Triarylboranes

Organomercury compounds undergo a similar reaction. Alkyl and aryl Grignard reagents can be converted to carboxylic esters with Fe(CO)5 instead of CO. Amides have been prepared by the treatment of trialkyl or triarylboranes with CO and an imine, in the presence of catalytic amounts of cobalt carbonyl ... 

In a series of remarkable papers, K. Tamao and S. Yamaguchi have shown that the physical properties of triarylboranes and triarylsilanes can be modified using structural changes induced by increasing coordination numbers [74]. In the same way, the tuning of the photophysical properties of PAr3 systems (96-99) (Scheme 27) according to the coordination number of the P-atom has been achieved [75]. 

Such 7r-bonding is evident, however, in the ground state of triarylborane radical-anions. The Na+ or K+ salt of [Ph3B] (43) has the free spin densities shown in 43a. Since fully 75% of the free spin is delocalized onto the phenyl groups, B—C ir-overlap as expressed by structure 43b is undoubtedly significant.40... 

An interesting cyclization is brought about in the two-step synthesis of compound 140 from the tribromide 141 (Scheme 56). The resolution of these complexes represents one of the first examples of optical resolution of triarylboranes. The crystal stmcture of 140b (R = Me, R = H) has been determined B-C distances are in the range 155.0-157.4 pm and the environment about the boron atom is close to trigonal-planar <1991TL6363>. 

Precedents for these reactions include insertion of a CHC1 group from (dichloromethyl)-lithium into the triarylborane C - B bond6 7, the preparation and reactions of achiral (dichloro-methyl)boronic esterss, and reactions of achiral boronic esters with (dichloromethyl)lithium9,10. 

The base-induced reaction of dichloromethyl methyl ether with trialkyl- and triarylboranes also provides a powerful method for the preparation of the corresponding tertiary carbinols.1617 fn this case, all three groups transfer readily from horon to carbon under mild conditions, and oxidation with alkaline hydrogen peroxide provides the tertiary alcohol ... 

Inherently, the orf/zo-phenylene spacer induces only a weak energy difference between the closed and open forms, but the ground-state structure of all MPB 7b-d was found to be well-defined, with or without P-B interaction. A different situation was observed for the related DPB 8a and TPB 9.24 The corresponding nB NMR signals (<5 = 43.1 ppm for 8a and 50.1 ppm for 9) are in between those of triarylboranes ( 70 ppm) and tetracoordinate derivatives thereof ( 0 ppm), suggesting a rapid equilibrium in solution between the open and closed forms (Figure 12). This hypothesis was further corroborated by low-temperature NMR experiments and DFT calculations. The minima associated with the open and closed forms were found to be almost isoenergetic (AG < 3 kcal/mol at 25 °C). 

Obviously, all tris-chelates whose stereochemistry is analyzable in terms of the above model are stereochemically correspondent similarly, compounds of the type ArsZ, such as triarylboranes and triarylcarbenium ions are also stereochemically correspondent. However, the more interesting correspondence is that between the class of tris-chelates and the class of AraZ molecules. Although these two classes of molecules differ enormously in their chemical properties and reactions, the concept of stereochemical correspondence tells us that their stereochemical attributes are necessarily closely related. For example, a tris-chelate and a stereochemically correspondent triarylborane will each have the same number and kinds of stereoisomers, interconvertible by the same number and kinds of rearrangement pathways. 

Selected Studies of Three-bladed Propellers 4.1. Triarylboranes... 

The stereochemistry of triarylboranes provides a typical example of isomerism and isomerization of molecular propellers with idealized... 

D3 symmetry. 32> This conformation is adopted by trimesitylborane (1 g) in the solid state, and the low-temperature H-nmr spectra of divers triarylboranes which have substituents other than hydrogen in all six ortho positions are consistent with this geometry in solution. At higher temperatures, the spectra of such molecules reflect rapid stereoisomerizations which are interpreted in terms of flip mechanisms. 

Variable temperature xH-nmr studies of other hindered (in both ortho positions) triarylboranes have revealed 32> that these compounds are also mixtures of propeller-like stereoisomers on the nmr time scale at low temperatures, and that interconversion of these stereoisomers occurs with activation energies of ca. 14—16 kcal/mol (Table 3). These isomerizations are also interpretable in terms of two-ring flip mechanisms. 

The stereochemically correspondent triarylcarbenium ions show similar behavior, and isomerizations in these systems can also be explained in terms of two-ring flips. 9>33> However, carbenium ions which have only hydrogen atoms in the ortho positions have activation energies of ca. 10—14 kcal/mol 34> (Table 3). It follows that in systems with comparable substituent patterns, i.e., in systems whose ortho ligands offer comparable steric encumbrance, triarylcarbenium ions have substantially higher barriers than the corresponding triarylboranes. No... 

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

The triarylborane-porphyrin conjugate receptor 45 [40] can bind a F ion concurrent with the change from sp2 to sp3 hybridization of the boron atom. The change in the hybridization interrupts both the electronic communication and the dipolar interaction between the triarylborane and porphyrin unit as well as the internal charge transfer in the presence of F". Bathochromic shifts... 

A new synthetic approach to fluorescent 4-amino-4 -boryl biaryls by a boronate selective Suzuki-coupling of / -(dimesitylboryl)phenylboronates with haloarenes under the employed reaction conditions has been reported. The triarylboryl unity is not attacked whereas non-sterically hindered triarylboranes like tri-l-naphthylborane gave coupling products in good yields 207.120... 

Arylboranes conjugated with an aromatic chain have been extensively studied due to their unique property as photonic and electronic molecular materials (25,49 2650,51 Scheme 3). In contrast to small triarylboranes, sterically hindered boranes containing at least two mesityl groups are stable to air and hydrolytic B-G bond cleavage. 

Aryl boron compounds, Ar,B(OH)3 (n = 1-3), bearing electron-withdrawing aromatic groups such as triarylboranes,... 

Cyclic voltammetry studies on 19 and 21 gave single reduction waves at —1.70 and —1.66 V, respectively, while two oxidation waves were observed for each at 1.14 versus 0.90 for the first wave and 1.56 versus 1.25 for the second (vs. SCE) <1994T6495>. These redox potentials were comparable to those of both triarylboranes and thiophenes. 

The reduction of PhsB is known to result in a radical anion PhsB- that is in equilibrium with the dimeric species [PhsB BPhs] ". However, if bulky substituents as in MessB are present and thus prevent attack of nucleophiles or dimerization, a blue-colored stable radical anion can readily be identified. While arylboranes are also known to form purple-colored dianions, the second reduction of simple triarylboranes is typically irreversible. In a recent study, Okada and Oda reported the formation of purple-colored solutions of dimesifylphenylborane dianions [Mes2BAr] (146 Ar = Ph, d-MesSiCeUi, biphenyl) upon extended reaction of Mes2BAr with Na K alloy in THE (Scheme 21). Formation of dianions was confirmed by multinuclear NMR specfroscopy. While the bulky mesityl groups provide chemical sfabihty to the dianions and thus prevent further chemical reactions, the presence of the unsubstituted phenyl group is beheved to allow for effective tt-interactions with the / -orbital on boron. 

Metal hydrides form addition compounds with triorganoboranes ", which are useful reagents e.g., LiH and NaH react with triorganoboranes to afford the species Li [HBRj]. However, long reaction times or reflux conditions are required, and with hindered triorganoboranes the reactions are slow. On the other hand, KH reacts quantitatively with trialkyl or triarylboranes in tetrahydrofuran (THF) at 25 C ... 

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