Complex reducing agents reduction

Activation by ClSi(CH ). The rate of hydrogenation of some alkenes catalyzed by this complex reducing agent is increased by ClSi(CH3)3, although reduction... 

A complex reducing agent (CRA), dubbed NiCRA-bpy, was 99% effective in converting thianthrene into diphenyl over 18 hrs over 89 hrs, benzene (83%), diphenyl (8%), and dibenzothiophen (3%) were the products. The reductant was a4 2 1 2 mixture of NaH, /-AmONa, Ni(OAc)2, and bpy (88TL2963). 

Closely tied to the photochemistry of these complexes is their behavior in the presence of reducing agents. Reduction of [Rh(bipy)3]3+ with BH4 or Zn amalgam was reported to yield [Rh(bipy)2]+.4 1... 

We chose to use reductions of these compounds as test reactions and we tried first to reduce halogeno naphthalenes. Quickly we observed that constituent ratios of complex reducing agents cannot be randomly taken. Thus, without data on the nature of the reducing agents, we determined empirically the best ratios giving the best yields in our conditions. Scheme 9 summarizes our main results13. ... 

Nickel is the most efficient reagent. On the other hand, more selective reductions should be expected with Co and Cu. With this latter which is the classical metal of Ullman reactions14, formation of binaphthyl is observed. This point is of interest because it shows that it would be possible to obtain new coupling reagents from complex reducing agents. 

Those results being obtained, we wanted to examine reductions of gem dihalo geno cyclopropane with the goal of comparing NaH in HMPA, NaH-f-AmONa in THF and complex reducing agents. Moreover we hoped to get some more informa tions about our new reagents. 

Results shown earlier show that complex reducing agents may be useful tools in performing reductions. Nevertheless a serious limitation would be anticipated with base-sensitive substrates if the classical basic properties of sodium hydride and sodium alcoholate were preserved in those reagents. 

Yields are between 90 and 100% except for 1-methyl cyclohexene. Thus it appears that the complex reducing agent is very sensitive to steric hindrance and will be able to perform selective reductions. 

Finally, when methylene cyclohexan is submitted to the action of NaH-f-AmONa—Ni(OAc)2 reduction yield is only 75% and 1-methyl cyclohexene appears. This isomerization as well as the reductions performed strongly suggest21,241 the presence of nickel hydride species in the complex reducing agent. 

In conclusion, all the work until now on complex reducing agents is only the opening of a new large field of investigations. Of course many points remain obscure and have to be studied. However, from our actual experiments we can say that we will publish in near future some new results, not only in the reduction area but also in the field of the numerous organic reactions by means of transition metal complexes. 

Finally we also showed that, contrary to NaH—FeCl2 or NaH-FeCl3, the iron containing complex reducing agent does not (or only very slightly) reduce carbonyl substrate. Thus, selective reductions are performable as exemplified in Scheme 4. 

A complex reducing agent was prepared from NaH, RONa and nickel(II) acetate This catalyst (referred to as Nic), similarly to the P-1 and P-2 nickel catalysts, is a selective catalyst in diene reductions. The reactive parts of Nic are metal hydrides and the key step in the hydrogenation is the formation of M—H bonds. The sodium salt of the alcohol added plays an important role as an activating agent in reductions using Nic. Whereas P-1 and P-2 nickels are selective and sensitive to the double-bond structure and show a rather low propensity toward isomerization, Nic has no propensity toward disproportionation. 

In terms of hydrogenation, the Sabatier-Senderens reduction has been extensively modified, as shown by the Fischer-Tropsch Synthesis (or process), the Adkins Catalyst, and Raney-Nickel Catalyst. In addition, the silica black-supported nickel catalyst, and nickel-based complex reducing agents (Nic, e.g. NaH-RONa-Ni(OAc)2), have also been developed, the latter is a heterogeneous hydrogenation catalyst that works at atmospheric pressure. 

In a series of papers, Caubere and co-workers have described their continued exploration of the use of complex reducing agents in the selective reduction of functional groups. For example, the readily prepared NaH-Bu ONa-FeCls reduces oct-l-ene to n-octane in 90—95% yield, and shows selectivity towards exocyclic double bonds. Aliphatic and aromatic halides are reduced to hydrocarbons in high yield by the same reagent, but ketones are unaffected. 

Thiols and thioethers are the most commonly encountered sulphur derivatives which can be directly reduced to alkanes. For this purpose, Raney Ni has been traditionally used. An example of its use is the synthesis of let- and 7j5-eremophilanes from a tricyclic thiophene precursor (equation 29) However, more recently nickel complex reducing agents ( NiCRA ) have been profitably and effectively used for this purpose, as indicated by the reduction of dodecanethiol to dodecane (equation 30)". Similarly, nickelocene in the presence of LAH has been used to effect desulphurization of thiols . A combination of NaEtaBH and FeCl2 also achieves the thiol-to-alkane transformation quite satisfactorily The increasing utility of nickel boride has also been exploited for the transformation of sulphides to alkanes as indicated in equation 31. ... 

Cram s model does not always predict the stereochemical result of kinetically controlled reductions with aluminum isopropoxide (Cram and Greene, 1953). For example, i -(—)-3-cyclohexyl-2-butanone is reduced to predominantly ZR,2R-erythro carbinol (erythro/threo = 1.9). Apparently special steric forces are important in this reduction. Recent work (Shiner and Whittaker, 1963) has shown that aluminum isopropoxide is trimeric or tet-rameric. It is therefore conceivable that some hydride transfers will involve Al(OR)3 units that are not coordinated to the carbonyl groups they reduce. These transfers may occur preferentially from the side opposite that exposed to a coordinated Al(OR).3 unit. Such competitive mechanistic pathways might well yield an isomer ratio not in agreement with that produced by less complex reducing agents. 

Further studies of complex reducing agents based on sodium hydride have shown that a mixture of sodium hydride, sodium t-amylate, and zinc chloride (ZnCRA) gives regioselective 1,2-reduction of a, 8-enones to allylic alcohols, in contrast to the 1,4-reduction preference shown by the earlier developed NiCRA (3,135). The activity is enhanced by the addition of MgBr2. 

Optimum conditions for the reduction of saturated ketones by the complex reducing agent formed from sodium hydride, sodium t-amylate, and Ni" acetate (NiCRA) have been delineated.Reoxidation of the secondary alcohol products is dramatically postponed by the addition of alkali- or alkaline-earth-metal salts, and catalytic ketone reductions are achieved with NiCRA-MgBr2 mixtures. Full details of the reducing properties of various complex metal hydrides (12) of copper, formed by reaction of UAIH4 with appropriate lithium methylcuprates [equation (1)], have been published for example enones are reduced pre-... 

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