Lewis acid lowest unoccupied molecular

In 1960, Yates and Eaton (192) demonstrated that Lewis acids can dramatically accelerate the Diels-Alder reaction. In principle, any transformation wherein coordination of a Lewis acid may reduce the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a given set of reactants should be susceptible to Lewis acid catalysis. Indeed, numerous important carbon-carbon bond-forming organic transformations have been shown to be amenable to rate acceleration by a Lewis acid. In many cases, the use... 

Lewis-acid catalysis of Diels-Alder reactions (Figure 7.5) in organic solvents leads to an enhancement of the reaction rate, because of the lowering in energy for the lowest unoccupied molecular orbital (LUMO) of the dienophile, and an improvement in the selectivity with specific ligands. 

The Catalysis Concept of Iminium Activation In 2000, the MacMillan laboratory disclosed a new strategy for asymmetric synthesis based on the capacity of chiral amines to function as enantioselective catalysts for a range of transformations that traditionally use Lewis acids. This catalytic concept was founded on the mechanistic postulate that the reversible formation of iminium ions from a,p-unsaturated aldehydes and amines [Eq. (11.10)] might emulate the equilibrium dynamics and 7i-orbital electronics that are inherent to Lewis acid catalysis [i.e., lowest unoccupied molecular orbital (LUMO)-lowering activation] [Eq. (11.9)] ... 

Klemperer and co-workers. 31) In this model the hydrogen bond is viewed as an electron donor-acceptor complex in which a pair of electrons from the highest occupied molecular orbital of the Lewis base is donated to the lowest unoccupied molecular orbital of the Lewis acid. If the donor electron pair is assumed to have the appropriate hybridization, and the acceptor orbital to be axially symmetric, the above structures can be rationalized as giving maximal overlap between the HOMO and LUMO. 

In certain cases, the reduction can take place without electrophilic catalysis (n-BU4NBH4 or phase-transfer conditions), but most frequently it requires the coordination of the carbonyl group by a Lewis acid before nucleophilic attack [S2]. The Lewis acid may be the cation associated with the reagent, an added acid, or even the boron or aluminum atom of tricoordinate reagents (AIH3, DIBAH, boranes). The importance of this phenomenon has been shown by the introduction of coordinating macrocyclic molecules into solutions of LAH and LiBH4. This considerably retards the reduction of carbonyl compounds in an ether medium [DCl, HPl], Electrophilic catalysis is more important when the lowest unoccupied molecular orbital (LUMO)... 

Lewis acid interacts by the lowest unoccupied molecular orbital (LUMO), Lewis base interacts via the highest occupied molecular orbital (HOMO). (See Chapter 6 for more details about HOMO/LUMO.)... 

The term species may mean a discrete molecule, a simple or complex ion or even a solid exhibiting non-molecularity in one or more dimensions (graphite as an example). Free atoms seldom act as Lewis acids and bases. They usually have one or more unpaired electrons and flieir reactions are more accurately classified as free radical. The donor orbital is usually die highest occupied molecular orbital HOMO, and the acceptor orbital is usually the lowest unoccupied molecular orbital or LUMO. The molecular orbital definitions have a number of important consequences ... 

In the case of TMP physisorbed on weakly acidic hydroxyl groups residing on the surfaces of solid acids (Fig. 2.IB), the formation of hydrogen-bonded complexes prevails, leading to P resonance with 5 P at ca. —60 ppm (Fig. 2.2). On the other hand, for TMP chemisorbed on Lewis acid sites (Fig. 2.1A), the lowest unoccupied molecular orbital of the Lewis acid centre (M) tends to share electron pairs with the phosphoms (P) atom on the TMP, and the formation of such M—P chemical bonds typically gives rise to P signals in a considerable range of 8 P within ca. —20 to —60 ppm... 

The DA reaction is controlled by the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the reactants (Fig. 13). The rate of the reaction can be significantly enhanced by using Lewis acid catalysis. An attractive feature of the DA reaction to biomolecule labeling is the observed rate enhancement in polar solvents, including water [71, 72]. The DA reaction fulfills the criteria of click reaction, and therefore plays a key role in modern bioorthogonal chemistry. 

Nature of the Activation Effect One of the principal questions that may be interpreted with the help of theoretical methods is the reasons for the activation of nitriles toward DCA upon their coordination to a metal center. Traditionally, the reactivity of dipoles and dipolarophiles in the DCA reactions is explained in terms of the frontier molecular orbital (FMO) theory and depends on the predominant type of the FMO interaction. The coupling of nitrones with nitriles is usually controlled by the interaction of the highest occupied molecular orbital (HOMO) of nitrone and the lowest unoccupied molecular orbital (LUMO) of nitrile centered on the C N bond (so-called normal electron demand reactions). For such processes, the coordination of N CR to a Lewis acid (e.g., to a metal) decreases the LUMOncr energy, providing a smaller HOMOjii -one - LUMOncr and, hence, facilitates the DCA reaction (Fig. 13.1a). 

Reactions such as these that involve polar molecules are best understood in terms of Highest Occupied Molecular Orbital—Lowest Unoccupied Molecular Orbital (HOMO-LUMO) orbital interactions. As we saw in Section 1.7, p. 41, when a filled occupied orbital overlaps an empty orbital, the two electrons are stabilized in the new, lower energy molecular orbital. The words Lewis bases react with Lewis acids are essentially equivalent to saying, The interaction of a filled and empty orbital is stabilizing. Indeed, this notion is one of the central unifying themes of organic reactivity, as essentially all reactions involving polar molecules can be understood this way. 

By definition, Lewis acid catalysts involve a metal center as an electron pair acceptor that accepts the electron pair from a nucleophile. This property makes them effective in many organic reactions and indispensable for the production of a large category of chemicals from simple alkylated compounds to complicated polymers or pharmaceuticals [6]. Also, the differences between the energy levels of the highest occupied molecular orbital (HOMO) of the reactant (nucleophile) and the lowest unoccupied molecular orbital (LUMO) of the Lewis acid (electrophile) make Lewis acid catalysis more complicated than the corresponding Br0nsted catalysis [7]. 

Metal Lewis acid catalyzed reaction is the most studied area in modern carbonyl chemistry [26]. The lone pair of carbonyl oxygen coordinates to the Lewis acid. The coordination lowers both the electron density of carbonyl oxygen and the energy of the lowest unoccupied molecular orbital (LUMO), the C=0 ti orbital, activating carbonyl towards nucleophilic attack. In general, hard and small Lewis acid is ideal in carbonyl activation because the lone pair of carbonyl oxygen is hard. Thus proton, which is the smallest and hardest Lewis acid, is one of the best for carbonyl activation [27]. 

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