Norbornyl complex

These are generally limited to what are termed kinetically stabilized alkyls, i.e. those devoid of protons p to the metal (Figure 4.5). These also include norbornyl and adamantyl examples since decomposition via p-metal hydride elimination (see below) would require the installation of an alkenic bond between the a and p carbons of the precursor alkyl. This is precluded for these two alkyls because of the prohibitive strain associated with forming a double bond to a bridgehead atom within a small cage structure (Brendt s rules). The tetrakis(norbornyl) complexes are also remarkable because some uncharacteristic oxidations states can be attained, e.g. Cr(IV), Co(IV) (low-spin e4t2°). The second factor which may confer stability is steric bulk bimolecular decomposition routes are thereby discouraged. 

The properties of the 7-norbornyl (18), 7-nobomenyl (19), and 7-norboma-dienyl (20) ions and radicals have aroused much interest in view of the evidence that the cations 19 and 20 have nonclassical -complex structures. We... 

More interesting is the Co(IV) complex [Co(l-nor)4] (nor = norbornyl), which also possesses a (distorted) tetrahedral geometry, Figure 81.125... 

The First example of a low spin tetrahedral complex of a first-row transition metal, lelrakisfl-norbornyl)cobalt(lV), was recently confirmed Sec Byrne, E K. Richeson, D. S.,ThcopoUI. K. H. Chem. Commufl. 1986. 1491-1492. 

Due to computational savings, the open and closed forms of the 6,6-dimethyl-6-sila-2-norbornyl cation, 42 and 43, the closed 5,6,6-trimethyl-6-sila-2-norbomyl cation (44) and their corresponding benzene complexes 45, 46 and 47, respectively, were optimized... 

In most cases chiral carbonyl compounds also afford low stereoselectivity. As for the related Passerini reaction, even the use of aldehydes that are known to give excellent asymmetric induction in the reaction with other kinds of C-nucleophiles, results in low or moderate diastereoisomeric ratios. For example, both norbornyl aldehyde 39 [47] and a-alkoxyaldehyde 40 [3, 48] gave drs lower than 2 1 (Scheme 1.16). The same happens with ortho-substituted chromium complex 41 [49], which usually leads to very high asymmetric induction in other nucleophilic additions. Finally, //-substituted aldehyde 42 [50] gave poor results as well. 

One of the most effective ligands for complexation of Ag(I) is the highly preorganised diphe-nylmethane-derived cyclophane 3.141. This compound incorporates a norbornyl bridgehead and alkene walls making the cavity extremely rigid, while the aromatic Jt-surface is complementary to... 

Transformations shown in Schemes 12-14 constitute the first examples of catalytic AROM reactions ever reported. Meso-triene 50 is converted to chiral heterocyclic triene 51 in 92% ee and 68% yield with 5 mol% 4a (Scheme 12) [21]. Presumably, stereoselective approach of the more reactive cydobutenyl alkene in the manner shown in Scheme 12 (II) leads to the enantioselective formation of Mo-alkylidene III, which in turn reacts with an adjacent terminal olefin to deliver 51. Another example in Scheme 12 involves the net rearrangement of meso-bicycle 52 to bicyclic structure 54 in 92% ee and 54% yield. The reaction is promoted by 5 mol% 4a and requires the presence of diallyl ether 53 [22], Mechanistic studies suggest that initial reaction of 53 with 4a leads to the formation of the substantially more reactive chiral Mo=CH2 complex (vs neophylidene 4a) which can react with the sterically hindered norbornyl alkene to initiate the catalytic cycle. 

Complex 5 (Scheme 1) follows a different pathway in reactions with aryl halides (R = aryl in RX). This time the aryl group reacts preferentially with the norbornyl site of the palladacycle rather than with the aryl site (probably via a palladium(IV) metallacycle of type 6). As a consequence, norbornene is no longer expelled and is incorporated into a condensed ring instead. The minor aryl portion that migrates on to the aryl site (complex 14) also retains norbornene and gives the same product 13. A C-H transformation process is thus operating in ring formation from 12 or 14 to 13 (Scheme 5) [2],... 

IV id ) 4 [Fe(l-norbornyl)4] Tetrahedral Iron(IV) is not common known in mixed oxides. Most easily prepared complexes are those with 17... 

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