Boron trifluoride complex formation

Table 8-11. Enthalpy Change for Boron Trifluoride Complex Formation with Donors al 25°C...

Primary nitroparaffins react with two moles of formaldehyde and two moles of amines to yield 2-nitro-l,3-propanediamines. With excess formaldehyde, Mannich bases from primary nitroparaffins and primary amines can react further to give nitro-substituted cycHc derivatives, such as tetrahydro-l,3-oxa2iaes or hexahydropyrimidines (38,39). Pyrolysis of salts of Mannich bases, particularly of the boron trifluoride complex (40), yields nitro olefins by loss of the amine moiety. Closely related to the Mannich reaction is the formation of sodium 2-nitrobutane-1-sulfonate [76794-27-9] by warming 1-nitropropane with formaldehyde and sodium sulfite (41). 

From Table III it is apparent that a number of different donors could be used to obtain very attractive fractionation factors. Indeed, at 30°C., the isotopic equilibrium constant was 1.03, or more, for phenetole, anisole, diethyl ether, ethyl formate, dimethyl selenide, dimethyl sulfide, and diethyl sulfide. However, all of these donors were not equally satisfactory for our purpose. The boron trifluoride complexes of the thioethers, the selenide, and the ester had a pronounced tendency toward irreversible decomposition and were too unstable to be seriously considered for an... 

The initial reaction is a Prins reaction, catalysed by the boron trifluoride complex. In order to achieve maximum overlap of the -orbitals of the olefin and aldehyde groups, the aldehyde must approach the olefin from below, as will easily be seen using molecular models. This means that the resultant alcohol function is located on the downward side of the molecule as shown in Figure S21. A 1,2-carbon shift followed by a transannular bond formation with concomitant loss of a proton, provides the skeletal rearrangement to the product. It may not be too obvious in the figure, but an experiment with molecular models will soon... 

The reaction of benzopentathiepin with alkenes [(fl-but- -ene, ( )-hex-3-ene, cyclopentene or cyclohexene] in the presence of the boron trifluoride-diethyl ether complex results in the formation of 3,4-dihydro-l,2,5-benzotrithiepins, e.g. formation of 3.407... 

The diastereofacial selectivity of Lewis acid promoted reactions of allylsilancs with chiral aldehydes has been thoroughly investigated58. Aldehydes with alkyl substituted a-stereogenic centers react with a mild preference for the formation of Cram products, this preference being enhanced by the use of boron trifluoride-diethyl ether complex as catalyst58. 

The use of boron trifluoride-diethyl ether complex as the Lewis acid in these reactions promotes silyl group migration and gives rise to the formation of tetrahydrofurans with excellent stereoselectivity82. 

These results may be explained either by Cram s cyclic model in the case of lithium alkyls or by Cornforth s dipolar model if copper-boron trifluoride reagents are used. Boron trifluoride causes double complexation of both nitrogen and oxygen atoms which results in the formation of an adduct with rigid antiperiplanar conformation due to electrostatic repulsion (see 4 and 5)9. 

Catalytic opening of diazabicyclohexane 331 in the presence of boron trifluoride-diethyl ether complex or concentrated hydrochloric acid leads to formation of a dimer 493 <1999RJO 44>, in contrast to the thermal process... 

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

The synthesis of the representative compound of this series, 1,4-dihydro-l-ethyl-6-fluoro (or 6-H)-4-oxo-7-(piperazin-l-yl)thieno[2/,3/ 4,5]thieno[3,2-b]pyridine-3-carboxylic acid (81), follows the same procedure as that utilized for compound 76. Namely, the 3-thienylacrylic acid (77) reacts with thionyl chloride to form the thieno Sjthiophene -carboxyl chloride (78). Reaction of this compound with monomethyl malonate and n-butyllithium gives rise to the acetoacetate derivative (79). Transformation of compound 79 to the thieno[2 3f 4,5]thieno[3,2-b]pyhdone-3-carboxy ic acid derivative (80) proceeds in three steps in the same manner as that shown for compound 75 in Scheme 15. Complexation of compound 75 with boron trifluoride etherate, followed by reaction with piperazine and decomplexation, results in the formation of the target compound (81), as shown in Scheme 16. The 6-desfluoro derivative of 81 does not show antibacterial activity in vitro. 

Adducts of type 13, arising from the rearrangement of the allylic intermediate, have never been observed. The product distribution in methanol depends, however, on the reaction conditions. When the addition of XeF2 is carried out in the presence of boron trifluoride as a catalyst, the formation of the complex b has been suggested. This complex would react with 2,3-dimethylbutadiene as a positive oxygen electrophile to give, besides 1,2- and 1,4-difluoro derivatives, 1,4- and 1,2-fluoromethoxy products with a predominance of the anti-Markovnikov adduct (equation 26). 

