Ring hydrogenation stereospecific

A longer-range coupling has been detected in 140, between the ringed hydrogen atoms, and the stereospecificity used to assign the conformation. (1988,117)... 

Any method of making such bicyclic compounds will automatically form this stereochemistry. An important method of stereochemical control that we have not used so far in this chapter is catalytic hydrogenation of alkenes, which adds a molecule of hydrogen stereospecifically cis. If the reaction also makes a fused ring system, it may show stereoselectivity too. Here is an example with 5/5 fused rings. 

Platinum oxide hydrogen chloride Reductive tetrahydro-l,2-oxazine ring opening Stereospecific synthesis... 

Palladous chloride/sodium tetrahydridoborate Preferential stereospecific ring hydrogenation with catalyst prepared in situ... 

Ketones and alcohols from a,y -ethyleneketones s. 12, 96 Ring hydrogenation of isocyclics of benzene rings s. 12, 98 stereospecific s. 11, 104 partial s. 13, 94... 

Since the stereochemical course of a catalytic hydrogenation is dependent on several factors, " an understanding of the mechanism of the reaction can help in the selection of optimal reaction conditions more reliably than mere copying of a published recipe . In the first section the factors which can influence the product stereochemistry will be discussed from a mechanistic viewpoint. In subsequent sections the hydrogenation of various functional groups in the steroid ring system will be considered. In these sections both mechanistic and empirical correlations will be utilized with the primary emphasis being placed on selective and stereospecific reactions. 

The rate of saponification of ethyl 2-thenoate, in contrast to ethyl 3-thenoate, was found to be considerably slower than predicted from the pKa of the acid, showing that the reactivities of thiophenes do not parallel those of benzene. The first explanation, that this was produced by a steric effect of the ring sulfur similar to the case in or /lo-substituted benzenes and in ethyl 1-naphthoate, could not be upheld when the same effect was found in ethyl 2-furoate. It was later ascribed to a stereospecific acid strengthening factor, involving the proper relation of the carboxylic hydrogen and the heteroatom, as the rate of saponification of 2-thienylacrylic acid was in agreement with that predicted from the acid constants. ... 

Stereospecificity was observed in the hydrogenation and hydroboration of the alkene (39) (38). Attack from below the furanose ring of both D-erythro-4-enosides (39 and 43) is hindered by the 2,3-O-isopropylidene ring and the products resulting from topside attack therefore predominate. 

The dimer of chloro(l,5-hexadiene)rhodium is an excellent catalyst for the room temperature hydrogenation of aromatic hydrocarbons at atmospheric pressure. The reaction is selective for the arene ring in the presence of ester, amide, ether and ketone functionalities (except acetophenone). The most useful phase transfer agents are tetrabutylammonium hydrogen sulfate and cetyltrimethylammonium bromide. The aqueous phase is a buffer of pH 7.6 (the constituents of the buffer are not critical). In all but one case the reaction is stereospecific giving cis products... 

The mechanism for the union of hydrogen with benzene also has been illuminated by stereochemical studies. Linstead and his students demonstrated that the hydrogenation of substituted benzenes occurred with considerable stereospecificity (J6). Thus the hydrogenation of diphenic acid yielded mainly compounds with either cis (one ring reduced) or cis-syn-cis (both rings reduced) configurations. 

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