Boron heterocycles Lewis acidity

Bromodimethylborane is also effective for the cleavage of cyclic ethers. Epoxides react at —78 °C while the analogous four- to seven-membered ring heterocycles react between 0 °C and rt. In contrast to other boron-containing Lewis acids, Me2BBr reacts via a predominantly Sn2 mechanism. Tetrahydrofuran derivatives which are substituted at the 2-position give rise to primary bromides as the major or exclusive products. The nature of the... 

Inverting the orientation of the C4-N3 imine unit of a 2,3,1-diheterabotine gives a boron heterocycle with a markedly different chemical reactivity. In effect, the weakly basic oxime- or hydrazone-type imine nitrogen in the 2,3,1-diheteraborine is replaced by a much more basic imidate- or amidine-type imine nitrogen in the 2,4,1-diheteraborine. Likely, the Lewis acid tendency of the boron is enhanced by the ready protonation of this basic N4, and the formation of a stable borate-based zwitterion becomes thermodynamically favored. 

Cationic mechanisms are much more characteristic of the polymerization of oxygen heterocycles, both ethers and acetals. A wide variety of catalysts has been used, including protonic acids, such Lewis acids as boron trifluoride, phosphorus pentafluoride, stannic chloride, antimony pentachloride, titanium tetrachloride, zinc chloride, and ferric chloride, and salts of carbocations or tri-alkyloxonium ions having anions derived from Lewis acids. Some complex, coordination catalysts that appear to operate by a mechanism... 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

Silyloxy-2-aldehydes can undergo hetero-Diels-Alder reaction with aldehydes to give useful heterocycles.350 A model reaction, H2C=C(OSiH3)-N=CH2 with formaldehyde, has been explored theoretically. Lewis acids such as boron trifluoride catalyse the reaction by coordinating to the aldehyde oxygen, making the aldehyde more electrophilic. Concerted and stepwise mechanisms for this process are considered. 

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. 

The N atom of the ketone-associated heterocycle B of the Corey-Itsuno reduction (Figure 8.21) is a Lewis base Other than in the ketone-free heterocycle A, its lone electron pair is not shared with the boron atom because the latter has already acquired a valence electron octet. Therefore, the electron pair is available for binding a molecule of the Lewis acid BH3, whereby the ternary complex C is produced. 

The formation of heterocycles 25-35, which are internal Lewis base-borane complexes, is dependent on the borane substituents. An increase in the Lewis acidity of the borane can result in ring formation. For example, bis(methoxy)borane 101 has no B-P bond, but treatment with boron trichloride results in the formation of 1,2-phosphoniaboratole 156, presumably via 155 (Scheme 12) <1997ZFA1093>. 

Boron-containing Heterocycles.—The enthalpies of adduct formation between pyridine, 2-picoline, 4-picoline, or 2,4,6-collidine and a number of heterocyclic boron derivatives (2-Br- or 2-organo-l,3,2-dihetero-borolans, -borinans, and -boroles) have been measured.282 The presence of Br greatly increases the Lewis acidity of the B atom. 

Both enantiomeric (R)- and (S)-oxazaborolidines are available so it is possible to obtain at will either enantiomeric alcohol. The selectivity depends upon the geometry of the complex formed by coordination of the carbonyl oxygen to the Lewis acidic heterocyclic boron atom, complex 3.72 being favored. The catalytic cycle is shown in Figure 3.25. 

Another interesting and synthetically useful behavior of aziridines is their tendency to open thermally to azomethine ylides, a process which can also be facilitated by Lewis acid catalysts. These reactive intermediates can be trapped by a variety of dipolarophiles to give new heterocyclic species. Methyl vinyl ethers convert aziridines such as 172 into a pyrrolidine derivative (i.e., 174) in the presence of a zinc(salen) Lewis acid catalyst 171 <04JA2294>. Similarly, nitriles (e.g., 175) lead to the formation of 2,4-disubstituted 177-imidazolines (e.g., 176) under the catalysis of boron triethyloxonium tetrafluoroborate <04TL 1137>. Under almost... 

Indoles react with epoxides and aziridines in the presence of Lewis acids (see 20.4.1 for reaction of indolyl anions with such reactants) with opening of the three-membered ring and consequent 3-(2-hydroxyethylation) and 3-(2-aminoethylation) of the heterocycle. Both ytterbium triflate and phenylboronic acid are good catalysts for reaction with epoxides under high pressure silica gel is also an effective catalyst, but reactions are slow at normal pressure and temperature. Reaction with aziridines can be catalysed by zinc triflate or boron trifluoride. °... 

Comparable acid- (or Lewis acid) -catalysed ring closures of 2-arylthio- and 2-aryloxy- -ketones, and -2-arylthio- and 2-aryloxyacetyl- chlorides lead to 3-substituted heterocycles and 3-oxygenated heterocycles, respectively. It is possible to combine the preparation of the arylthio-ketone and the ring closure steps utilising two solid-supported reagents in a one-pot procedure, as illustrated. Formation of 3-aryl-benzothiophenes by this route can be complicated by partial or complete isomerisation to the 2-aryl-heterocycle,however using boron trifluoride as the Lewis acid produces only the 3-aryl-isomer. 3-Tosylamino-benzofurans can be prepared from aryl glyoxal hydrates. ... 

The acid—base properties of the five-membered heterocyclic dioxa— and dithia-boron systems (similar to 20) with trimethylamine and tri-methylphosphine have been considered,and it was claimed " that there was a reversal of base strength. In this work Shore et al. did not make use of the AB values, although these did in fact correspond to the relative acid—base strengths found from the study of homogenous displacement reactions and from dissociation tendencies. Similarily, from dissociation pressures it was concluded ° that the five-membered dioxaborolane (42) was a stronger Lewis acid than the corresponding six-membered compound (43) the AB values obtained for the complexes of (42 AB= -I-21-2) and (43 AB= -I-9-3) are in agreement with this conclusion. 

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