[89805-98-1] · C11H11NS2 · 2-(2-Butenylthio)benzothiazole · (MW 221.37)
Solubility: sol all solvents except water.
Form Supplied in: white oils or solids; not commercially available.
Preparative Methods: 2-allyloxybenzothiazoles are prepared by reaction of potassium allyloxides with 2-chlorobenzothiazole in dry ether at rt.2 2-Allylthiobenzothiazoles are prepared by reaction of allylic alcohols with 2,2´-dithiobis(benzothiazole) and triphenylphosphine or with benzothiazole-2-thiol, diethyl azodicarboxylate, and triphenylphosphine in dry toluene at rt.3
Handling, Storage, and Precautions: store at 5 °C. Safety and handling data not available.
The reaction of these substrates with organocopper reagents, prepared by reaction of Grignard reagents with Copper(I) Bromide, leads to C-C coupling products (eq 1).
Efficient regiocontrol of this process (a- or g-substitution) depends on the nature of the organometallic reagent. High ratios of RMgX:CuX, which favor the formation of organocuprates, give only a-substituted products, whereas lower ratios, which favor the formation of organocopper reagents RCu, afford exclusively g-substituted products (eq 2).3 g-Substitution occurs independently of the steric hindrance of the R2 group in the allylic sulfide (eq 3).
These features have been applied to a simple synthesis of California red scale pheromone4 starting from the benzothiazole sulfide/benzyl ether of (S)-(-)-citronellol (eq 4).
Stereodivergent pathways relative to other allylic systems (e.g. allylic acetates)5 have been found6 in the reactions of cyclic allylic sulfides or ethers of benzothiazole with organocopper reagents. In this case the SN2´ process occurs with syn stereochemistry (eq 5) and is due to the anchimeric coordination of the organometallic reagent with the heterocyclic moiety. This has been utilized for regio- and stereocontrolled access to branched-chain sugars.
The importance of coordination in dictating the stereo- and regioselectivity is further demonstrated7 by the reactions of a,b-enoates g-substituted by a S-benzothiazolyl group with organocopper reagents to give, in a anti-Michael fashion, a-alkylated-b,g-enoates (eq 6).
The control of chemo-, regio-, and stereochemistry in the reactions of allylic electrophiles with two different leaving groups with organocopper reagents is difficult to achieve since many chemo-, regio-, and stereochemical reactions occur together with the desired one.8 These difficulties can be overcome by using the benzothiazolyl group as one of the leaving groups which is selectively replaced. This has been used for a synthesis of homoallylic pivalates (eq 7).1
Coupling of these reagents with allylic Grignard reagents in the presence of CuBr allows regioselective synthesis of 1,5-dienes, thus overcoming the difficulties found for different allylic electrophiles.9 The C-C coupling occurs exclusively in a head-to-tail fashion. Contrary to this the same electrophiles, when complexed with CuBr before the addition of the allylic Grignard reagent, give only 1,5-dienes derived from head-to-head coupling process (eq 8).10
University of Bari, Italy