Allyltrimethylsilane cross-metathesis

Some further examples of the allyltrimethylsilane cross-metathesis reactions, originally investigated by Crowe, with more complex substrates were also in-... 

Attacking the problem of cross-metathesis selectivity from a different angle, Crowe and co-workers explored the reactivity of a more nucleophilic partner for the Ji-substituted alkenes. They chose to use allyltrimethylsilane since they proposed that the CH2SiMe3 substituent should have a negligible effect on alkyli-dene stability, but enhance the nucleophilicity of the alkene via the silicon P-ef-fect (Fig. 1). 

As expected, there was no formation of stilbenes or a dinitrile product and, more surprisingly, in all of the reactions reported only 5-7% of the allyltrimeth-ylsilane self-metathesis product was observed. It was proposed that this lack of allylsilane self-metathesis was due to the steric bulk of the TMS group reducing the reactivity of the Me3SiCH2 substituted alkylidene. In a more recent report by Blechert and co-workers it was noted that allyltrimethylsilane and its hydrocarbon equivalent (4,4-dimethylpent-l-ene) had comparable reactivities in the cross-metathesis reaction [28], further suggesting that the selectivity arises from steric rather than electronic effects. 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

After successful completion of all rearrangement reactions, the incorporation of the different side chains of the tetraponerines was attempted by employing a cross metathesis reaction. However, the cross metathesis of 19 and 22 with allyltrimethylsilane in the presence of 10% [Ru-1] was unsuccessful due to the formation of a carbene with low reactivity. The use of Schrock s molybdenum catalyst26 [Mo] (Figure 7) also failed to show any conversion. The terminal double bonds of 19 and 22 were assumed to be too hindered for cross metathesis. An alternative route to incorporate the different alkyl chains of the tetraponerines was necessary (Scheme 8). 

With certain ruthenium complexes sueh a [RuCl2(p-eymene)]2, allyltrisubstituted silanes (allyltrimethylsilane, allyltriethoxysilane) undergo isomerization to the corresponding 1-propenyltrisubstituted silane followed by cross-metathesis between the two isomers (Marciniec 1994a). 

The ruthenium NHC complex (53) has been reported to catalyse the enantioselective ring-opening cross-metathesis of norbomenes with allyltrimethylsilane (<98% ee)P ... 

Lesma and associates began their synthesis of (-b)-monomorine I (1562) with the N-Boc -protected amine 1623, a simple derivative of (5)-pyroglu-tamic acid (Scheme 205). Reaction with allyltrimethylsilane in the presence of boron trifluoride according to a reported procedure and replacement of the Boc group by Cbz produced the 2,5-n s-disubstituted pyrrolidine (—)-1624, cross-metathesis of which with methyl vinyl ketone... 

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