Bonding site selectivity

A cyclobutene ROCM sequence was also used in a synthesis of racemic sporochnol (410), a naturally occurring feeding deterrent toward herbivorous fish (Scheme 80) [170]. Exposing cyclobutene 406 (0.01 M in boiling 1,2-dichloroethane) in the presence of ethylene to second-generation catalyst C (8 mol%) led to 1,5-diene 407 in 73% yield, along with 9% of the homodimer derived from 407 by involving the less hindered double bond. Site-selective hy-... 

A related unprecedented double insertion of electron-deficient alkynes has also been reported in the reactions of the linear Pt2Pd heterotrimetallic complex 64 with 65 (RO2CCSCR) (Scheme 24) [95,96]. A series of unsymmetri-cal A-frame clusters 68 has thus been obtained in which a first insertion of the alkyne takes place site-selectively into the Pt-Pd bond vs the Pt-Pt bond (66). After a zwitter-ionic polar activation of the resulting inserted alkene (67), a subsequent reaction with the phosphine unit of the dpmp allows one to obtain the products 68 via the nucleophilic migration of the terminal P atom from the Pd center to the CH terminal carbon (formation of the P-C bond). 

The site selectivity in this reaction Is excellent -70 - 80% yield of (3) - and It depends on the stability of cyclic transition state (6). This looks at first like a six-membcrcd ring, but N—H—CII2 is linear and shorter than two bonds so that it is somewhere between a five and a six-membered ring.Cyclisation to (3) Is via a conventional five-membered cyclic transition state. Problem ... 

The ONIOM protein system contains the substrate, methylmalonyl-CoA, bound to the active site, the cofactor (AdoCbl) and all amino acids within a 15-A radius from the cobalt atom. The active-site selection contains a truncated AdoCbl and the imidazole ring of its lower ligand. The QM part was calculated using the BP86 functional [31, 72] because it gives better agreement with experimental Co—C bond energies [73, 74], This a different choice of functional compared to the other studies in the present review. 

When a larger excess of the azomethine ylide precursors was used and the reflux was prolonged for 40 hours, the reaction afforded isobacteriochlorin 63 as the major adduct together with a small amount of bacteriochlorin 64 (Figure 5). This result demonstrates that the attack of the second azomethine ylide species to the chlorin macrocycle occurs preferentially at the adjacent P-pyrrolic double bond, yielding the isobacteriochlorin with high site selectivity. 

Another convenient entry to fused cyclobutene-1,2-diesters was via site selective modification of the norbomene rt-bond in Smith s fe-alkene 49, e.g. treatment with 3,6-di(2 -pyridyl)-s-tetrazine 51 followed by DDQ oxidation afforded the cyclobutene-derivative 53 <97AA119>, while direct coupling with 3,5-f> (trifluoromethyl)-l,3,4-oxadiazo]e 54 furnished the tas(cyclobutene-l,2-diester) 55 (Scheme 6) <97SL196>. 

The intramolecular dipolar cycloaddition of a nitrone with an unactivated allene was also studied [76], Treatment of 5,6-heptadien-2-one with N-methylhydroxyl-amine in refluxing ethanol yielded allenyl nitrone 78, which cyclized with the terminal allenic C=C bond to give an unsaturated bicyclic isoxazolidine. On the other hand, the site selectivity decreased with an allenic ketone having a trimethylene tether. 

With 141, the allenic Jt-bond that is distal to the ester group added to furan spontaneously upon concentration [122]. The observed site-selective addition can be attributed to steric reasons. The transition state geometry 143 is destabilized by a repulsive non-bonding interaction between H-3 and the exo-methylene group. 

Monofluoroallene (24) underwent site-selective [4+ 2]-cycloadditions at the C2-C3 bond with some 1,3-dienes, although with little face selectivities [21,129]. 

Dienophiles may contain more than one double or triple bond. This might result in multiple product formation, but in most instances the diene will attack one bond with high site specificity. This site selectivity is often controlled by substitution patterns and electronic or steric parameters. 

Marchand and coworkers102 reported a difference in site selectivity between the thermodynamically and kinetically controlled Diels-Alder reactions of cyclopentadiene with 2,3-dicyano-p-benzoquinone (126) (equation 37). Under kinetic conditions, the more reactive double bond of 126 reacted with cyclopentadiene affording 127, whereas the less substituted double bond reacted under thermodynamic conditions affording 128. Both reactions proceeded with complete endo selectivity. These findings were in agreement with ab initio HF/3-21G calculations. 

In general, 2,3-dialkyl-p-benzoquinones exhibit site selectivity in that they tend to give predominantly Diels-Alder adducts resulting from diene attack on the external rather than the internal double bond. This external site selectivity is, however, dramatically reversed when a (substituted) cyclobutane ring is fused to p-benzoquinone. Paddon-Row and coworkers103 studied the reactions of p-benzoquinones such as 129 with several... 

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