Molybdenum substitution pattern

In particular, ruthenium carbenes 1 are more sensitive to the substitution pattern of the alkenes than the molybdenum catalyst 24 [19]. While the latter reacts readily even with di- and tri-substituted double bonds and is apparently the only catalyst capable of producing tetrasubstituted cycloalkenes (cf. Table 2, en-... 

The idea of determining the site of initiation via the substitution pattern of the olefin has also been used by Blechert et al. during the course of a stereocon-trolled RCM process (Scheme 10). Again, the reaction starts most likely at the terminal olefin site in 33 independent of whether 1 or 24 is used as catalyst however, due to the different coordination geometries of ruthenium and molybdenum, the evolving carbene reacts with either diastereotopic olefin attached to... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

The regioselectivity is dependent not only on the substitution patterns of the substrates, but also on catalysts used. Rhodium can also catalyze the reaction efficiently with allenes, and it complements molybdenum in terms of... 

In addition, allenes can act as the olefinic part of the reaction [32], Al-lenynes like 12 may react with both double bonds. Brummond established the substitution patterns for the reaction with either the external or the internal bond of the allenic fragment, that give products with different ring sizes (13— 14) [33]. This group has applied these studies to the synthesis of hydroxy-methylfulvalene (17), a potent anticancer agent related with illudines, a natural sesquiterpene family. The key step was the synthesis of 16 from 15 with a PKR mediated by molybdenum carbonyl (Scheme 6) [34,35]. In addition they have developed an asymmetric version of the reaction. They have transferred efficiently chirality from a non-racemic allene to an a-alkylidene and an a-silylidene cyclopentenone in a molybdenum mediated reaction [36-38]. 

The most recent application of olefin metathesis to the synthesis of polyenes has been described by Tao and Wagener [105,117], They use a molybdenum alkylidene catalyst to carry out acyclic diene metathesis (ADMET) (Fig. 10-20) on either 2,4-hexadiene or 2,4,6-octatriene. The Wagener group had earlier demonstrated that, for a number of nonconjugated dienes [118-120], these polymerizations can be driven to high polymer by removal of the volatile product (e. g., 2-butene). To date, insolubility limits the extent of polymerization of unsaturated monomers to polyenes containing 10 to 20 double bonds. However, this route has some potential for the synthesis of new substituted polyacetylenes. Since most of the monomer unit is preformed before polymerization, it is possible that substitution patterns which cannot be incorporated into an alkyne or a cyclic olefin can be built into an ADMET monomer. 

The first examples of the synthesis of functionalized acetylenes (283) by metathesis, using the molybdenum catalyst (282) on the 1-phenylbutynes (281), have been reported, the yields being dependent on the substitution pattern. ... 

Molybdenum hexacarbonyl has been added to the synthetic arsenal of reagents for the synthesis of specifically alkylated cots. Thus, for example, the propellane (28) isomerises at lower temperature and to a different product with this catalyst than by the action of heat alone (Scheme 5). The reaction pathway is, however, sensitive both to the substitution pattern and the degree of unsaturation of the starting material. 

Compound 12 displays an altogether different reactivity pattern upon treatment with iodine in the presence of an ether or a thioether. Thus, the sole product of the reaction of 12 in THF with iodine was the anionic Mo complex [N(PPh3)2][2,2,2-(CO)3-2-I-7-0(CH2)4-c/oxo-2,1-MoCBioHio] (67), with the carborane cage having undergone a substitution at a boron vertex. The substitution occurs at a p boron atom in the CBBBB face that ligates the molybdenum atom. When 67 is treated with further iodine in THF no reaction occurs. However, treatment with iodine using... 

The tetracarboxylatodimolybdenum dimers that result from the reaction of molybdenum hexacarbonyl with a wide variety of carboxylic acids are the initial source of quadruply bound molybdenum units for many further chemical investigations. The initial isolation of Mo2(02CC6H5)4 (2), Mo2(02CCH3)4 (14), and the higher alkyl carboxylate analogues (235) by Wilkinson et al. established a reactivity pattern that has been successfully exploited to prepare many related molybdenum dimers. In addition to those carboxylates enumerated above, a series of substituted aromatic carboxylate derivatives has been prepared by refluxing the appropriate acid with molybdenum hexacarbonyl in diglyme (155). 

Efforts to explore and categorize the reaction patterns of triply-bonded dimers of molybdenum and tungsten of the types M2Lg (L = R, NR2 or OR) and (r -CsHs )2M2(C0K, i.e., those of classes (C) and (D), have been pursued 8-13) Reactions which may be viewed as examples of ligand substitution, Lewis base association,... 

Examples for an isolobal relationship (P is isolobal to CpMo(CO)j) and for the lack of an isolobal relationship (P is not isolobal to Nidppe) are provided in Figs. 7.50 and 7.51. It is quite obvious that the two frontier orbitals of the nickel fragment are not equivalent to the three frontier orbitals of the molybdenum fragment. The consequence of the present case, of course, is that substitution of P with CpMolCO) would result in retention of all of the bonds in the P tetrahedron, whereas substitution with Nidppe would result in a different bonding pattern, as a bond is missing . In fact, [(dppeNi)j( j,-Pj)] does not possess a Ni— Ni bond, whereas [ CpMolCOl l lft-Pj)] does possess a Mo— Mo bond. 

The IR spectrum of W(CO)3(dmtc)2 was typical of substituted cis tricarbonyls in the absence of threefold symmetry. The Ai and E normal modes generated by a C3 axis produce two allowed absorptions. Departure from the threefold symmetry splits the degenerate E band into two components. Figure 1 reproduces this simple pattern that was observed in the carbonyl region for W(CO)3(dmtc)2. The solid state IR spectrum obtained as a KBr pellet for purposes of comparison with similar molybdenum data displayed four distinct absorptions (2010, 1932, 1909, and 1880 cm ) while only three were reported for Mo(CO)3(dmtc)2 (2020, 1920, and 1882 cm" ) (12). The additional solid-state absorption band is probably the result of solid-state splitting of the three fundamental modes, but the possibility of more than one isomer also exists. 

TaB -MoSi composite This composite started to shrink at 1530°C, but at 1680°C no further movement of the rams was measured. The X-ray diffraction pattern of TBM composite showed the presence of hexagonal TaB and traces of MoSij and quartz. The TaB lattice parameters were the same as those reported in the PDF card 65-1462, no evidence of cell shrinkage could be observed, in contrast to the previously presented hexagonal transition metals borides containing MoSi or TaSij. Indeed, the atomic radii of tantalum and molybdenum are very close, 1.43 and 1.36 A, respectively hence, even if Mo substituted in Ta sites in the TaB lattice, it would be very difficult to detect a peak shift by this technique. 

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