Hydride donors catalysts

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

Enantioselective 1,4-reduction of enones can be done using a copper-BINAP catalyst in conjunction with silicon hydride donors.158 Polymethylhydrosilane (PMHS) is one reductants that is used. 

Krische et al. demonstrated intramolecular reaction with Co(dpm)2 (5mol%) and PhSiH3 (120 mol %) as a hydride donor (Scheme 8) [14-16]. Addition of aldehyde-enone 17 to a solution of the Co catalyst and phenylsi-lane resulted in the formation of the corresponding aldol cyclization product... 

The most common catalysts for the Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation are Alm and Lnm isopropoxides, often in combination with 2-propanol as hydride donor and solvent. These alkoxide ligands are readily exchanged under formation of 2-propanol and the metal complexes of the substrate (Scheme 20.5). Therefore, the catalytic species is in fact a mixture of metal alkoxides. 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

Scheme 20.16 Alkene reduction with dioxane (39) as hydride donor and a Wilkinson-type catalyst (18).

With both liquid acid catalysts, but presumably to a higher degree with sulfuric acid, hydrides are not transferred exclusively to the carbenium ions from isobutane, but also from the conjunct polymers 44,46,71). Sulfuric acid containing 4-6 wt% of conjunct polymers produces a much higher quality alkylate than acids without ASOs (45). Cyclic and unsaturated compounds, which are both present in conjunct polymers, are known to be hydride donors (72). As was mentioned in Section II.B, these species can abstract a hydride from isobutane to form the -butyl cation, and they can give a hydride to a carbenium ion, producing the corresponding alkane, for example the TMPs, as shown in reactions (7) and (8). 

Fast hydride transfer reduces the lifetime of the isooctyl cations. The molecules have less time to isomerize and, consequently, the observed product spectmm should be closer to the primary products and further from equilibrium. This has indeed been observed when adamantane, an efficient hydride donor, was mixed with zeolite H-BEA as the catalyst (78). When 2-butene/isobutane was used as the feed, the increased hydride transfer activity led to considerably higher 2,2,3-TMP and lower 2,2,4-TMP selectivities, as shown in Fig. 5. 

Cyclization of a 1,6-enyne to a methylenecyclopentane can be effected with a Pd(0) catalyst complexed with a phosphine ligand in combination with a trialkyl-silane as a hydride donor (equation I).4 Under these conditions a 1,7-enyne can be cyclized to a methylenecyclohexane. 

Initial methane formation from methanol on the fresh catalyst is proposed to proceed on Bronsted acid sites as a reaction with a hydride donor - in... 

When a mixture of cis- and frares-decalins 24 (61.5 38.5) are treated with the same solid superacid at 0°C, the thermodynamic equilibrium is rapidly achieved110 [Eq. (5.48)]. In the reaction, decalin 24 serves as the solvent, substrate, and the hydride donor. When the equilibrium is reached, the hydrocarbon can be separated from the catalyst by simple filtration. Perhydroindane 25 was also isomerized under... 

Catalytic reduction of alkynes to ds-alkenes. This reduction is not possible with 10% Pd/C alone because this metal is too reactive and the alkane is formed readily. The selective reaction is possible if the Pd/C is deactivated by either Hg(0) or Pb(0), obtained by reduction of metal acetate with NaBH4. Sodium phosphinate, H2P02Na, is the preferred hydride donor. Since this donor is not soluble in the Organic solvents used, a phase-transfer catalyst, benzyltriethylammonium chloride, is added.3... 

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

Treatment of halides or sulfonates with hydride donors such as tetrabutylammonium borohydride,38 lithium aluminium hydride,39 lithium triethylborohydride46 or sodium borohydride generate deoxy sugar derivatives (Scheme 3.8c).41 When sodium borohydride is employed, a transition metal catalyst (PdCk or NiC-b) may be added. 

