Alkyl group cross-metathesis

The reaction tolerated a variety of functionality, including ester and ether groups on the alkyl-substituted alkene at least two carbons away from the double bond, and raefa-nitro or para-methoxy substituents on the styrene. As expected, cross-metathesis occurred selectively at the less hindered monosubsti-tuted double bond of dienes also containing a disubstituted alkene (Eq. 8). 

In this strategy, an ally 1 group is attached to the substrate bound silicon that then reacts in a cross metathesis fashion with (2a). Then this surface bound Ru-alkylidene catalyzes ROMP of norbomene to give alkyl silicon terminated polymer film that is thus attached to the surface in controllable density and polymer thickness. 

The total synthesis of Stemona alkaloid (-)-tuberostemonine was accomplished by P. Wipf. Late in the synthesis, the introduction of an ethyl sidechain was required. This could be achieved in a novel four-step sequence. First, the allyl sidechain was introduced by the Keck radical allylation. To this end, the secondary alkyl phenylselenide substrate was treated with allyltriphenyltin in the presence of catalytic amounts of AIBN. This was followed by the introduction of a methyl group onto the lactone moiety. The allyl group then was transformed into the desired ethyl group as follows the terminal double bond was isomerized to the internal double bond by the method of R. Roy. This was followed by ethylene cross metathesis and catalytic hydrogenation to provide the desired ethyl sidechain. 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

It was thought that a new type of analog that does not contain the endocyclic double bond could be synthesized in almost exactly the same manner. Instead of the mono-esterified pimelic acid undergoing the aldol reaction with acrolein, an allyl group could be installed via a simple enolate alkylation reaction. Subsequent methylation, elimination, decarboxylation, cycloaddition, and cross-metathesis steps could be performed in the same manner as before with the result being a more saturated analog (76 Scheme 15). The synthesis to generate diester 73 proceeded as planned however, numerous decarboxylation conditions failed. 

In 2009, Cai et al. reported a cascade olefin cross-metathesis/intramolecular Friedel-Crafts alkylation for the construction of polycyclic indoles 133 (Scheme 5.89) [90], A Ru complex (134)/chiral Brpnsted acid [(S)-135] binary system was used in this relay catalysis. Recently, the same group reported an enantioselective intramolecular aza-Michael addition of indoles using a similar binary catalyst system (Scheme 5.90) [91]. 

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