Cross-metathesis compounds

Allylboronates are attractive reagents for the highly diastereoselective ally-lation of carbonyl compounds. A sequential cross-metathesis-allylation reaction has recently been developed by Grubbs et al. [88c] and by Miyaura et al. [103]. The sequence is illustrated in Scheme 23 for the formation of homoallylic alcohol 114 from allylboronate 112, acetal 113, and benzaldehyde [88c]. 

Table 12 Compounds TsRs obtained by cross- -metathesis or silylative coupling of T8[CH = CH2]8 w ith alkenes ... 

A similar strategy served to carry out the last step of an asymmetric synthesis of the alkaloid (—)-cryptopleurine 12. Compound 331, prepared from the known chiral starting material (l )-( )-4-(tributylstannyl)but-3-en-2-ol, underwent cross-metathesis to 332 in the presence of Grubbs second-generation catalyst. Catalytic hydrogenation of the double bond in 332 with simultaneous N-deprotection, followed by acetate saponification and cyclization under Mitsunobu conditions, gave the piperidine derivative 333, which was transformed into (—)-cryptopleurine by reaction with formaldehyde in the presence of acid (Scheme 73) <2004JOC3144>. 

Applications of the cross-metathesis reaction in more diverse areas of organic chemistry are beginning to appear in the literature. For example, the use of alkene metathesis in solution-phase combinatorial synthesis was recently reported by Boger and co-workers [45]. They assembled a chemical library of 600 compounds 27 (including cisttrans isomers) in which the final reaction was the metathesis of a mixture of 24 oo-alkene carboxamides 26 (prepared from six ami-nodiacetamides, with differing amide groups, each functionalised with four to-alkene carboxylic acids) (Eq.27). 

Two repeated exposures of resin 38 to the catalyst (9% mol) for 18 h in dichloromethane at room temperature afforded the expected allyl lactoside in an encouraging isolated yield of 81% from resin 35 (90% per step). Traces of dimerized compounds resulting from cross-metathesis were detected as the only side products. Extension of the oligosaccharide chain was subsequently performed first by deacetylation (excess NaOMe in 4/1 CH2Cl2/MeOH at r.t.) and glycosylation with known lactosyl donor 40 in conditions similar to those mentioned above. Cleavage was performed twice as described above, but with a reduced reaction time of 6 h in this case tetrasaccharide 42 was isolated in 51% yield from 35 (84% per step). No dimerized products were detected. 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

As indicated by the examples, ruthenium catalysts 5-7 can be used for compounds containing a broad range of functional groups, especially those containing oxygen, and they are also tolerant to water. The greater challenge is the use of these catalysts for cross-metathesis for which SHOP (Eq. 15) is one example, and Eq. 19" represents a recent success. 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

Where there is no spacer group between the C=C bond and the functional group, productive self-metathesis does not occur, but cross-metathesis reactions with other olefins are still possible. Recent impressive examples of this are the cross-metathesis reactions of acrylonitrile (equation 19). The reaction occurs with a wide variety of R groups. For 15 different compounds the yield of the new nitrile after 3 h at room temperature is 40-90%, with the cis isomer always strongly preferred (75-90%). Only minor amounts of RCH2CH=CHCH2R are formed, and no NCCH=CHCN182. The fact that acrylonitrile... 

Cross-metathesis applications, 11, 200 enynes, 11, 282 in ethenolysis, 11, 198 Lewis-basic substrates, 11, 193 in one-pot reactions, 11, 197 for reagent synthesis, 11, 188 as simple metathesis reaction, 1, 171 Crotyltributyltins, with aldehydes, 9, 352 Crown ether clathrates, diorganozinc compounds, 2, 335 Crown ether-pendant polysilanes, preparation, 3, 577 Crown-ethers, as hosts, 12, 813... 

Heterochalcogenides, with chromium, 5, 312 Heterocoupling reactions in olefin cross-metathesis, 11, 181 Pd-catalyzed, alkynes, 8, 274—275 Heterocubanes, reactions, 3, 8 Heterocumulenes in insertion reactions, 1, 107 nickel metallacycle reactions, 8, 103-104 Heterocyclic compounds... 

Ruthenium-catalyzed olefin cross-metathesis (ring-closing metathesis, RGM) between terminal alkenes and vinyl-boronic acid or esters has recently been developed for the synthesis of ( )-l-alkenylboron compounds from alkenes.459,460 The efficiency of protocol was proved in the synthesis of a key intermediate of epothilone 490 292 461 (Equation (84)). The vinyl boronate was given almost exclusively the trans-adduct. 

The synthesis of unsynunetrical nonadjacently linked tetrahydrofurans was accomplished by Mootoo through (4a) catalyzed cross metathesis of the protected substituted-tetrahydrofuran components. These compounds are potential intermediates for acetogenins having potent antitumor activity... 

Grubbs used (4a) to efficiently install functional groups that might otherwise be added via C H activation or allylic oxidation pathways (Scheme 12). Among the breadth of compound types accessible through these cross metathesis strategies were substituted vinyl-phosphonic, boronic, and carbonyl esters, which were synthesized in good to excellent yields. ... 

Cross metathesis of chiral allyhc alcohol or amine moieties and styrene to create phenyl analogues of the parent compounds was achieved using (4a). The phenyl group, in conjugation with the parent molecule, lends a stronger extinction coefficient to the molecules, making the species more amenable for analysis by circular dichroism. ... 

Hoveyda and coworkers recently developed sequential catalytic cross metathesis/asymmetric conjugate addition utilizing (4b) to make acyclic aliphatic enones (equation 31)3 Blechert developed a synthetic route toward bicyclic N-heterocycles that hinged on cross metathesis and double reductive amination to access compounds like... 

Cross metathesis of ethylene with internal alkenes provides a facile route to terminal alkenes. A number of processes have been described that use this transformation however, the only products, besides neohexene,that appear to be important are the a,o>-dienes that result from metathesis of cyclic alkenes with an excess of eAylene. This family of compounds should find a wide variety of applications. 

Keywords cross-metathesis, silylative coupling, tmn.s-silylation, ruthenium catalysts, vinylsilicon compound... 

Summary Two catalytic reactions, i.e. silylative coupling (mms-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, II, V, VI) and cross-metathesis (CM) catalyzed by mthenium-carbene (i.e. 1st and 2nd generation mthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with conunercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds with vinylsilicon functionality. 

In the last 15 years we have developed two new catalytic reactions between the same parent substances, i.e. silylative coupling (SC) (also called tmns-silylation or silyl groiq> transfer) and cross-metathesis (CM) of alkenes, which have provided an universal route for the synthesis of well-defined molecular compounds with vinylsilicon functionality. While the cross-metathesis is catalyzed by well-defined Ru and Mo carbenes, the silylative coupling is catalyzed by complexes initialing or generating M-H or M-Si bonds (where M = Ru, Rh, Ir). For recent reviews see Refs. [4-6],... 

On the other hand, our recent study on the highly efficient cross-metathesis of vinyltrialkoxy-and vinyltrisiloxy-silanes with various olefins, for example, with styrene [12] allyl eth [13] and esters [14] as well as octavinylsilsesquioxane [IS] with several olefins have opened a new opportunity for the use of alkene-cross-metathesis in the synthesis of unsaturated organosilicon compounds (see also Refs. [5] and [6]). In this p r new examples of the two reactions involving hetero(N,S,B)organic olefins have been overviewed. 

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