Frontier molecular orbital analysis

Fig. 3.26 Frontier molecular orbital analysis for the Simmons-Smith cyclopropanation. [Dargel, T.K. Koch, W.J. Chem. Soc., Perkin Trans. 1996, 2, 877. Reproduced by pemiission of The Royal Society of Chemistry]...

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

Table 2 Frontier Molecular Orbital Analysis of Intermolecular Diyl Trapping...

Frontier molecular orbital analysis of a [4 + 2] cycloaddition reaction. The HOMO of either of the reactants can be used with the LUMO of the other. Both situations require suprafacial overlap for bond formation. 

Frontier molecular orbital analysis of a [2 + 2] cycloaddition reaction under thermal and photochemical conditions. 

Frontier molecular orbital analysis validates the proposed transition state 133, where the predisposed arrangement of the enamine secures a favorable interaction between C-N c and n of alkene effectively lowering the LUMO energy of this conformation. ... 

The frontier molecular orbital analysis led to the conclusion that butadiene will react with ethylene to give cyclohexene if both butadiene and ethylene collide in a suprafacial manner. However, ethylene will not react with ethylene in an analogous manner. We can extend these conclusions to the reaction of any conjugated diene with any alkene and to any alkene with another alkene. Presented below are several examples and practical considerations of the allowed 4 + 2 reaction (the Diels-Alder reaction). The suprafacial approach of both reactants has important consequences on the stereochemistry of the Diels-Alder reaction. 

The 2 + 2 cycloaddition with one suprafacial and one antarafacial interaction is allowed. Show this conclusion via a frontier molecular orbital analysis. Although the reaction is allowed, it is seldom seen. Can you think of a reason not based upon an orbital analysis of why this reaction is difficult ... 

Although [2 + 2] and [4 + 4] cycloadditions bear a formal relationship to the Diels-Alder reaction, neither, in fact, takes place under the thermal conditions required for Diels-Alder reactions (see Section 20.4) because they are forbidden as determined by the frontier molecular orbital analysis. 

To derive the frontier molecular orbital analysis for any [3,3]-shift, we ll use 1,5-hexatriene as the model, just as we used butadiene and ethylene as models for the frontier molecular orbital analysis of all Diels-Alder reactions. As always, in a frontier molecular orbital analysis, we first identify a proposed geometry for the reaction. Let s propose a chairlike transition state in which the carbons on the ends of the chain react from the top of one ir bond and the bottom of the other. 

Frontier molecular orbital analysis shows that the reaction is allowed in a geometry that creates a chairlike transition state, although boatlike transition states can also occur. 

Write the frontier molecular orbital analysis for the cycloaddition of butadiene with butadiene when both interact in a suprafacial manner. Is this reaction allowed ... 

Write the frontier molecular orbital analysis for a [3,3]-sigmatropic shift in the analogous fashion as presented in the chapter except that you are using a geometry that would lead to a boatlike conformation for the transition state. As a hint, you should find that the reaction is allowed. However, why would this geometry be less favorable ... 

S.E. Obrien, H.L. Browne, T.D. Bradshaw, Antitumor benzothiazoles. Frontier molecular orbital analysis predicts bioactivation of 2-(4-aimnophenyl)benzothiazoles to reactive intermediates by cytochrome P4501Al, Org. Biomol. Chem. 1 (2003) 493-497. 

Suishu, T, Tsuru, S., Shimo, T, and Somekawa, K., Singlet and triplet photocycloaddition reactions of 2-pyridones with propenoate and 2,4-pentadienoates and the frontier molecular orbital analysis, J. Heterocycl. Chem., 34, 1005-1011,1997. 

Ab initio SCF MO calculations and frontier molecular orbital analysis (FMO) reveal a preference for an inversion mechanism for the topomerization of the cation over a torsional process. The calculations gave for planar inversion (i.e. changes from trans position to cis position towards without rotation around the N-N bond) a considerably lower barrier (123 kJ/mol) than for rotation around the N-N bond (309.8 kJ/mol) [16, 23] for earlier studies, see [24, 25]. 

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