Sc-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data Methods (reference in  

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The discussion above has been more or less empirical and descriptive. However, considerable effort has been made to interpret 3-SCS on a more physical basis. Electric-field effects (71-75) were invoked to explain signal shifts of 3-carbon atoms induced by protonation of amines (157,158) (cf. Section II-B-3). This approach was later extended to other functionalities by Schneider and coworkers, who assumed that the SEF component (E2) rather than inductive properties of the substituents should be responsible for 3-SCS (113). They found fairly linear correlations of 3-SCS(X ) and 3-SCS(X ) in cyclohexyl derivatives (76) and attributed the difference between these for a given X to a widening of the C -Cp-Cv bond angle by 2.2° in the axial conformer (114,159). The decrease of 3-SCS in the order primary Cp —> secondary Cp — tertiary Cp — quaternary Cp was explained by electron-charge polarization in the Cp-C" bond(s) induced by the LEF component of the C -X dipole, which is already of significance at this distance, though ( 2) still dominates (160). Such an electron flow toward the 3 carbon is expected to be much more pronounced in C-C than in C-H bonds because of the polarizability difference (aCH = 0.79 acc = 1.12) (150,151,160). 

The observed ya-SCS(X) values in 7-exo-substituted norcaranes 97 (225) and those of M(CH3)3 (M = Si, Ge, Sn, or Pb) in cyclohexyl and bicy-clo[2.2. l]heptane derivatives (133) were later interpreted on the same basis. The back-lobe-overlap treatment was further supported by interpretations of H and 13C contact shifts of aliphatic amine signals in the presence of nickel acetyl-acetonate and by INDO calculations (226,227). Additional support came from extensive investigations of the structure dependence of three-bond coupling constants 3JCX (X = H, C, or F) (228,229), although the interpretation of these data has been subjected to criticism (230). 

In bridgehead-substituted bicyclo[2.2.2]octanes 113 (R = H, Ph) the LEF treatment failed to rationalize observed 8-SCS(X) values (189,248). Since the X-C and C -R bonds are collinear, one expects upheld shifts, with absolute values decreasing in the order F > Cl > Br > I. The opposite, however, is found experimentally, so it was concluded that magnetic-field effects are probably responsible for these 8-effects (189). 

First developments in the Friedel-Crafts alkylation were concentrated on the use of stoichiometric amounts of Lewis acids, such as A1C13, BF3 or TiCl4, to produce stoichiometric amounts of salt by-products [5-9]. However, in recent years more and more catalytic methods have been developed. In particular, rare earth metal triflates, including Sc(OTf)3, La(OTf)3 and Yb(OTf)3, have been extensively used as Lewis acid catalysts in various C-C and C-X bond forming reactions [10-13], Despite the benefit of their versatility for organic synthesis, these Lewis acids possess major drawbacks. They are expensive, rather toxic [14], and air- and moisture-sensitive. 

Ionization Energy Data (eV) for Predominantly Metal MOs and Metal-Carbene o-bonding MOs of Cr(CO)sC(X)Y Complexes0... 

Reaction of the anhydrous SCX3 (X = Cl, Br) with pyridine yields Sc(py)4X3, which possibly have one molecule of pyridine in the lattice, as it is lost in vacuo. Sc(py)3(NCS)3 also exists. Using bidentate ligands, SCL2X3 (L = phen, bipy) have been made, sometimes in two forms they are believed to be [ScL2X2]+X and it is possible that cis and trans-isomers may exist, but there is no stmctural confirmation. Similar ethylenediamine complexes have been made by removal of a ligand molecule in vacuo from Sc(en)3X3 (X = Cl, Br). Of the complexes of tridentate N-donors, the best characterized is Sc(terpy)(N03)3, which has nine-coordinate scandium, but with one rather long Sc-0 bond. 

Another compound featuring Sc— P bonding is the phosphomethanide Sc[C(PMe2)2X]3 ((8) X = SiMe3, PMe2)... 

Figure 1 Variation of Sc—O bond lengths (x-axis) with the number of observations taken mainly from...

Analogous trends, although to a lesser extent, appear in selenocarboxylic acid derivatives as soon as third- and/or higher-row elements (sulphur or selenium) are involved, the 8 C Se) reach up to 5 = ca. ISOO. It seems that the interaction of the lone-pairs of these chalcogen atoms with the C=Se 7i-bond (formula II in Scheme 24) is less effective.2 These trends may be demonstrated in a series of selenoesters Ph-C(=Sc) X X = NMc2, 5 = 733 X = OMe, 5 = 910 X = SeMe, 5= 205 correlations of Se... 

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