ChemInform Abstract: O‐ UND C‐ELEMENTORGANISCHE ISOMERE 19. MITT. ELEMENTORGANISCHE (SI, GE, SN)DERIVATE DES METHYLPHENYLACETATS

Abstract

Phenylessigsäuremethylester (I) wird mit Natrium‐bis‐(trimethylsilyl)‐amid (II) intermediär in die Natriumverbindung (III) übergeführt; auf Zusatz von TrimethylchloF silan (IV) erhält man das Ketenacetal (V), das, in Abhängigkeit von den Arbeitsbedingungen, entweder als reine trans‐Verbindung oder als Cis‐trans‐Gemiseh (22:78) anfällt.

Chemistry of Trichlorosilyl Enolates. 1. New Reagents for Catalytic, Asymmetric Aldol Additions

“…While substantial literature exists on the generation and reactions of trichlorostannyl 6 and trichlorotitanium 7 enolates, the chemistry of trichlorosilyl enolates 8 is embryonic by comparison . For the initial studies, the use of isolated, purified trichlorosilyl enolates was deemed essential, and we followed the general procedure of Baukov and Lutsenko,8a Scheme . Thus, from methyl tributylstannylacetate ( 1 ) we could obtain the trichlorosilyl ketene acetal 2 8a as a distillable liquid (bp 25 °C/5 mmHg), which thermally isomerized to methyl trichlorosilylacetate 8d…”

Preparation of Chlorosilyl Enolates

“…Enolates Derived from Esters and Ester-Surrogates. The first chlorosilyl enolates reported in the literature were those derived from methyl acetate, prepared by the metathesis of alkyl (trialkyl)stannyl acetates with chlorosilanes . We have also found this to be the most general procedure for the generation of such acetate-derived chlorosilyl enolates.…”

“…When the reaction was complete, excess chlorosilane was removed and the products were distilled at 0−25 °C under reduced pressure. These manipulations must be carried out at the lowest possible temperature, as the chlorosilyl enolates 2 are unstable with respect to their C -silyl isomers and isomerization occurs rapidly at 70−90 °C 2d. These C -(chlorosilyl) esters are inert in the subsequent aldol reactions and do not revert back to the O-bound trichlorosilyl enolates under common reaction conditions.…”

“…Presumably, this is due to a catalyst deactivation pathway, though the process involved is not clear at the present time. We feel that, from the precedent of both House 11 and Baukov, the mercury salt electrophilically adds to the starting enol ether, forming a transient α-mercurio ketone, with loss of the trimethylsilyl group, Scheme . Coordination of SiCl 4 to the carbonyl and assisted loss of the mercury electrofuge would then form the trichlorosilyl enolate and reform an active mercury salt for the next catalytic cycle.…”

“…More importantly, these reactions are accelerated by the addition of catalytic quantities of Lewis bases (chiral phosphoramides). When cyclic enolates are used, aldol adducts are formed with high syn-diastereoselectivity in the uncatalyzed reactions, suggestive of a closed, boatlike transition structure. 1b,e In contrast, reactions catalyzed by chiral phosphoramides typically provide products resulting from closed, chairlike transition structures, often with good to excellent enantioselectivity. 1b,e Although several chlorosilyl enolates were known , when our work commenced, new, more efficient methods of synthesis were needed to provide access to certain classes of enolates and to enhance the general utility of this aldol reaction. We describe herein general methods for the synthesis of these unique and useful reagents.…”