Boron Trifluoride Etherate condensation reactions

Boron trifluoride etherate is used as a catalyst in many organic reactions namely, alkylation, polymerization and condensation reactions. 

Sulfoxides can act as an S—O two-atom fragment in [4 + 2] condensations. Thus reaction of DMSO with benzoyl isocyanate in the presence of boron trifluoride etherate (75IZV1206) gives the 1,3,2,5-dioxathiazine (248). Boron trifluoride induced condensation of DMSO with Ar-benzoyl-S,5-bis(trifluoromethyI)sulfimide (249) gives the 1,3,2,4,5-dioxadithiazine (117) (78JA985), and the iV-acylimine (250) also gives a 1,3,2,5-dioxathiazine (251) on reaction with DMSO (80ZOR463). 

This borane reagent may be prepared as follows.6 In a dry 500-ml flask equipped with a condenser, a thermometer and a pressure-equalising dropping funnel are placed 80 ml of diglyme, 23.1 g of 2-methylbut-2-ene (0.33 mol) in 20ml of diglyme and 4.7 g of sodium borohydride (0.125 mol). The flask is immersed in an ice bath and 23.5 g of boron trifluoride-etherate (0.165 mol) is added dropwise over a period of 30 minutes. The semi-solid reaction mixture containing 0.165 mol of disiamylborane is permitted to remain an additional 15 hours at 0-5 °C, and then used for hydroboration. Examples of the use of disiamylborane are described in Sections 5.4.3 p. 542 and 5.7.1, p. 588. 

Condensation polymerization of functional ferrocenes generally yields medium- or low-molecular-weight polymers with broad molecular-weight distributions.12 For example, ferrocenylcarbinol, 6.9, has been condensation-polymerized to polymers 6.10 and 6.11 in the presence of boron trifluoride etherate or zinc chloride (reaction (5)).910 Species 6.11... 

The reaction vessel is designed to fit an ultrasound probe , and is fitted with a reflux condenser and a septum side-arm, and furnished with an inert atmosphere. Magnesium turnings (0.97 g, 40 mmol), a crystal of iodine, boron trifluoride etherate (1.42 g, 10 mmol) and dry ether (34.5 ml) are introduced into the flask. The ultrasound is switched on, and 1-bromopropane (4.31 g, 35 mmol) is added dropwise via the septum during 5 min, during which time an exothermic reaction starts. After 10 min the ultrasound is switched off and the magnesium salts are allowed to settle. The clear ether layer is transferred to another flask. The magnesium salts... 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

Reactions of free ketones with imines are far more restricted than those with preformed iminium salts. Examples of ketone-imine condensations are included in several reviews - and are limited to the use of arylimines, mainly benzylideneaniline. Blatt and Gross have noted that the reproducibility of uncatalyzed additions of ketones to Schiff bases is poor but can be improved by the addition of small amounts of hydrochloric acid. The highest yields of 3-amino ketones are obtained using 10 mol % of concentrated hydrochloric acid and 95% ethanol as solvent, and by performing the reaction at room temperature. The reaction, however, is of limited applicability, as it is restricted to methyl ketones and cycloalkanones. Deamination leading to unsaturated ketones is a major side reaction. In addition to hydrochloric acid, boron trifluoride etherate has been used as a catalyst in reactions of aromatic imines with methyl ketones, cyclic ketones and an epoxy ketone. ... 

---