The boron trifluoride alkyl fluoride (chloride) complexes gave no evidence of alkylcarbonium ion formation. It must, however, be emphasized that (a) the physical investigation of the binary system was carried out at such low temperatures (generally below —100°) that ionization of the halides could hardly be expected (with exception of highly reactive tertiary halides) (b) the methods used could not he relied on to detect a small ionization equilibrium even if it existed. 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

In this procedure, the ketone is first converted to its enol acetate by reaction with acetic anhydride in the presence of a proton acid. Since this enol acetylation is performed under equilibrating conditions, the more stable enol acetate (usually the more highly substituted isomer) is produced. Acetylation of this enol acetate, catalyzed by the Lewis acid boron trifluoride, usually leads to the formation of the enol acetate of a /3-diketone which is cleaved by boron trifluoride to form acetyl fluoride and the borofluoride complex of the /3-diketone. Thus, this procedure offers a convenient and general synthetic route... 

Figure 5.19. Illustration of the formation of a donor-acceptor complex by the sequential adsorption of trimethylamine and boron trifluoride. The Lewis-basic trimethylamine forms a dative bond with the electrophilic Si dimer atom, and the Lewis-acidic boron trifluoride bonds to the nucleophilic Si dimer atom [278].

Cao, X. P. and Hamers, R. J. Formation of a surface-mediated donor-acceptor complex Coadsorption of trimethylamine and boron trifluoride on the silicon (001) surface. Journal of Physical Chemistry 106, 1840-1842 (2002). 

Boron trifluoride and boron trifluoride-diethyl ether complex can be used as a source of fluoride ions in the presence of hypobromites and hypochlorites, e.g. methyl hypobromitc, tert-butyl hypobromite, methyl hypochlorite in carbon tetrachloride at 25 C. The addition of bromine monofluoride" and chlorine monofluoride" to various alkenes is accompanied by the formation of the corresponding alkoxybromides and alkoxychlorides which hinder the isolation of the halofluorinated products.57 jV-Bromo- and A -chloro-substiluted alkyl- and arylamines. -amides, and -imides, A -chloro-A,-methylamine, A -bromo-A -methylamine, A -chloro-A, /V-dimethylamine, A-bromo-A.A-dimethylamine, ACV-dichloro-A -methylamine, V,fV-dibromo-,V-mcthylaminc, A -bromosuccinimide, -V-chlorosuccinimide, Af-bromoacct-amide, A.A -dichlorourethane, can be used in the reaction instead of the hypohalites. The reactions with various alkenes conducted in dichloromethane at room temperature in the presence of boron trifluoride-diethyl ether complex produce bromofluoro and chlorofluoro addition products in 40-80 % yield. However, the reactions are complicated by the addition of A -halo-succinimides and Af.A-dichlorourcthane to the C = C bonds.58... 

There are some reactions in which boron trifluoride and boron trifluoride-diethyl ether complex are used as fluorinating agents, but they are not so frequently used and widespread. The best-known reaction is the decomposition of fluoroformates. In this type of reaction boron trifluoride or pyridine are essentially required as catalysts for the decomposition process. The formation of fluoroformates is established4 5 and the decomposition proceeds cither by heating in pyridine at higher temperature6 or by addition of boron trifluoride-diethyl ether complex at 0-50°C.7... 

The reaction of norbornene (5) with xenon difluoride in dichloromethane at various temperatures and in the presence of various catalysts results in the formation of seven products.45 Table 6 shows the significant effect of boron trifluoride-diethyl ether complex as catalyst in the product formation. 

Studies by Nakane et al. also support the two-step mechanism when alkylation is carried out with alkyl halides under substantially nonionizing conditions. It was further shown that in nonpolar organic solvents carbocations rather than the polarized complexes participate directly in the formation of the first n complex. BF3—H20 catalyzes ethylation,130 isopropylation,131 and benzylation132 through the corresponding carbocations. Accordingly, ethylbenzene equally labeled in both a and p positions was obtained when [2-14C]-ethyl halides were reacted in hexane solution in the presence of boron trifluoride, BF3—H20, or aluminum... 

The reaction is irreversible and can be used to synthesize aliphatic and aromatic esters. In addition, there are no complications involving water removal or azeotrope formation. Boron tribromide can be used in place of boron trichloride, but the bromide has a stronger tendency to halogenate the alkyl group of the alcohol (26). Boron trifluoride does not give the ester, but gives either a complex or dehydrated product. 

Kurabayashi and Grundmann have reported the preparation of the 1,2,4-oxadiazoles (34) from 1,3,5-triazine and aryl nitrile oxides in the presence of boron trifluoride. The mechanism has not been fully elucidated, but it is most likely that the initial stage is the formation of the complex (33) (78BCJ1484). 

The direct formation of racemic a-tocopherol from trimethylhydroquinone and isophytol occurs at low temperature in the presence of boron trifluoride or aluminum chloride (71JOC2910). It is important that the solvent should not be able to complex with the Lewis acid rather, it is the phenol-catalyst complex which is alkylated. 

In the presence of boron trifluoride and methanesulfonyl chloride, polyhydric acetophenones react with DMF to give 3-substituted chromones. It is postulated that ring for-mylation is prevented through deactivation by complex formation between the substrate and boron trifluoride (76CC78). 

Acylation of thiochromanones with boron trifluoride and acetic anhydride readily affords (92), which may be elaborated into (93) (equation 36) (79CJC3292), while conversion of the keto function to an oxime (94) permits the formation of the palladium complex (95) as a first step towards functionalization of the aromatic ring (equation 37) (78BCJ3407), or ring expansion by Beckmann rearrangement (equation 38) (76JCS(P1)2343). 

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