Boron trifluoride geometry

Scheme 6.40. Influence of alkene geometry on stereoselectivity of allylic substitution of mesylates 189 and 192 with boron trifluoride-modified lower order cyanocuprate reagents.

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

A basically new type of synthesis of dihydrooxazines was applied for the preparation of 177 and 179 (86JOC3248). Boron trifluoride catalyzed the intramolecular [4+2] cycloaddition of the A-acyliminium compounds derived from 176 and 178, resulting stereospecifically in the rrani-fused cyclo-pent[d][l,3]oxazines and partly saturated 3,1-benzoxazines 177 and 179, respectively. The steric orientation of the 4-methyl substituent of the product is controlled by the Z and geometry of the starting bisamides 176 and 178 (86JOC3248). 

The stereoselectivity of any particular reaction depends on the details of the structure of the transition state. The structures of several enone-Lewis acid complexes have been determined by X-ray crystallography.11 The site of complexation is the carbonyl oxygen, which maintains a trigonal geometry, but with somewhat expanded angles (130-140°). The Lewis acid is normally anti to the larger carbonyl substituent. Boron trifluoride... 

Enolate species 6, derived from 1-oxopropyl complex 5, reacts similarly with monosubstituted epoxides. Under the influence of diethylaluminum chloride, only the diastereomers 7 and 8 were observed in the reaction mixture 7 was the major product. The use of boron trifluoride - diethyl ether complex instead of diethylaluminum chloride caused a complete loss of stereocontrol at C , producing a 50 50 mixture of diastereomers 7 and 8, but stereocontrol at C was retained as no other diastereomers were produced. The major diastereomer produced is consistent with the intermediacy of a transition state like that represented in Newman projection C which has the usual anti-E-snolate geometry and lacks the R methyl gauche interaction of structure D. 

Hybridization. A satisfactory description of covalent bonding should also be able to account for molecular geometry, that is, for the mutual directions of bonds. Let us take for an example boron trifluoride, which is a trigonal planar molecule. Boron uses three orbitals to form three completely equivalent bonds to fluorine atoms. 

The structure of the complex (96) between benzaldehyde (95) and boron trifluoride (equation 15) was investigated by X-ray crystallography169. In 96, BF3 is in the anti position to the phenyl ring and this geometry remains also in solutions, as tested by the 19F-NMR spectrum in CD2CI2. An ab-initio study170 on interactions between formaldehyde and boron trihalides showed that these complexes (mainly donor-acceptor complexes) affect spectroscopic properties and the reactivity of the carbonyl group the polarization of the C=0 bond favours the attack of nucleophiles. 

Figure 2-2. Changes in geometry occurring upon the formation of an adduct between boron trifluoride and dimethyl ether.

Boron trifluoride has a plane trigonal shape a 2p orbital on each fluorine atom overlaps with a boron sp2 hybrid. In general, we can expect that all molecules in which a central atom uses three equivalent sp2 hybrid orbitals will exhibit plane trigonal geometry, since this represents the most symmetrical, and hence equivalent , arrangement of the three bonds. 

FIGURE 1.10 Representations of the trigonal planar geometry of boron trifluoride (BF3). There are 6 electrons in the valence shell of boron, a pair for each covalent bond to fluorine. The three pairs of electrons are farthest apart when the F — B—F angle is 120°. 

Next we will look at the BF3 (boron trifluoride) molecule, known to have planar geometry based on VSEPR. Considering only the valence electrons, the orbital diagram of... 

Only the E,3E stereoisomer of the 2-azadiene was found to be reactive under the reaction conditions. Consequently, each of the piperidine products was found to possess the cis 2-phenyl, 5-methyl relative stereochemistry. Moreover, the cycloaddition products derived from reactions of enol ethers possessed the all-cis 2-phenyl-4-alkoxy-5-methyl stereochemistry necessarily derived from exclusive endo cycloaddition. Dienophile geometry is maintained during the course of the [4 + 2] cycloaddition, and no products derived from a potential stepwise, addition-cyclization reaction were detected. Representative neutral and electron-deficient dienophiles failed to undergo cycloaddition. A related boron trifluoride etherate-catalyzed [4 + 2] cycloaddition of simple 2-aza-l,3-butadienes with carbonyl compounds provides 5,6-dihydro-2//-l,3-ox-azines and appears to proceed preferentially through an endo [4 + 2] transition state although evidence supporting a stepwise, addition-cycli-zation was occasionally detected.630... 

The boron trifluoride-catalyzed silicon-iodine(III) exchange reaction of alkenylsilanes 301 with iodosy-larenes followed by treatment with aqueous NaBp4 is the most general method for the synthesis of alkenyl(aryl)iodonium tetrafluoroborates 302 (Scheme 2.86) [394,439], This reaction proceeds under mild conditions and in a stereoselective manner with retention of the geometry of organosilanes. 

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