected, thus indicating bond order higher than unity for the acetyl-ring C—C bond, due to a fulvenolate character of the acetylcyclo-pentadienyl ligand. 

P-. As-, and Sb-donor ligands. H N.m.r. measurements at 60, 100, and 220 MHz have been made for the complexes [PdCl2 Ph3 nP(CH2Ph)n 2] (n = 1, 2, or 3).82 The different spectra observed at these three frequencies have been explained in terms of hindered rotation about the P—C bonds when n = 2 and 3 and probably also for n = 1. The square-planar complexes [PdX2L] [L = cis-1,2-difluoro-1,2-bis(diphenylphosphino)ethylene X = Cl, Br, I, or SCN] have been obtained by treatment of [PdX4]2- with L.83 The complexes were characterized by chemical analysis and their visible spectra recorded. In addition, difluorophosphonato-complexes [M(PF20)4]2 (M = Pd or Pt) have been isolated as their JV-allyl-pyridinium salts (equation 16) and their n.m.r. and i.r. spectra reported.84... 

In Section IV,C,2 octa- and pentaphenyl-, as well as penta(isopropyl)-and penta(neopentyl)-Cp, were cited as examples for hindered rotations about substituent-C5 bonds. Half-sandwich metal complexes or sym-metallocenes with these ligands will be chiral when there is a barrrier for the interconversion of the enantiomers created by the phenyl canting or the isopropyl and neopentyl directionalities of the substituents (clockwise or counterclockwise) around the C5 ring. For penta(isopropyl)- and penta(neopentyl)-Cp this difference in directionalities is commonly sketched (24) by showing the methine proton only and omitting the methyl or ethyl groups for clarity (14). 

The model requires hindered rotation about the V—R bond. Considering the force fields of the Me of the last added unit and of the X ligands, this requirement is fulfilled at low temperatures. 

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