Hydrogenolytic selectivity

Somorjai offers the picture presented in Table V. Although the order of is rather well established experimentally, there is no definitive theoretical explanation of it. However, we have already mentioned two factors, probably not independent of each other, which might be of importance here. A high hydrogenolytic selectivity seems to be related to the following factors (1) the ease with which a metal forms multiple bonds (159) and (2) the depth of the dehydrogenation of adsorbed intermediates at the given temperatures (216). The second factor is also supported by data reviewed by Tetenyi (222). Most likely, a third factor can also be added to the list (3) the extent and structure of the carbon layer on the surface. This third factor will be discussed further in Section V. 

Cyclopropane rings are opened hydrogenolytically, e.g., over platinum on platinum dioxide (Adam s catalyst) in acetic acid at 2 - 4 bars hydrogen pressure. The bond, which is best accessible to the catalyst and most activated by conjugated substituents, is cleaved selectively (W.J. Irwin, 1968 R.L. Augustine, 1976). Synthetically this reaction is useful as a means to hydromethylate C—C double bonds via carbenoid addition (see p. 74f. Z. Majerski, 1968 C.W. Woodworth, 1968). 

The extremely high selectivity for tandem cycloaddition, the ease of manipulation of the nitroso acetals, and the release of the vinyl ether appendage in the hydrogenolytic cleavage constitute ideal features for asymmetric modifications of the cycloadditions with chiral vinyl ethers. As discussed in Section 8.3.2.1 (Inter [4+2]/inter [3+2] cycloadditions of nitroalkenes), the stereochemical course depends on the Lewis acids. The results are summarized in Scheme 8.38.179 The high levels and complementary selectivity with three chiral vinyl ethers and two kinds of Lewis acids (Ti- and Al-based Lewis acids) are presented in this scheme. 

The product selectivity was assumed to be markedly different as explained in Section I. The relations between the structures of asym DAMs and their hydrogenolytic behaviors, as well as their reactivities, are the next point for discussion. In our studies, a fixed catalyst was used in order to maintain the same effect on the hydrogenolysis of asym DAMs (53). 

A catalyst with different properties can be obtained by changing the treatment conditions of the catalyst. The relationships between the hydrogenolytic behavior and the treatment conditions of a catalyst are discussed here in order to clarify the interaction between an asym DAM and the catalyst. A M0O3-AI2O3 catalyst was selected because it is the most selective catalyst for the dearylation of asym DAM as shown in Table II (S4). 

The formation of spirocyclopropanes from the reaction of diazodiphenylmethane and ( )-8-phenylmenthyl esters of acrylic acid and methyl fumarate occurred with a modest level of diastereofacial selectivity (136). In contrast, diastereoselectivities of 90 10 were achieved in the cycloadditions of diazo(trimethylsilyl)methane with acrylamides 65 derived from camphor sultam as the chiral auxiliary (137) (Scheme 8.16). Interestingly, the initial cycloadducts 66 afforded the nonconjugated A -pyrazolines 67 on protodesilylation the latter were converted into optically active azaproline derivatives 68. In a related manner, acrylamide 69 was converted into A -pyrazolines 70a,b (138). The major diastereoisomer 70a was used to synthesize indolizidine 71. The key step in this synthesis involves the hydrogenolytic cleavage of the pyrazoline ring. 

In the early synthesis of deamino-dicarba-oxytocin, the intermediate Z-Asu(OMe)-OH was used which requires a saponification step prior to cyclizationJ1-2 Subsequently, a synthesis more consistent with the general protection strategies in peptide synthesis was developed with the intermediate Z-Asu(OtBu)-OH.12,24 As outlined in Scheme 9, upon selective deprotection of the side-chain carboxy group of the Asu residue by exposure to TFA, the octapeptide derivative 26 is converted into the 2,4,5-trichlorophenyl ester 27 using the tri-fluoroacetate method.129,20 Hydrogenolytic Na-deprotection of 27 in dilute solution leads to... 

Table 2. Products of Selected Hydrogenolytic Reactions of Fluorohydro and Fluorohalocompounds...

The first step of the total synthesis of 31 is the (7v )-proli nc-calal yzcd aldol reaction between 4 and 32, which gave the aldol adduct 33 with a good yield (69%) and nearly perfect stereocontrol (>96% de, >99% ee, Scheme 10). The same results were observed when the reaction was carried out on a 40-mmol scale yielding 5.22 g of 33 without a decrease of selectivity. The free hydroxyl group of 33 was quantitatively protected as MOM-ether. After hydrogenolytic debenzylation the aldehyde-ketone was obtained after Dess-Martin oxidation followed by a double Wittig reaction to provide the bisolefine 34 in 41% yield over 4 steps (Scheme 10). 

Due to the high stability of the benzyloxycarbonyl group under most conditions of peptide synthesis and its mild and selective hydrogenolytic cleavage, it is generally used for intermediate a-annino protection in chain-elongation steps in combination with the more acid-... 

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