[21329-14-6] · C6H10CuLi · Lithium Diisopropenylcuprate · (MW 152.65)
(isopropenylating reagent; undergoes conjugate addition reactions,1a 1,2-addition reactions,3 substitution reactions with oxiranes4 and alkyl, vinyl, and aryl substrates,1e carbocupration of alkynes,5 and acylation reactions1e)
Physical Data: chocolate-brown slurry in Et2O at -78 °C.2
Solubility: sol THF, Et2O.
Preparative Methods: prepared in situ from CuI salts (Copper(I) Iodide or CuBr.SMe2 (see Copper(I) Bromide)) under N2 or argon atmosphere.2 Rate of cuprate formation is temperature- and CuI salt-dependent, and reagent prepared from CuI halides is critically dependent on the purity of the salt used (see Lithium Dimethylcuprate for purification of CuI and CuBr). Dark-colored solutions are indicative of decomposition.
Handling, Storage, and Precautions: stable in solution for several h below -60 °C; however, at elevated temperatures of 0 °C to -15 °C, thermal dimerization occurs and only 2,3-dimethyl-1,3-butadiene can be detected.2 Air and moisture sensitive.
Lithium diisopropenylcuprate (1) displays the characteristic reactivity patterns of lithium diorganocuprates (see Lithium Di-n-butylcuprate, Lithium Dimethylcuprate, Lithium Diphenylcuprate).
Reaction of (1) with a,b-alkenyl ketones (eq 1),1,6 esters,1e and lactones7,8 results in 1,4-transfer of the isopropenyl ligand. Several natural product syntheses employ this particular methodology as a key step.6,7,9 A homoconjugate addition reaction of (1) with 1-(arylsulfonyl)bicyclobutanes (eq 2) has been observed.10 The resultant substituted cyclobutane derivative arises from addition across the central bond of the bicyclobutane from the endo side. Acylvinylcyclopropanes also afford homoconjugate addition products, and dienyl systems generally yield 1,6-addition products (eq 3).11 2-Pyrones can be converted to 2-siloxypyrylium salts,12 which undergo regioselective addition at the 4-position (eq 4).
The key step in the synthesis of the C(7)-C(13) subunit of erythronolide B involved the reaction of (1) with (R)-3-benzyloxy-2-methylpropionaldehyde to afford the 1,2-addition product.3 The stereochemistry of addition was controlled by chelation effects.
[CH2=C(Me)]2CuLi effects carbocupration of acetylene in good yield. Generally, only those vinylic cuprates having a substituent geminal to copper afford satisfactory yields.5
Although isopropenyl transfers in epoxide substitutions are rare, (1) reacts with b,g-epoxy alcohols4 and epoxy esters (eq 5).13 Reaction regioselectivity and yields are temperature dependent, and CuI-catalyzed Grignard additions are preferred. Cyanocuprates [e.g. (CH2=C(Me))CuCNLi and 2 (CH2=C(Me))Li + CuCN] afford better results in oxirane cleavage reactions.
The title reagent participates in general substitution reactions with alkyl (e.g. halides14,15 and tosylates15), vinyl (eq 6),16 and aryl (eq 7)14 substrates.1a,e Acylation of (1) with acid chlorides is a poor synthetic route to 1-methylvinyl ketones.2 Mixtures result from conjugate addition of the organocopper reagent to the product a,b-enone.
Janice W. Dieter & R. Karl Dieter
Clemson University, SC, USA