Solvent-Mediated Allylation of Carbonyl Compounds with Allylic Stannanes

Abstract

Methanol promotes the addition of allyltrimethylstannane (1a) to isobutyraldehyde (2a, 30 degrees C) yielding the corresponding homoallylic alcohol (3a), without the necessity for added catalyst. The corresponding reaction of aldehydes 2a-eor activated ketone 2f with tetraallyltin (1b, 0.25 equiv) is substantially faster and proceeds in high yield (81-98%) and with easy separation of the product from tin residues. Aliphatic ketones 2g and 2h also react, but require more forcing conditions. Competitive experiments involving equimolar mixtures of selected aldehydes and ketones with 1b indicates very high aldehyde chemoselectivity. The reaction of 1b with aldehydes proceeds slowly at first, followed by a rapid acceleration which may be attributable to a build up of partially soluble tin(IV) methoxide. The increased rate of carbonyl allylation by 1a and 1b in methanol, relative to dimethyl sulfoxide, suggests that the primary activating influence of the solvent is via hydrogen bonding to the carbonyl oxygen. There is no NMR spectroscopic evidence for a significant change in the ground state structure of these allylic stannanes in methanol, relative to other solvents.

High Atom Efficiency in Sc(OTf)3‐Catalyzed Allylation of Aldehydes with Tetraallyltin

“…The analogous increase of yield by adding Ac 2 O was reported for the Sc(OTf) 3 -catalyzed allylation of electron-rich aldehydes with allyltrimethylsilane. [11] In this case, the coexisting Ac 2 O serves for suppressing the secondary reactions (like double allylation or ether formation) of the resulting homoallyl alcohols through in situ acetylation to furnish homoallyl acetates.…”

“…Along this line, the reaction between 1 and PhCHO with a 1 : 4 molar ratio in CH 3 CN at 0 8C was investigated (Table 1). With a 5 mol % catalyst concentration relative to the aldehyde that is the most common level in various Sc(OTf) 3 catalyzed reactions, the yield was 80% (entry 1), a figure comparable with that obtained in aqueous media. [6] Upon gradual reduction of the catalyst concentration, it turned out that the yield was not significantly decreased until the 0.1 mol % level where an 82% yield was obtained (entry 2).…”

“…This is much more advantageous than more popular protocols using allyltrialkylsilyl or alkyltrialkylstannyl reagents which transfer only one allyl moiety out of four organic groups on the silicon or tin. The reaction between 1 and an aldehyde in a 1 : 4 molar ratio was reported to be promoted by HCl [2] or methanol, [3] yet the yields of desired homoallyl alcohols were not constantly high: more than 90% in some cases but less in others. Ionic liquids were employed as reaction media in which the yields were 70 ± 93%.…”

“…[8] As part of our project to explore highly atom-efficient processes catalyzed by Lewis acid, [9] we have disclosed that the yield of Sc(OTf) 3 -catalyzed allylation of aldehydes with 1 is intensively improved simply by addition of acetic anhydride (Ac 2 O) (Equation 1).…”

“…Needless to say, Ac 2 O itself cannot work as a promoter. When a mixture of 1 and PhCHO was exposed to Ac 2 O in the absence of Sc(OTf) 3 , no reaction took place.…”