Finally, there are also some special NHC-mediated transformations that do not completely fit into the classification, such as triazolylidene-catalyzed hydroacylations (Chan and Scheldt 2006). Aldehydes can serve as hydride donors for activated ketones partly following a standard 1,2-addition of the NHC to the aldehyde, but instead of the usual carbonyl umpolung a hydride ( H-umpolung ) transfer is initiated. A related Cannizzaro-type transformation has been described for indazole-derived carbene catalysts (Schmidt et al. 2007). 

Although catalytic hydrogenation in the presence of H2 and a catalyst such as Pt, Pd, Ni or Ru, reaction with diborane, and reduction by lithium, sodium or potassium in hydroxylic or amine solvents have all been reported to convert carbonyl compounds into alcohols, the most common reagents used for the reduction of carbonyl compounds are hydride donors. 

This reaction is believed to proceed via a hydride shift from the a-carbon of an alcohol component to the carbonyl carbon of a second component, which proceeds via a six-membered transition state involving the metal center of the catalyst (Figure 6.20). Isopropanol has been frequently used as the hydride donor because the resulting acetone can be continuously removed from a reaction mixture by distillation. 

As vinyl ethers were known to be poor substrates in Ru-catalyzed olefin metath-eses, it has been difficult to obtain cydic enol ethers by RCM of the vinyl ethers. Recently, a novel method to obtain cyclic enol ethers has been reported, which afforded cydic enol ethers directly from easily prepared dienes containing an allyl ether moiety [46]. Treatment of 70 with diene 99 in CH2CI2 in the presence of small amount of H2 resulted in a formation of dihydropyran 101 (Eq. 12.40). Treatment of 70 with H2 has been thought to produce an active catalyst for the olefin isomerization, and only metathesis products are formed until a small amount of H2 is introduced in the reaction. These results implied that this reaction most likely proceeded by way of a formation of the cyclic olefin 100, which was subsequently converted to dihydropyran 101 by the newly formed isomerization catalyst. In addition to the tandem reaction shown in Eq. 12.40, another method for obtaining cydic enol ethers from allyl ethers has also been demonstrated [46b]. This method induded addition of the hydride donor, such as NaBH4, to the reaction solution after the metathesis reaction had been completed. Although attempts to observe an active species for olefin isomerization in the presence H2 failed, these results suggested participation of hydride species in the olefin isomerization. 

Oxazaborolidines have been found to be a unique catalyst for asymmetric borane reduction of ketones and imines [35,36]. Coordination of BH3 to the nitrogen atom of 24 serves to activate BH3 as a hydride donor and to increase the Lewis acidity of the boron atom (Eq. 9). The Lewis acidity of the boron atom in the oxazaborolidine plays an important role in the reduction. Several types of polymer-supported oxazaborolidine have been reported and are considered to be polymer-supported boron-based Lewis acids. 

Group 14 metal hydrides, especially those of silicon and tin, are satisfactory nonreactive hydride donors, as in the absence of a catalyst they are, generally, poor reducing agents. Transition metal complexes are attractive transfer agents because they insert readily into Si—H or Sn—bonds and they also bind specifically to various functional groups. 

The most striking product result is the extensive formation of propane over very active catalysts. Venuto et al. (99) reported analogously that dealkylation of rf-butylbenzene over rare earth-exchanged X zeolite at 260° gave isobutane as the major gaseous product. Such paraffin formation is presumably the result of hydride transfer reactions to the car-bonium ions formed by initial electrophilic cleavage of the alkylbenzene 100) or by protonation of the olefin. Reasonable hydride donors are cumene and propylene the resultant hydrogen-deficient species are then precursors of residue formation (32, 89). Parafiin formation by treatment of alkylbenzenes with aluminum halides in the presence of cyclohexane or decalin has been known for 30 years 47), and there is ample evidence for hydride transfer between carbonium ions and hydrocarbons 10, 22, 27,53). 

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