Haptotropic

Structural analysis of the reaction products clearly shows that the ligand on silicon undergoes a haptotropic rearrangement from if -C5MC5 in 50 to in1 -C5MC5 in 52 and 53 as in the cases of oxygenation and thionation of 50.36 Apparently, silaneselone 51 is not kinetically stable enough to be isolated under normal conditions. 

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

Finally, theoretical studies of haptotropic rearrangements of polyene-MLn complexes were reported together with detailed literature surveys144,145. 

This approach has been discussed previously for the cyclopenta-dlenyl ring and has been referred to as a haptotropic search (11,6). The geometry definitions were as follows ... 

Flve-Membered Ring Systems. In terms of haptotropic searches. 

The following perhapto structures emerge from haptotropic searches (see also Table III). 

Jemmis and Schleyer (3a) have found previously that Li, LIH, and Beir " are capable of binding in the hexahapto fashion to a benzene ring. On the other hand, no minimum could be discerned for hexahapto binding of Be to GgHe utilizing the ST0-3G basis set. Our MNDO haptotropic search indicates that prefers to... 

In none of the known tricyclic carbanions is the metal haptotropic tt-coordinated on top of the ring. Obviously, the ring is too small to suit even the small lithium cation. 

Chromium-Templated Benzannulation and Haptotropic Metal Migration... 

Such an experiment is not yet possible for the OFCOT analogues, leaving us with the rather unsatisfactory qualitative conclusion that the activation free energy for haptotropic shifts appears to be higher for these complexes than for 57b. The quantitative effect of perfluorination on the magnitude of AG and any effect of perfluorination on the mechanism of any such shifts remain a mystery. 

The formal oxidation state of silicon changes from +11 to +IV in nearly all of the reactions with decamethylsilicocene (82) described so far, and the hapticity of the cyclopentadienyl ligand changes from /j5 to jj1. The latter phenomenon raises the question about the possibility of a haptotropic shift in the ground state molecule, which should lead to quite different silicon species in equilibrium, as shown in equation 81. 

Moreover, after transfer the free ligand can be recovered by crystallization. The mechanistic proposal for the transfer reaction is based on an initial r 4 to p1 haptotropic migration (Scheme 1.22). 

A special application of the haptotropic rearrangement is the reaction of diastereopure complex 59a (Section 8.3.4, Scheme 23). Upon warming to 90 °C in di-n-butyl ether, haptotropic migration of the chromium tripod occurs intramolecularly along the same face of the naphthalene system to produce pure diastereomer 59c (Scheme 25) [57d]. 

Scheme 25. Stereoselective haptotropic rearrangement in naphthalene tricarbonylchromium complexes.

Scheme 30. Synthesis and haptotropic rearrangement of meta-benzonaphthohydroquinonophane tricarbonylchromium complexes.

No such complex of this type has been isolated to date. In fact, attempts to photolyze the well-characterized Re( -C7H7)(CO)s in order to induce haptotropic jj ring slippage leads to the dimer [Re(jj -C7H7)(CO)2]2 Similarly,... 

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