Meso triene

Transformations shown in Schemes 12-14 constitute the first examples of catalytic AROM reactions ever reported. Meso-triene 50 is converted to chiral heterocyclic triene 32 in 92% ee and 68% yield in the presence of 5 mol % 4a (Scheme 12) [21]. Presumably, stereoselective approach of the more reactive cy-clobutenyl alkene in the manner shown in Scheme 12 (II) leads to the enantioselective formation of Mo-alkylidene III, which in turns reacts with an adjacent terminal olefin to deliver 51. Another example in Scheme 12 involves the net rearrangement of meso-bicycle 52 to bicyclic structure 54 in 92% ee and 85% yield. The reaction is promoted by 5 mol % 4a and requires the presence of di-... 

In 2007, Collins and coworkers [53] reported the use of halide additives to improve the reactivity and selectivity of a class of Ru-based catalysts in which the NHC ligand is C -symmetric and monodentate (Scheme 12.29). Similar to the above example, the catalysts were again evaluated in the ARCM reaction of meso trienes. Based on the knowledge that the size of the Al-substituents on the NHC ligand of Ru-based catalysts can influence catalyst reactivity [54], the... 

Scheme 12.28 Desymmetrizations of meso trienes catalyzed by chiral Ru-based catalysts in the presence of Nal as an additive.

All three topological forms (cis-a., cis-j8 and trans) have been isolated769 for CoCl2(trien)+ and the formation of the optically active trans isomer from the chiral cis-fi establishes the (RR,SS) configuration for the racemic form of the latter. The alternative R,S configuration for the chiral cis- S would lead to an inactive R,S-meso) trans form. 

The (2e<5) mesa configuration is adopted in the Ni11838 and rra/w-(R,5)-Co(CN)2 (trien)+ 839 complexes while the Pd11779 complex adopts the (sss) meso arrangement. Hg(trien)Cl2 is also believed to be five coordinate.840... 

Co(trien)(NH3)2] + has been isolated, and only the meso trans isomers (197-198) with two different axial ligands, remain to be distinguished. There is also an extensive chemistry of N and C-alkylated derivatives of (178 180) as cA-[Co(OH)(trien)(OH2)] assists the hydrolysis of amino acid esters, amino acid amides, and peptides to form cis-fi (194) and cA-jS2-[Co(OA0(trien)] + (195)(( A = amino acid) complexes. Chiral alkylated trien ligands have the potential for chiral stereospecificity in such reactions. 

However, (DHQD)2PYR, the pseudo-enantiomer of (DHQ)2PYR (see chapter 25), would prefer to form two R chiral centres. The substrate and reagent are in competition here—the mismatched case. The selectivity is turned over to 75 25 in favour of the C2 symmetric product 91 this time. So the reagent wins but not outright there is still much of the meso diastereomer formed. Now we can return to the original question which concerns the reaction of triene 89. 

An early report of the stereospecific reaction between (/ )(-f )-tartaric or (l )(- -)-malic acid and [Co(C03)(phen)2]Cl at ambient temperature has been refuted and both A- and A-[Co (i )L (phen)2] ions (233) are formed. The distribution ratio A(J )/A(/ ) = 0.38 for the (i )-tartrate complex. Using [Co(Cl)2(phen)2]Cl at 60 °C and Na(- -)-tartrate the ratio is 0.70, but thermodynamic distributions remain unknown. A(Ji) and A(i ) diastereoisomers of both acids have recently been isolated and characterized by CD and 360 MHz H NMR. The reaction of meso-tartaric acid results in four diastereoisomeric possibilities (depending on which asymmetric carbon is in the chelate ring) and all four have been separated (J S)R. M S)R A K)S A R)S = 26 24 27 23). From these results it appears likely that the reported stereospecific reaction of [Co(C03)(en)2]Cl with (5)(—)-tartaric acid resulting in only A-[Co (S)tart (en)2]CP is in error. The reaction of sodium citrate with [Co(Cl)2(trien)]Q also leads to a preference for the five-membered chelate involving the hydroxyl group (234) as has recently been shown by a crystal structure of )3-[Co(cit)(trien)] 5H20. ... 

The dihydropentalene could arise by a 3,3-shift from the cis isomer to give the meso-cyc c bisallene which cyclizes to a biradical which is also a vinylcarbene that can insert into an adjacent CH bond. However, the trans isomer apparently must isomerize to the cis material, presumably via a biradical, before undergoing the 3,3-shift, and the biradical can also undergo cleavage to vinylacetylene. Also possible from the trans isomer is a 3,3-shift to a threo-cycXic bisallene which could undergo a conrotatory cyclization to the 4.2.0 triene that is formed to a greater extent in the pyrolysis of the trans isomer. 

So far, enantioselective olefin metathesis has the largest impact on organic synthesis in the desymmetrization of achiral polyenes [81] an illustrative example refers to the total synthesis of endo-brevicomin [82]. The catalytic asymmetric cycli-zation of achiral trienes and meso-tetraenes via ARCM proceeds in excellent enan-tioselection (e.e. > 99 %) as demonstrated for dihydrofuran 163 (Scheme 11.40). 

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