Aldehydes continued unsaturated

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

Figure 5.10 Continuing reaction of an unsaturated oxyphosphorane with an aldehyde.

Continued investigation revealed that the principal epoxidizing agents for combined oxidation of unsaturated compounds and aldehydes are not the corresponding peracids, but the radicals of acyl peroxides. 

The scope of catalytic hydrogenations continues to be extended to more difficult reductions. For example, a notoriously difficult reduction in organic synthesis is the direct conversion of carboxylic acids to the corresponding aldehydes. It is usually performed indirectly via conversion to the corresponding acid chloride and Rosenmund reduction of the latter over Pd/BaS04 [65]. Rhone-Poulenc [30] and Mitsubishi [66] have developed methods for the direct hydrogenation of aromatic, aliphatic and unsaturated carboxylic acids to the corresponding aldehydes, over a Ru/Sn alloy and zirconia or chromia catalysts, respectively, in the vapor phase (Fig. 1.18). 

In a continuation of this work, Breit and Smejkal (43) showed that when a-p unsaturated carboxylic acids are exposed to hydroformylation conditions in the presence of supramolecular catalyst 5, the reaction takes a completely unexpected path, yielding a product corresponding to a decarboxylative hydroformylation (Scheme 1, bottom). Under standard hydroformylation conditions, moderate activity for the hydrogenation of the double bond was observed, but no aldehyde product was detected (Scheme 1, top). 

The reactions (forward and reverse) are acid-base-catalyzed and complicated in basic solutions at higher concentrations of reactants by continued condensation to form polymers of the general formula H—[CH2—C(R)(OH)]n—OH. Depending on R the final diol may revert to the aldehyde or ketone form. In acid solution dehydration of the tertiary alcohol leads to the formation of a-0 unsaturated ketones [for example, R—C(CH3) = CH—CO(R)], which may condense further. 

The continued search for methods to effect lj4-reductions using catalytic quantities of CuH produced several reports late in the last decade. The basis for these new developments lies in an appreciation for the fadlitj with which various silyl hydrides undergo transmetalation vdth copper enolates. Thus a limited amount of (PhjP)CuH (0.5-5 mol%) in the presence of PhSiHj (1.5 equivalents relative to substrate) reduces a varietj of unsaturated aldehydes and ketones in high yields (Eq. 5.14) [29]. Limitations exist with respect to the extent of steric hindrance in the educt Similar results can be achieved using BusSnH in place of PhSiHj although the latter hydride source is the appropriate (albeit expensive) choice from the environmental perspective. 

---