Hydrometalation cyclic

Deliberate production of (vinyl)polystyrene from (toluenesul-foxyethyl)polystyrene or (haloethyl)polystyrenes was best accomplished by quaternization with N,N-dimethylaminoethanol, followed by treatment with base beta-deprotonation is encouraged in the cyclic zwitterionic intermediate. Reaction was faster and cleaner than with other reagents recommended (64, 76, 77) for eliminations, such as alkoxide, diazabicycloundecene or quaternary ammonium hydroxide this new and efficient procedure may find application elsewhere. Hydrometallation or other additions to polymer-bound olefin may prove useful steps in future syntheses by polymer modification. 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

SCHEME 11. Cyclic anft -hydrometallation and anft -carbometallation reactions for Pd-catalyzed... 

The ruthenium-catalyzed cycloisomerization of a variety of <5-enallenes was also achieved, forming cyclic 1,3-dienes or 1,4-dienes depending on the substrates and reaction conditions [32] (Eq. 22). This intramolecular coupling of the C=C bond and allenes can be envisioned by the initial hydrometallation of the allene moiety followed by intramolecular olefin insertion and isomerization. 

Reactions of the hafnium analog of (6), [Cp2HfHCl], differ little from those of the zirconium com-plex. It was suggested that the hafnium reagent is somewhat more sensitive to steric inhibition, as cyclohexene and related cyclic alkenes failed to react but this might well be due to the specific preparation of the solid (see above), rather than to any inherent molecular differences. There are some changes in the stereochemical outcome of hydrometallation of bicyclic alkenes (see Section 3.9.3.3.S), but it is far from clear that hafnium will provide any features sufficient to overcome the availability and cost advantages of zirconium. 

Cycloisomerization. Activation of a C—H bond by the Rh complex for intramolecular hydrometallation of a proximal double bond can lead to valuable cyclic products. Examples for such reactions include elaboration of dehydrobenzosuberones from o-formylbenzylide-necyclopropanes and of cyclopentane derivatives through addition of an azadiene." ... 

Scheme 3.13 Cyclic anti hydrometallation and anti carbometallation reactions for Pd-...

The postulated mechanism involves palladium(II) hydride complexes as the catalytically active species. They are assumed to initiate the cycloisomerization process through hydrometalation of the acetylenic unit, which converts the intermediate 7t-complex I into the cr-bonded complex II (Fig. 10.6). The subsequent step is the conversion of II into the cyclic species III by carbometalation of the olefin function. This step creates the stereogenic carbon. 

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