Transition Metals as Protecting, Activating, and Directing Groups

TRANSITION METALS AS PROTECTING, ACTIVATING, AND DIRECTING GROUPS 

In Section VI, the use of transition metals to stabilize otherwise unstable molecules was demonstrated. Transition metals can also be used in the reverse sense to destabilize otherwise relatively stable systems [e.g., the palladium(II) chloride derivative of Dewar(hexamethyl)benzene, decomposes quantitatively at 33° in 20 min into hexamethylbenzene and palladous chloride, whereas the free ligand has a half-life of 105 hr at 120° (Koser and St. Cyr, 1974)]. 

Another partial hydrogenation reaction which would not be easy to achieve by normal procedures is the quantitative conversion of tra j,/ra/ij-2,4-hexa-diene into c/j-3-hexene by Cr(CO)e under photolytic conditions, which takes place selectively in the presence of cis.trans and cis.cis isomers [Eq. (173) (Platbrood and Wilputte-Steinert, 1974a,b)]. This 1,4-addition of hydrogen to 1,3-dienes can also be effected using tris(acetonitrile)tricarbonylchromium(0) as the catalyst (Schroeder and Wrighton, 1974). 

Just as the tricarbonyliron group has been used as a protecting group for dienes, the tricarbonylchromium group has been used in a similar fashion to stabilize dihydropyridines which are normally difficult to handle and to isolate. The dihydropyridine is readily liberated from the complex by treatment with pyridine at room temperature (Kutney et ai., 1974). Dihydropyridines can also be generated from the corresponding tricarbonyliron complexes, recently synthesized by Alper (1975). 

Use of Cr(CO)3 as a blocking group is illustrated by the stereo-selective deuteration of the olefin XXVI in which one side of the olefin is shielded by the Cr(CO)3 group [Eq. (174) (Trahanovsky and Baumann, 1974)]. 

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