On the use of mixtures of organotin species for catalytic enantioselective ketone allylation—a detective story

Abstract

In the presence of enantiopure MTBH(2)(monothiobinaphthol, 2-hydroxy-2[prime or minute]mercapto-1,1[prime or minute]-binaphthyl; 0.2 eq.) quantitative allylation of ArC([double bond]O)Me takes place with impure Sn(CH(2)CH[double bond]CH(2))(4)(prepared from allyl chloride, air-oxidised magnesium and SnCl(4)) to yield tert-homoallylic alcohols in 85-92% ee. In the same process highly purified, or commercial, Sn(CH(2)CH[double bond]CH(2))(4) yields material of only 35-50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH(2)CH[double bond]CH(2))(3), ClSn(CH(2)CH[double bond]CH(2))(3) ClSnEt(CH(2)CH[double bond]CH(2))(2) in the tetraallyltin sample and the presence of traces of water (which inhibits achiral background reactions). All the triallyl and diallyl species enhance the stereoselectivity in the catalytic allylation reaction, the chlorides more so than the ethyl compound. Hydrolysis of ClSnEt(CH(2)CH[double bond]CH(2))(2) affords crystallographically characterised Sn(4)(mu(3)-O)(mu(2)-Cl)(2)Cl(2)Et(4)(CH(2)CH[double bond]CH(2))(4). Reaction of this latter compound with MTBH(2) leads to the most potent catalyst.

Enantioselective catalytic allylation of arylmethylketones using tetraallyltin and tin(IV) chloride mixtures

“…Neat PhC(O)Me (100 mol%) was added to these solutions at room temperature and the dependence of product e.e. versus yield by GC over 16 h. With 2-3 mol% of the SnCl 4 promoter selective catalysts were formed but at 1 mol% the insufficient chiral catalyst was produced to compete with racemic background autocatalysis from the Sn(allyl) n [OC-Me(Ph)(allyl)] 4Àn (n = 2-3) kinetic products [2]. The conversion dependent enantioselectivity of Fig.…”

“…); 1 H NMR (CDCl 3 , 400 MHz): d 1.53 (s, 3H), 2.12 (s, 1H), 2.49 (ddt, J = 13.7, 8.2 and 0.8 Hz, 1H), 2.64 (ddt, J = 13.7, 6.6 and 1.0 Hz, 1H), 5.10-5.13 (m, 1H), 5.15-5.18 (m, 1H), 5.60 (dddd, J = 14.7, 9.7, 8.2 and 6.6 Hz, 1H), 7.31 (dd, J = 8.7 and 2.0 Hz 2H), 7.46 (dd, J = 8.7 and 2.0 Hz, 2H). These data were concordant with published values[2]. The enantiomeric excesses were determined by GC (6-Me-2,3-pe-c-CD, 90°C initial, 4 deg min À1 to 125°C).…”

“…Toluene solutions of SnCl 4 solutions were freshly prepared using commercial SnCl 4 (>98%) and toluene distilled from sodium wire. The allylated products were identified by comparison with authentic samples [2].…”

“…Subsequent careful work revealed the selective catalyst is formed from trace amounts of EtSnCl(CH 2 CH@CH 2 ) 2 (3) derived from incomplete reaction of the SnCl 4 and the presence of EtBr (used for Grignard preparation) [2]. Compound (3) undergoes facile allyl hydrolysis with trace water to provide the crystallographically characterized bis(diallyldiethyldichlorodistanoxane) (4a).…”

Chloramine-T-Mediated Oxidation of Benzylic Alcohols Using Indium(III) Triflate

“…However, the electron-donating methoxy group, where the oxygen atom can withdraw electron density inductively, at the para-position gave a reduced yield of 52% (entry 10). 32,33) Furthermore, comparison of entries 4 and 11 could suggest the steric hindrance of methyl group at the ortho-position was quite significant. …”