[367-46-4] · C6H16BFO3 · Fluorodimethoxyborane Diethyl Etherate · (MW 166.03)
(electrophilic borylating agent, precursor of allylboronic esters)
Physical Data: bp 46-48 °C/760 mmHg.1
Solubility: sol common organic solvents (THF, Et2O, toluene, CH2Cl2, etc.). The title compound is moisture sensitive and will hydrolyze when exposed to water.
Analysis of Reagent Purity: 1H (d 3.62 (br m, 4H), 3.51 (br s, 6H), 1.23 (6 H, t, J = 7.2 Hz)) or 11B NMR.
Preparative Methods: by the disproportionation reaction of two moles Trimethyl Borate and one mole of Boron Trifluoride Etherate. Distillation of the reaction mixture produces the title reagent as the diethyl etherate (76%).1
Handling, Storage, and Precautions: reagent reacts with moisture and other protic solvents. It should be handled under an inert atmosphere in a well-ventilated hood. Contact with the eyes and skin should be avoided.
The reagent has been used extensively as an electrophilic borylating agent in the synthesis of substituted allylboronic esters (eqs 1-5). Allyl anions used in these reactions include Allylmagnesium Bromide (eq 1),2 crotylpotassium reagents (eqs 2-4),1,3-5 and g-alkoxyallyllithium reagents (eq 5).6 The resulting allylboronate reagents undergo highly diastereoselective reactions with aldehydes (see Crotyldimethoxyborane). The diastereoselectivity of these reactions depends on the isomeric purity of the reagent.7
This procedure was used initially for the synthesis of the tartrate ester modified crotylboronates (7) and (8) (eq 6).5 On scales up to 100 mmol, it was found that crotylboronates (7) and (8) were obtained in 70-88% yield with excellent isomeric purity. However, as the scale increased to 200 mmol the yields fell to 14-35% and (7) and (8) were obtained with much lower isomeric purity. These problems have been attributed to the competitive formation of tricrotylborane.5 Much more consistent results, particularly for large scale reactions (up to 400 mmol), have been achieved by using Triisopropyl Borate as the borylating agent. A similar improvement in yield has been reported for the preparation of the tartrate ester modified allylboronate (9) (eq 7).2
A systematic study of the synthesis of alkyl boronic esters from the reaction of organometallic reagents and electrophilic borylating agents has been reported by Brown.8 This study clearly identifies (i-PrO)3B as the best reagent for these purposes. For example, the reaction of FB(OMe)2 with Methyllithium gives only 10% yield of methylboronic ester and 28% of lithium tetramethylborate. However, treatment of triisopropyl borate with MeLi gives almost exclusively the methylboronic ester (&egt;98%) (eq 8).
Metalation of homoallylsilane (10) with n-Butyllithium and Potassium t-Butoxide followed by treatment of the resulting allyl potassium derivative with FB(OMe)2 gives the (Z)-d-(trimethylsilylmethyl)allylboronate exclusively, which is further oxidized by using basic peroxide to give the (Z)-allylsilane (11) (61% yield) (eq 9).9
Thomas G. Marron
Indiana University, Bloomington, IN, USA