Classical boron Lewis acids

The most commonly used traditional Lewis acids are halides of aluminum, boron, titanium, zinc, tin, and copper. However, there are also more complex Lewis-acids that are quite effective catalysts that can be easily modified for carring out enantioselective processes, by incorporating chiral ligands. These can overcome some limitations associated with the use of classical Lewis acids [47]. 

This electrophile/nucleophile dichotomy can be looked upon as a special case of the acid/base idea. The classical definition of acids and bases is that the former are proton donors, and the latter proton acceptors. This was made more general by Lewis, who defined acids as compounds prepared to accept electron pairs, and bases as substances that could provide such pairs. This would include a number of compounds not previously thought of as acids and bases, e.g. boron trifluoride (39),... 

The use of ethers as cocatalysts for the cationic polymerisation of alkenyl monomers induced by Lewis acids has received little systematic attention and the mechanism through which these compounds operate is not well understood. The complex diethyl-ether-boron fluoride has been extensively used as a very convenient cationic initiator, but mostly for preparative purposes. As in the case of alcohols and water, ethers are known to act as inhibitors or retarders in the cationic polymerisation of olefins, if used obove cocatalytic levels, because they are more nucleophilic than most rr-donor monomers. Imoto and Aoki showed that diethyl ether, tetrahydrofuran, -chloro-diethyl ether and diethyl thioether are inhibitors for the polymerisation of styrene-by the complex BF3 EtjO in benzene at 30 °C, at a concentration lower than that of the catalyst, but high enough (0.5 x 10 M) to quench the active species formation for a time. Their action was temporary in that the quenching reaction consumed them, and therefore induction periods were observed, but the DP s of the polystyrenes were independent of the presence of such compounds, as expected from a classical temporary inhibition. 

The 1,2-azoniaboratolidines 26 are internal amine-borane complexes. Analogous acyclic complexes have been studied since the classical work of H. C. Brown in the 1940s <1947JA1332>. Qualitative data suggest that many 1,2-azoniaboratolidines are in equilibrium with their open-chain isomers 130, as illustrated in Equation (5). The position of the equilibrium is a function of the Lewis acidity of the boron atom and steric hindrance about both boron... 

Boron trifluoride is a classic Lewis acid. Therefore, an accurate molecular orbital picture of BF3 should show, among other things, an orbital capable of acting as an electron pair acceptor. The VSEPR shape is a planar triangle, consistent with experimental observations. 

More recent research has uncovered some unusual attributes of the transition states for Lewis acid-catalyzed Diels-Alder reactions. Of note, a [4-t3] transition state has been postulated as the low energy pathway for the borane-catalyzed Diels-Alder reaction between acrolein and 1,3-butadiene [ 17] that is, a stabihzing interaction between the terminal carbon of the diene and the carbonyl carbon of the dienophile appears to be more important than the classically-invoked interaction between C-2 of the diene and the carbonyl carbon (Fig. 3). While this argument was originally advanced for only the endo s-trans transition state, it has been subsequently broadened in scope to include each of the four possible diastereomeric transition states for the boron trifluoride-promoted process [ 10] thus, the energy differences for these reaction pathways are determined by the strength and number of the secondary interactions. 

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

Boron-nitrogen and boron-phosphorous compounds are classical textbook examples of donor-acceptor complexes. The chemical bonds of the Lewis-acid Lewis-base complexes are usually explained in terms of frontier orbital interactions and/or quasiclassical electrostatic attraction in the framework of the Hard and Soft Acid and Base (HSAB) model [73]. We were interested in seeing if the differences between the bond strengths of boron-nitrogen and boron-phosphorous complexes for different boranes, amines and phosphanes can be explained with the EDA method. 

Does the presence of excessive steric bulk when a Lewis acid and a Lewis base attempt to form an adduct automatically render these species inert towards each other The unique behavior of sterically frustrated Lewis pairs (FLPs), pioneered by Stephan, is a vigorous research area with applications for small molecule activation and catalysis." The highly Lewis acidic and sterically bulky tris(pentafluorophenyl)borane" plays a role in many FLP reactions. The great promise of FLP chemistry was revealed by reactions between tris(pentafluorophenyl)borane and tertiary and secondary phosphines, where sterics preclude formation of classic adducts. A seminal example of a frustrated Lewis pair is that of the secondary phosphine di(2,4,6-trimethylphenyl)phosphine that is precluded from forming a classic adduct with tris(pentafluorophenyl)borane. The phosphine Lewis pair is frustrated since it cannot interact with boron to form the adduct. 

