Diastereoselective Allylation and Crotylation Reactions of Aldehydes with Potassium Allyl- and Crotyltrifluoroborates under Lewis Acid Catalysis

Abstract

Potassium allyl-and crotyltrifluoroborates react with aldehydes in a process catalyzed by a variety of Lewis acids, to give the corresponding homoallylic alcohols. Of the Lewis acids examined, BF 3 ◊OEt 2 , used either stoichiometrically or catalytically, was found to most efficiently catalyze this reaction. The air and moisture stable potassium organotrifluoroborate salts react with a variety of alkyl, a-or b-substituted alkyl, and aryl aldehydes, and lead to the adducts in high yield and with high diastereoselectivity.Synthetically versatile homoallylic alcohols 2 are typically prepared by the allylation and crotylation of aldehydes. 1,2 A variety of organometallic reagents have been used for these transformations, but allyl-and crotylboron compounds 1 are particularly useful because of the high yields and excellent levels of stereo control they provide (Scheme 1). 3 Of the two main classes of 1, allyl/crotyl dialkylboranes 1a and allyl/crotyl boronates 1b, only one reagent, allyl pinacol boronate, is commercially available at the present time. 4 A degree of inconvenience is often associated with other allyl-and crotylboron compounds, which are usually prepared immediately prior to use due to their sensitivity to air and/or moisture, and the difficulties involved with their subsequent storage and handling. As part of our ongoing research in organoboron chemistry, 5 we recently reported a new class of allyl-and crotylboron compounds, potassium allyland crotyltrifluoroborates, and their BF 3 ∑OEt 2 catalyzed addition to carbonyl compounds. 6 We now report the use of other Lewis acid catalysts, and the reaction of potassium allyltrifluoroborate with aldehydes bearing a-or b-stereocentres. Scheme 1Organotrifluoroborate salts (RBF 3 M, M = alkali metal) 5a,7 have been demonstrated to be more air and water stable than the corresponding boronic acids. 8 Vedejs and co-workers recently showed potassium aryltrifluoroborates (ArBF 3 K) to be useful precursors for the formation of arylboron difluoride Lewis acids, 7a and reported their preparation using aqueous KHF 2 . Similarly, we reasoned that potassium allyl-and crotyltrifluoroborate 3 should be convenient precursors for the in situ formation of allylboron difluoride, a species theoretically predicted to have higher reactivity in allylation processes. 9The preparation of the potassium allyl-and crotyltrifluoroborates 3 was achieved via the boronic acids in a manner analogous to that used for the synthesis of other potassium trifluoroborate salts (Scheme 2). The addition of either allylmagnesium bromide 10 or crotylpotassium 11 to trimethylborate or triisopropylborate respectively, followed by acidic hydrolysis, was used to prepare the requisite allyl-and crotylboronic acids, respectively. Conversion to the potassium allyl-or crotyltrifluoroborate 3 was then achieved by treatment with aqueous KHF 2 , followed by recrystallization from acetonitrile. Particular care should be taken to avoid isomerization of (E)-crotylpotassium during the preparation of 3c. 11 The salts...

Acyclic Stereocontrol in the Additions of Nucleophilic Alkenes to α‐Chiral N‐Sulfonyl Imines

“…To that end, ar ange of chelatable metal halides and triflates were screened ( Table 3). Inspired by Batey's additions of allyltrifluoroborates to achiral aldehydes [29] and N-sulfonyl imines, [30] as wella sd iastereoselective additions to chiral aldehydes, [31,32] we substituted allylBF 3 Kf or allylTMS. [20] Our calculations predicted ah ighly organized zinc chelate structure that is consistent with the high The relative configuration was established by X-ray crystallography (see SupportingInformation).…”

A Convergent, Modular Approach to Functionalized 2,1-Borazaronaphthalenes from 2-Aminostyrenes and Potassium Organotrifluoroborates

“…15 Adapting this protocol would allow the use of air- and moisture stable organotri-fluoroborates in the borazine synthesis, 16 which was expected to expand the range of functional groups and subunits that could be incorporated. Thus, aryl as well as alkyl-, alkenyl-, and alkynyltrifluorobo-rate substrates were envisioned as suitable precursors.…”

Expanded Substrate Scope and Improved Reactivity of Ether-Forming Cross-Coupling Reactions of Organotrifluoroborates and Acetals

“…Following the pioneering work by Petasis on the 1,2-addition of alkenyl- and arylboronic acids to imines 20 and further complemented by outstanding related work including that of Batey, 21 Langlois, 22 and Raeppel 23 , the nucleophilic addition of organoboronic acid derivatives to oxocarbenium ion has gained much interest. 24 Furthermore, recent reports have shown that such reactions can be rendered enantioselective.…”