Mo complex

In attempts to understand the photochemical reactions of Fischer carbene complexes, several matrix isolation and flash photolysis studies have been conducted using both Cr and W (but not Mo) complexes [5-11]. Although the complexes studied and conditions used varied, several general conclusions were drawn ... 

In a different type of reaction, alkenes are photooxygenated (with singlet O2, see 14-8) in the presence of a Ti, V, or Mo complex to give epoxy alcohols formally derived from allylic hydroxylation followed by epoxidation, for example, ... 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

Likewise, pyridines such as methyl isonicotinate 1999 or quinolines are readily oxidized by BTSP 1949 in the presence of HOReOs in CH2CI2 to give, after 6 h at 24°C, 98% yield of, e.g., methyl isonicotinate N-oxide 2000 [174] (Scheme 12.49). The oxidation of diphenylsulfide with BTSP 1949 and triphenylphosphine dichloride in acetonitrile results, after 60 h at room temperature, in only 12% diphenyl sulfoxide 2001 and 88% recovered diphenyl sulfide [175] (Scheme 12.49), whereas thianthrene 5-oxide 2002 is oxidized by the peroxy-Mo complex 2003 to give 58% of a mixture of 2004 to 2007 in which the trans 5,10-thioxide 2005 predominates [176] (Scheme 12.50). 

Dirheniumheptoxide 2154 is converted by TCS 14, in the presence of 2,2 -dipyri-dine, into the dipyridine complex 2160 [77]. Free ReCls, NbCls, and WCI5 react with HMDSO 7 and 2,2 -bipyridine to form bipyridine oxochloride complexes 2161 and TCS 14, with reversal of the hitherto described reactions of metal oxides with TCS 14. The analogous Mo complex 2162 undergoes silylahon-amination by N-trimethylsilyl-tert-butylamine 2163 to give the bis-imine complex 2164 and HMDSO 7 [77] (Scheme 13.22). 

We will discuss each topic below. Later, we will discuss the key reaction, asymmetric nucleophilic addition of a n-allyl Mo complex, in great detail. 

Asymmetric Nucleophilic Addition of a ir-Allyl Mo Complex route... 

The chiral center would be installed from either Unear carbamate 15 or branched carbamate 16 via the asymmetric addition of malonate anion to the 7i-allyl Mo complex reported by Trost et al. [11] to afford the branched chiral malonate derivative 17. Decarboxylation of 17 should provide the mono-carboxylic acid 18. Masa-mune homologation with 18 affords our common precursor 14. Linear carbamate 15 was obtained from the corresponding cinnamic acid, and branched 16 was prepared in one pot from the corresponding aldehyde. 

A newly developed asymmetric nucleophilic addition of malonate to 7i-allyl Mo complex was the cornerstone for this preparative campaign. 

The results with Mo(CO)s were similar to Trost s report. Reactions with S,S-ligand 31 yielded the S-products predominantly. It is interesting to point out that entry 1 and entry 2 should give the same result if this reaction was going through the same tr-allyl Mo complex. However, branched carbonate (entry 1) gave slightly... 

As stated earlier, this reaction did not match perfectly with the Curtain-Hammett postulate. The chiral Mo complex can select the favored face (either A or B) from L-C. However, facial selection of B-C on formation of the tr-complex (A or B) should be dictated by the orientation of the carbonate itself not by the chirality of the Mo complex. At the same time, we would expect the chiral Mo complex to... 

In contrast, the mismatched R-carbonate B-C-R needs slower nucleophilic substitution for better selectivity, allowing the initially formed undesired tt-allyl Mo complex B to convert to A, prior to substitution. For instance, when all the malonate was added at the beginning with the mismatched B-C-R, the ee was only 70%. On the other hand, when malonate was added to the reaction mixture over six hours, the ee was dramatically improved to 92%. Previously, we reported that the reaction in toluene gave better selectivity than in THF with branched carbonate as the starting material. We monitored the progress of the reaction in toluene and TH F with chiral HPLC and the results are summarized in Figure 2.4. 

