Allyl systems

Let us follow the first few iterations for the allyl system by hand calculations. We subtract the matrix xl from the HMO matrix to obtain the matrix we wish to diagonalize, just as we did with ethylene. With the rotation block in the upper left conrer of the R matrix (we are attacking an and aai), we wish to find... 

Draw bond order and free valency index diagrams for the butadienyl system. Write a counter into program MOBAS to detemiine how many iterations are executed in solving for the allyl system. The number is not the same for all computers or operating systems. Change the convergence criterion (statement 300) to several different values and determine the number of iterations for each. 

That no degenerate molecular orbitals arose in the above examples is a result of the fact that the C2v point group to which H2O and the allyl system belong (and certainly the... 

Interestingly, some nucleophiles attack the central carbon of the 7r-allyl system to form a palladacyclobutane 316 and its reductive elimination gives... 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

No 0-allylation is observed in formation of the six-membered ring compound 79 by intramolecular allylation of the /3-keto ester 78(15,57]. Intramolecular allylation is useful for lactone fonnation. On the other hand, exclusive formation of the eight-membered ring lactone 81 from 80 may be in part derived from the preference for the nucleophile to attack the less substituted terminus of the allyl system[58]. 

The formation of 1-and 2-aIkenes can be understood by the following mechanism. In the presence of formate anion, the 7r-allylpalladium complex 572 is converted into the 7r-allylpalladium formate 573. The most interesting feature is the attack of the hydride from formate to the more substituted side of the (T-allylic system by the cyclic mechanism shown by 574 to form the 1-alkene 575[367]. The decarboxylation and hydride transfer should be a concerted... 

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

Desulfonylation of equally substituted allylic sulfones with NaBH4 and LiBHEt3 usually yields a mixture of regioisomeric alkenes[406,407]. However, the regioselective attack of the less substituted side of the unsymme-trically substituted allylic system with LiEtjBH has been utilized for the removal of the allylic sulfone group in synthesis of the polyprenoid 658[408],... 

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

Reactions of allylic systems that yield products m which double bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift... 

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