Boron empty orbital

This solvent is called tetrahydrofuran, or THF for short. Even though it somewhat stabilizes the empty p orbital on the boron atom in BH3, nevertheless the boron atom is very eager to look for any other sources of electron density that it can find. It is an electrophile—it is scavenging for sites of high electron density to fill its empty orbital. A pi bond is a site of high electron density, and therefore, a pi bond can attack borane. In fact, this is the hrst step of our mechanism. A pi bond attacks the empty p orbital of boron, which triggers a simultaneous hydride shift ... 

Boron-based Lewis acids are often used in organic syntheses. Since the boron atom has an empty / -orbital, many boron compounds can function as Lewis acids. Typical boron Lewis acids are boron trihalides, for which Lewis acidity increases according to the order of fluoride < chloride < bromide < iodide, the reason for this order being the relative abilities of the different halogens to act as 7r-donors to boron. 

However, these compounds proved to be unstable and difficult to characterize. The authors reasoned that the source of the instability was likely to be the trialkylboron moiety. Boronates are more stable than trialkylboranes since the lone-pairs of electrons on an oxygen atom can donate to the empty orbital of a boron atom. The corresponding gem-boriozirconocenes should also be more stable. Thus, hydrozirconation of the alkenyl-... 

The characteristics of the vinylboranes as dienophiles can be rationalized in terms of a strong interaction of the diene with the empty n orbital at boron. Molecular orbital calculations show a strong interaction between B and C-l in the transition state, and the transition state shows little charge separation, accounting for the relative insensitivity to substituent effects. As for regiochemistry, the para-like selectivity would also be expected to be reduced because the LUMO of the dienophile is nearly equally distributed between B and C-2.36... 

Evidence for the reverse process, donation of electron density from the nucleophilic dimer atom to an electron-deficient molecule, also exists. Konecny and Doren theoretically found that borane (BH3) will dissociatively adsorb on Si(100)-2x1 [293]. While much of the reaction is barrierless, they note an interaction between the boron atom and the nucleophilic atom of the Si dimer during the dissociation process. Cao and Hamers have demonstrated experimentally that the electron density of the nucleophilic dimer atom can be donated to the empty orbital of boron trifluoride (BF3) [278]. XPS on a clean Si(100)-2 x 1 surface at 190 indicates that BF3 dissociates into BF2(a) and F(a) species. However, when BF3 is exposed on a Si(100)-2 x 1 surface previously covered with a saturation dose of trimethylamine, little B-F dissociation occurs, as evidenced by the photoelectron spectrum. They conclude that BF3 molecularly adsorbs to the nucleophilic dimer atom and DFT calculations indicate that the most energetically favorable product is a surface-mediated donor-acceptor complex (trimethylamine-Si-Si-BF3) as shown in Figure 5.19. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene s pi orbital. 

This theory explains why BF3 reacts instantaneously with NH3. The nonbonding electrons on the nitrogen in ammonia are donated into an empty orbital on the boron to form a new covalent bond, as shown in the figure below. 

Complex B can be isolated, characterized and stored. The B atom of the complex B of the Corey-Itsuno reduction (Figure 10.26) is a Lewis acid. Unlike heterocycle A, its boron atom does not possess a neighboring lone pair of electrons and is adjacent to an atom with a formal positive charge. Therefore, the empty orbital is available for binding a carbonyl group to form the ternary complex. 

Cyclic boric acid esters derived from triethanolamine (Figure 9.11) or diethanolamine can be stabilized toward hydrolysis by an intramolecular, boron-nitrogen coordination bonding. The blockage of the empty-orbital on the boron atom can alleviate hydrolysis. This effect has been used to prepare... 

P orbital is not needed and contains no electrons. Do not be tempted by the alternative structure with tetrahedral boron and an empty sp3 orbital. You want to populate the lowest energy orbitals for greatest stability and sp2 orbitals with their greater s character are lower in energy than sp3 orbitals. Another way to put this is that, if you have to have an empty orbital, it is better to have it of the highest possible energy since it has no electrons in it and doesn t affect the stability of the molecule. 

But we have not told you the whole story about BF3. Boron is in group 3 and thus has only six electrons around it in its trivalent compounds. A molecule of BF3 is planar with an empty p orbital. This is the reverse of a lone pair. An empty orbital on an atom does not repel electron-rich areas of other molecules and so the oxygen atom of acetone is attracted electrostatically to the partial positive charge and one of the lone pairs on oxygen can form a bonding interaction with the empty orbital. We shall develop these ideas in the next section. 

More often, reaction occurs when electrons are transferred from a lone pair to an empty orbital as in the reaction between an amine and BF3. The amine is the nucleophile because of the lone pair of electrons on nitrogen and BF3 is the electrophile because of the empty p orbital on boron. 

There is a nasty trap when a charged atom has its full complement of electrons. Since BH4 and NH4 are isoelec Ironic with methane and have four o bonds and hence eight electrons, 110 new bonds can be made to B or N. The following attractive mechanisms are impossible because boron has no lone pair in BHJ and nitrogen has no empty orbital in NH4. 

You will notice that the boron atom always adds to the end of the alkene. This is just as well otherwise, three sequential additions would give rise to a complex mixture of products. The boron always becomes attached to the carbon of the double bond that is less substituted. This is what we should expect if the filled % orbital of the alkene adds to the empty orbital of the borane to give the more stable cationic intermediate. 

The oxidation occurs by nucleophilic attack of the hydroperoxide ion on the empty orbital of the boron atom followed by a migration of the alkyl chain from boron to oxygen. Do not be alarmed by hydroxide ion as leaving group. It is, of course, a bad leaving group but a very weak bond—the 0-0 <5 bond—is being broken. Finally, hydroxide attacks the now neutral boron to cleave the B-O-alkyl bond and release the alcohol. 

Of particular sigiuficance in the chemistry of organoboranes is their behavior as Lewis acids, which is a result of the empty / -orbital of tricoordinate boron. Much recent effort has been devoted to the synthesis and development of highly Lewis-acidic organoboron compounds through introduction of fluorinated organic substituents. Boron can reach the... 

In a different stndy, a series of snbstitnted 3-borabicyclo[3.3.1]nonanes (27) have been prepared and investigated by X-ray crystallography. Intramolecnlar interactions between the empty / -orbital on boron and the snbstitnent R were shown to play an important role in the stabilization of the chair-like conformation in these species. ... 

Boronates are more stable since they are stabilized owing to the donor effect of oxygen lone-pairs to the empty orbital of the boron. The two different carbon-metal bonds afford particnlar reactivity. For example, addition of propargylbromide on (94) in presence of a catalytic amount of copper cyanide nndergoes a carbon-carbon bond formation with exclusive cleavage of the C-Zr bond. The subsequent borylallene, by treatment with a ,/3-unsaturated aldehydes, affords two trienes, depending on the reaction conditions (Scheme 20). 

The most important interaction in hydroboration is between the HOMO of the alkene (here i diene) and the empty orbital (LUMO) on boron (p. 1279). Here the ends of the diene have th larger coefficients. The reaction occurs on the opposite face of the diene to the one occupied by tbt substituent, especially as it must occur next to that substituent. In the oxidation, the less substitute group migrates with retention. 

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