[51522-97-5] · C3H3Cl2Li · 1,1-Dichloroallyllithium · (MW 116.90)
(ambient allyl anion; reacts with a variety of electrophiles to give a- and/or g-substituted products;1-5 can serve as a ketone annulating agent to form cyclopentenones;5 an equivalent of chlorovinyl carbene6)
Solubility: sol THF.
Preparative Methods: 1,1-dichloroallyllithium (2) can be prepared1 by the lithium-halogen exchange reaction between n-butyllithium and 3,3,3-trichloropropene (1) at -105 °C (eq 1), but transmetalation of 3,3-dichloro-2-propenyltriphenyllead (3) with n-butyllithium in THF at -95 °C is cleaner (eq 2).1
Handling, Storage, and Precautions: highly unstable; generated only at -95 °C, in the presence of electrophiles in an inert atmosphere. Use in a fume hood.
Reaction of (2) with Chlorotrimethylsilane gave exclusively the a-silylated product (4) in 99% yield (eq 3). In contrast, the g-isomer (5) was the sole product when (2) was treated with Me3SnBr (eq 4).
Similar regioselective reaction was observed with HgCl2 to afford a g-mercurated product (i.e. CCl2=CH-CH2HgCl) in 83% yield, while its reaction with iodomethane yields the corresponding a-alkylated product (i.e. MeCCl2-CH=CH2) in 98% yield. Only trimethylchlorogermane gives a mixture (5.66:1) of a- and g-isomers.1
The reaction of (2) with ketones is governed by the ambident nucleophilic character of (2), whose two reacting termini differ substantially in terms of both steric and electronic factors. Several ketones and aldehydes give either exclusive a- or g-products, with two exceptions (PhCHO, PhCOMe) where a mixture of both isomers is obtained in each case (eq 5). The regioselectivity can be explained on the basis of HSAB principle.2,3 There is also a strong effect of the counterion. When (2) is generated by treatment of 3,3-dichloro-1-propene with Lithium Diisopropylamide, benzaldehyde gives mostly g-product, whereas with Potassium t-Butoxide present, mostly a-product is obtained (eq 6).4
The reaction of (2) with both cyclic and acyclic ketones gives dichlorohomoallylic alcohols (11) exclusively. The alcohols have been transformed to the cyclopentenones (14) in good yields (eq 7).5 This conversion is a modified
The anion (2) generated by treatment of 3,3-dichloropropene with Lithium 2,2,6,6-Tetramethylpiperidide in the presence of alkenes yields, after hydrolytic work-up, the corresponding gem-chlorovinylcyclopropanes (15) (eq 8) in good yields.6 With 1,3-butadiene a mixture of cis- and trans-1-chloro-1,2-divinylcyclopropanes is formed. The cis isomer (16) (eq 9), however, rapidly undergoes Cope rearrangement to the chloroheptadiene (17) in 31% yield.
1,1-Difluoroallyllithium (19) is of interest as a reagent for the introduction of fluorine into organic molecules. It was first generated7,8 at -95 °C in THF by treating 3,3-difluoroallyltrimethyltin (18) with n-Butyllithium (eq 10). The reagent is unstable, even at -95 °C to -130 °C, in the absence of an electrophile. However, when (19) is generated in the presence of Me3SiCl, the corresponding a-silylated product (Me3SiCF2-CH=CH2) is obtained in 75% yield. A more effective route to (19) involves the Li/Br exchange reaction of CH2=CH-CF2Br with BuLi (eq 11) in the presence of various electrophiles.9
Chlorosilanes, aldehydes, ketones, and esters give a-substituted products such as (18) from esters and (20) from ketones and aldehydes (eq 12).10
H. Junjappa, H. Ila & J. Satyanarayana
North-Eastern Hill University, Shillong, India