Methyl acetate, LUMO

One way to investigate the electrophilic properties of these molecules is to examine the orbital that each uses to accept electrons from a nucleophile. This orbital is the lowest-unoccupied molecular orbital (LUMO). Examine the LUMO for methyl acetate (Z=OCH3), acetaldehyde (Z=H), N,N-dimethylacetamide (Z=N(CH3)2) and acetyl chloride (Z=C1) (acetaldehyde is not a carboxylic acid derivative, but is included here for comparison). What is the shape of the LUMO in the region of the carbonyl group Is it a o or 7U orbital Is it bonding or antibonding What other atoms contribute to the LUMO Which bonds, if any, would be weakened when a nucleophile transfers its electrons into the LUMO ... 

A molecule with a low energy LUMO can accept electrons more readily than a molecule with a higher energy LUMO. The LUMO energies (in au) for the above molecules ai e 0.192 (methyl acetate), 0.161 (acetaldehyde), 0.212 (N,N-dimethylacetamide), and 0.132 (acetyl chloride). Order these molecules from most electrophilic to least electrophilic. 

LUMO of methyl acetate reveals the likely site of nucleophilic attack. 

LUMO for protonated methyl acetate reveals likely site of attack by water. 

LUMO (acetone) = a-0.878p LUMO (acetic acid)= a— 0.917ft LUMO(methyle acetate) = a— 0.925/ ... 

It is noteworthy that the LUMO of a Fischer type carbene resembles the LUMO (determined using semi-empirical MO theory) of a typical ester such as methyl acetate (Figure 10-9a) or a ketone such as acetone (Figure 10-9b). In fact, the analogy between Fischer carbenes and carbonyl compounds should be useful to readers already familiar with organic chemistry. Indeed, the chemistry of these two types of compounds, seemingly so different in structure, is similar in several ways. 

Whereas 260 does not react with electron-rich dipolarophiles, the more delocalized isomiinchnone 261 does react with both electron-rich and -deficient dipolarophiles (154). A detailed FMO analysis is consistent with these observations and with the regiochemistry exhibited by diethyl ketene acetal and methyl vinyl ketone as shown in Scheme 10.36. The reaction of 261 with the ketene acetal to give 262 is LUMO-dipole HOMO-dipolarophile controlled (so-called lype III process). In contrast, the reaction of 261 with methyl vinyl ketone to give 263 is HOMO-dipole LUMO-dipolarophile controlled (so-called lype I process). In competition experiments using a mixture of A-phenylmaleimide and ketene acetal only a cycloadduct from the former was isolated. This result is consistent with a smaller energy gap for... 

The LUMOdiene HOMOalkene interaction for butadiene and methyl vinyl ether is lower than the comparable HOMOdiene-LUMOaikene interaction in dienes bearing an electron-withdrawing group. In many cases, but not all, this leads to the inverse electron demand reaction mentioned above. Fleming uses the example of azonia salt 86, which reacted with diethyl ketene acetal to give 87. Similar reaction with allyl alcohol gave 88, and acrylonitrile gave 89.97a in all cases, the reaction proceeded to 75% completion, and it is clear from the relative rates of reactions provided with each transformation that the electron-rich alkenes reacted faster. The concept of inverse electron demand applies to a Diels-Alder reaction that is controlled by the LUMO of the diene and the HOMO of the alkene, which usually means that an electron-rich alkene reacts faster than an electron-poor alkene, the opposite of what is normally observed (see above). 

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