Aldehydes hydride addition

The 10-57-5-hydridosiliconate ion 62 is known in association with lithium,323 tetrabutylammonium,101 and bis(phosphoranyl)iminium93 cations. It is synthesized by hydride addition to the 8-.S7-4-silane 63, which is derived from hexafluoroacetone.101 Benzaldehyde and related aryl aldehydes are reduced by solutions of 62 in dichloromethane at room temperature101 or in tetrahydrofuran at 0°96 within two hours. The alkyl aldehyde, 1-nonanal, is also reduced by 62 in tetrahydrofuran at O0.96 Good to excellent yields of the respective alcohols are obtained following hydrolytic workup. The reactions are not accelerated by addition of excess lithium chloride,96 but neutral 63 catalyzes the reaction, apparently through complexation of its silicon center with the carbonyl oxygen prior to delivery of hydride from 62.101... 

The mechanism of the hydroformylation reaction suggests that aldehyde regioselectivity is determined in the hydride addition step, which converts the five-coordinated H(alkene)-Rh(CO)L2 into either a primary or a secondary four-coordinated (alkyl)Rh(CO)L2. For the linear rhodium alkyl species, this... 

There is a rather important difference between chemical reductions using complex metal hydrides and enzymic reductions involving NADH, and this relates to stereospecificity. Thus, chemical reductions of a simple aldehyde or ketone will involve hydride addition from either face of the planar carbonyl group, and if reduction creates a new chiral centre, this will normally lead to a racemic alcohol product. Naturally, the aldehyde primary alcohol conversion does not create a chiral centre. 

The a complex (a) obtained by cis addition of the cobaltcarbonyl hydride (path A) can either lead to the expected threo aldehyde (path B) or isomerize through a new tt complex ( tt ) path C, to give aldehyde 27. The other possible o- complex (hydride addition (path D) must isomerize to undergo a CO insertion. It can do this through a tt complex (tt") (path E) to give as a final product the erythro aldehyde or through a tt complex ( tt " ) (path F) leading to aldehyde 26. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

MacMillan s catalysts 56a and 61 allowed also the combination of the domino 1,4-hydride addition followed by intramolecular Michael addition [44]. The reaction is chemoselective, as the hydride addition takes place first on the iminium-activated enal. The enamine-product of the reaction is trapped in a rapid intramolecular reaction by the enone, as depicted in Scheme 2.54. The intramolecular trapping is efficient, as no formation of the saturated aldehyde can be observed. The best results were obtained with MacMillan s imidazolidinium salt 61 and Hantzsch ester 62 as hydride source. As was the case in the cyclization reaction, the reaction affords the thermodynamic trans product in high selectivity. This transformation sequence is particularly important in demonstrating that the same catalyst may trigger different reactions via different mechanistic pathways, in the same reaction mixture. 

Just as addition of a Grignard reagent to an aldehyde or ketone yields an alcohol, so does addition of hydride ion, H (Section 17.4). Although the details of carbonyl-group reductions arc complex, LiAll-14 and N aBH4 act as if they were donors of hydride ion in a nucleophilic addition reaction (Figure 19.7).. Addition of water or aqueous acid after the hydride addition step protonates the tetrahedral alkoxide intermediate and gives the alcohol product. 

Any explanation of facial selectivity must account for the diastereoselection observed in reactions of acyclic aldehydes and ketones and high stereochemical preference for axial attack in the reduction of sterically unhindered cyclohexanones along with observed substituent effects. A consideration of each will follow. Many theories have been proposed [8, 9] to account for experimental observations, but only a few have survived detailed scrutiny. In recent years the application of computational methods has increased our understanding of selectivity and can often allow reasonable predictions to be made even in complex systems. Experimental studies of anionic nucleophilic addition to carbonyl groups in the gas phase [10], however, show that this proceeds without an activation barrier. In fact Dewar [11] suggested that all reactions of anions with neutral species will proceed without activation in the gas phase. The transition states for reactions such as hydride addition to carbonyl compounds cannot therefore be modelled by gas phase procedures. In solution, desolvation of the anion is considered to account for the experimentally observed barrier to reaction. 

Eine schnelle Addition des Hydrids an die Carbonyl-Gruppe wird angenommen. Eine nochmalige Hydrid-Addition erfolgt nicht oder nur sehr langsam. Die anschlieBende saure Spaltung des 0,N-AcetaIs fiihrt zum Aldehyd. 

We now come to the primary value of tlic.se reagents their ability to react as. nucleophiles toward the electrophilic carbon in carbonyl compounds ( C = 0). In a reaction mechanistically analogous to the hydride additions of Section 8-6. orgaiiomelallic reagents add nucleophilic carbon to aldehydes and kcioncs. rcsulliiig in alcohols, and making a new carbon-carbon bond in the process. Ai die end of the next seciion, you will find a summary chart of the major types of reactions that convert carbonyl compound.s to alcohols. 

Diastereoselective hydride addition is quite versatile, and it provides facile synthetic access to ( —)-pinidine (661), an alkaloid isolated from several species of Pinus, as well as its unnatural isomer (+ )-pinidine (660b). The unstable aldehyde 655, prepared in four steps from 624 [202], undergoes Grignard addition with 4-pentenylmagnesium bromide followed by Swem oxidation to afford ketone 656 in 90% yield for the two steps. Stereoselective hydride addition with L-Selectride provides the -yn-alcohol 657 (91 9), while zinc borohydride reduction provides almost exclusively the anri-alcohol 658 (>99 1) (Scheme 144). 

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