Thus, we prepared Mo complex 70 having four carbon monoxides from (nob) Mo(CO)4 with ligand 64. 

Next we examined the reaction between the Mo complex 70 and linear carbonate L-C in an NMR tube [26]. The result was quite interesting, as summarized in Scheme 2.21. 

Mo complex 70 (2 mole) were reacted with 1 mole of carbonate L-C to generate lmole of tt-allyl Mo complex 71, lmole of free ligand 64, lmole of Mo(CO)6,... 

The isolated salt 72 was reacted with carbonate L-C to regenerate the tr-allyl Mo complex 71, releasing 1 mole of carbon dioxide, and sodium methoxide, and 2 mole of carbon monoxide (Scheme 2.23). Then, sodium dimethylmalonate reacts with the regenerated tr-allyl Mo complex 71 in the presence of 2 mole of carbon monoxide. 

As previously mentioned, the nucleophilic substitution on the Mo complex 71 most likely occurs with retention of configuration based on the stereochemistry outcomes of the product and 71. The retention-retention mechanism was confirmed with labeling experiments in collaboration with Professor Lloyd-Jones, as shown in Scheme 2.25 [27]. 

Stoichiometric reaction with matched S-carbamate having the D atom in the Z-position 733) in the presence of S,S-ligand 64 without a nucleophile solely formed (no other isomer was observed by NMR) the Mo-complex 74 without transposition of the label. The structure of 74 was probed based on NMR studies by comparison with NMR studies and the X-ray structure of the protio complex 71. Nucleophilic attack of sodium malonate on the Mo complex 74 provided the S-product 75, where the D atom remained at the Z-position. On the other hand, stoichiometric reaction with mismatched R-carbamate having the D atom in the Z-position 76 without a nucleophile generated the Mo complex 80 as sole product, based on NMR studies. The structure of the complex 80 was elucidated by NMR. In 80, Mo is located on the same face as in 74 but the D atom is transposed from the Z to the E position. The transposition could be explained as follows. Initially the n-allyl Mo-complex 77 (unobserved) must form with retention. Mo complex 77 is equilibrated into the more stable Mo complex 80, where the D atom is moved... 

There was still some room for uncertainty on this retention-retention mechanism. The argument was, if the unobserved tt-allyl Mo complex (such as 77 or B in Scheme 2.18) was more highly reactive towards sodium malonate than experimentally observed tt-allyl Mo complexes (such as 71, 74, and 80), the reaction should proceed through inversion (since there is an equilibrium between the two tt-allyl Mo complexes via the o-allyl complex.) If so, when the isolated Mo-complex 71 was subjected to the reaction, 71 must be equilibrated to the enantiomer of 71 via the o-allyl complex prior to reaction with a nucleophile. Therefore, reaction from the Mo complex 71 should proceed with less stereoselectivity than that from a mismatched branched carbonate. This hypothesis was examined, as shown in Scheme 2.26. 

The catalyst effectiveness decreased upon increasing the concentration of the intercalated complexes via aggregation of the closely associated complexes.164 Sulfides were also oxidized by heterogeneous Co(II) complexes.163 Catalytic oxidation of thiols was mediated by Mo complex intercalated in a layered double hydroxide.166... 

The dry tungsten and molybdenum complexes are extremely explosive when dry handle no more than 10 mg portions. The corresponding Mo complexes with Br or I replacing H are also explosive. 

Hoveyda and coworkers [227] used a domino process to give chromanes 6/3-8 by treatment of 6/3-7 in the presence of ethylene. One of the first-generation Grubbs catalyst 6/3-9 and one of Blechert s [228] early examples allowed the synthesis of bicyclic compounds of different sizes, depending on the length of the tether thus, the reaction of 6/3-10 led to 6/3-11 using 30 mol% of the Schrock Mo complex 6/3-12. 

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