[75583-56-1] · C9H11CuLiNSi · Lithium Cyano(dimethylphenylsilyl)cuprate · (MW 231.79)
(organometallic reagent for the silylcupration of enones, alkynes, and strained alkenes)
Solubility: sol ether, THF from -78 to 25 °C.
Preparative Method: the title reagent (1), as well as lithium cyano(trimethylsilyl)cuprate [81802-36-0], can be prepared from the corresponding silyllithium species and 1 equiv Copper(I) Cyanide in ether or THF at -78 to -5 °C;1,2 the precursor silyllithium (2) is generated by the reaction of Chlorodimethylphenylsilane with Lithium metal in THF or ether (eq 1).
Handling, Storage, and Precautions: thermally stable up to ambient temperatures; oxygen- and moisture-sensitive; use in a fume hood.
Low-temperature 29Si, 13C, and 1H NMR spectroscopic techniques have been used to probe the structure of (1),2 which is referred to as a lower-order cuprate. When 2 equiv of the silyllithium (2) are combined with 1 equiv copper(I) cyanide, a new reagent, referred to as a higher-order cuprate, is generated (see Dilithium Cyanobis(dimethylphenylsilyl)cuprate).3 Evidence for the different cuprate structures comes from separate signals in the 29Si and 13C spectra.2
Reagent (1) effectively adds the dimethylphenylsilyl group to a,b-unsaturated ketones, esters, and alkynic analogs. In a typical case, (1) adds to cyclohexenone in ether at -78 °C in 58% yield (eq 2).2
The analogous trimethylsilylcuprate (3) adds to ethyl-2-butynoate at 0 °C to give a mixture of stereoisomers, while the higher-order species reacts at -78 °C to yield a single geometrical isomer (eq 3).4
The lower-order silyl cuprates are apparently less reactive than the higher-order cuprates in conjugate addition reactions. Reagent (1) has been added in an addition-elimination sequence to an alkylidenemalonate system as a route to vinylsilanes (eq 4).5
This reaction was a prelude to a more elaborate scheme for producing more substituted vinylsilanes. The product of eq 4 further reacted with a geminal diorganometallic species to yield an (E)/(Z) (86:14) mixture of a vinylsilane (eq 5).
The enhanced reactivity of allyl halides is reflected in the reactions of both (1) and (3). Thus (1) reacts with 2-t-butylsulfonylallyl bromide at low temperatures (eq 6).6
Reagent (3) can be employed in the preparation of the 2-bromoallylsilane (eq 7).7
Carbocupration of alkynes is a major synthetic route to substituted alkenes. With (1), additions to terminal alkynes usually lead to regioisomeric vinylsilanes (eq 8).2,8 In contrast to the lower-order cuprates, the corresponding higher-order cuprates generally are regioselective for the generation of the 1-silyl-substituted vinylsilanes (98:2 ratio).
The regioselectivity of the addition of the title reagent (1) to 1,2-allenes, such as 1,2-undecadiene, is much more favorable for silylation at the 1-carbon atom. In fact, the major regio- and stereoisomer produced is the (Z)-1-dimethylphenylsilyl-2-undecene (eq 9).9
In general, (1) does not add to unactivated alkenes. However, strained bicyclic alkenes do undergo silylcupration with the reagent. The [2.2.1] bicyclic ethers (4) react with (1) at 0 °C, ultimately giving substituted 1,3-cyclohexadienes (7) via intermediates (5) and (6). The latter intermediate undergoes a Peterson alkenation to give (7) (eq 10).10
Silylcupration of the oxabicyclic ketone (8) takes a different course, in that the bridged oxygen system is not cleaved. Instead, the initial exo adduct (9) produces a cyclobutanol product (10) exclusively (eq 11).11
In the same [3.2.1] bicyclic ether system with methyl substitution a to the carbonyl, (1) adds to the double bond without cyclobutanol formation (eq 12). With the monomethylated ketone (11) an 8:1 mixture of regioisomers is produced, with (12) predominating.
By allowing the above reaction to proceed for extended periods of time, the corresponding cyclobutanol product derived from (11) is favored, while with the dimethyl ketone (13) (R1 = R2 = Me), no cyclobutanols are formed. Intermediate cuprates (9) derived from higher-order cuprate additions may be trapped with Iodomethane, acid, or hydroxyl groups (eq 13).
In summary, the lower-order cyanotrialkylsilylcuprates (1) and (3) react as do most carbon-based cuprates in conjugate additions and carbocupration processes. These reagents are less basic than higher-order cyanocuprates, and less reactive with some less electrophilic partners.
Joseph P. Marino & David P. Holub
University of Michigan, Ann Arbor, MI, USA