When a Br0nsted-Lowry acid such as acetic acid (18) is compared with a classical Lewis acid such as boron trifluoride (BFg, 61), it is important to look for differences as well as similarities. In acetic acid, the atom that accepts electron... 

Boron trifluoride (BFg) is a classic Lewis acid. Borane, BHg, is a highly reactive boron compound that also functions as a Lewis acid in the presence of a suitable electron-donating species. 

In the field of frustrated Lewis pair chemistry, organoaluminum compounds are often employed as the Lewis acid of the pair to great effect Just as aluminum is a substitute for boron in this case, carbenes are often substituted for the standard phosphines as mentioned in Section 15.5.1. Zhang et al. [137] used several frustrated Lewis pair combinations (A1(C6F5)3 with a variety of phosphines and carbenes) to facilitate the polymerization of methyl methacrylate, a-methylene-y-butyrolactone, and y-methyl-a-methylene-y-butyrolactone. Even combinations that form classical Lewis acid/base adducts rather than FLPs could still exhibit excellent polymerization activity. 

Non-classical compounds.- The boron-carbon monoxide molecule, BCO, prepared by the reaction of B atoms with CO, has been trapped in solid Ne matrices at 4 K, It has a ground electronic state of with a zero-field-splitting parameter D = 0.074 cm The bonding is almost that of a classic Lewis acid-base complex with the three spins distributed in essentially and sp, orbitals on B. Evidence exists for the formation of fullerenes in which a boron atom substitutes for carbon in the soccerball cage C59B and KC59B were reported... 

One of the differences between nonclassical and classical structures (as defined above) was the existence of vacancies on boron atoms (i.e., a trivalent boron with an empty or quasi-empty p-orbital). This property makes the clas.sical structure a better Lewis acid than the nonclassical structure. Thus, when a Lewis base coordinates to a boron hydride or carborane, often the classical structure is obtained. Table 6 indicates some of the boron hydrides and carboranes for which an adduct is known. 

Following the use of boron acids by Brown et al. (1953-1955) [91] in his classical work on steric effects in the complexation of amines with trimethylboron, an extensive set of calorimetric measurements was provided by the studies of BF3 complexes by Gal, Maria et al. (1970-1992) [80]. Boron trifluoride is the archetype of Lewis acids in the original Lewis definition. The promising BF3 affinity scale is presented in Chapter 3. 

Figure 4.15 Boron trifluoride is a classic Lewis acid. The hybridization of boron is sp and boron, therefore, also has a p-orbital that is vacant, and can accept a pair of electrons.

In the chemistry of polyhedral boron hydrides, boron-centered cations were postulated to be key intermediates of an electrophile-induced nucleophilic substitution mechanism that is responsible for the formation of a variety of boron-substituted derivatives [14], Such boron-centered cations can be easily generated by abstraction of a hydride by the treatment of polyhedral boron hydrides with Lewis or Bronsted acids [15], Similar to the classical chelate-restrained borinium cations based on 3-coordinate boron, these species, which we called quasi-borinium cations, have an unstabilized p orbital and are strong electrophiles (Scheme 6.1). Such quasi-borinium cations are highly reactive and react with even weak nucleophiles, such as ether or nitrile solvent molecules giving the corresponding oxonium and nitrilium derivatives whose properties are close to those of similar complexes of transition metals [15-17]. 

Lewis proposed his stiU broader and more useful definition of acids and bases in the late 1920s and early 1930s. Classifying acids as electron-pair acceptors and bases as electron-pair donors, he thereby liberated acid—base theory entirely from its former dependence on the presence of hydrogen. The advantage of the Lewis definition is that a larger number of reactions can be classified as acid-base than under either the Arrhenius or Bronsted-Lowry definitions. The classic example used to demonstrate the more general nature of the Lewis definition is the gas-phase reaction between boron trifluoride and ammonia, as represented in Equation (4.1) ... 

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