Alkylations of alkynols with organoaluminum reagents promoted by bis(.eta.5-cyclopentadienyl)titanium dichloride

Abstract

We have been interested in adapting the chemistry of titanium-aluminum based Ziegler-Natta catalyzed olefin polymerization to effect the alkylation of isolated unsaturated carbon-carbon bonds in a manner useful for ordinary (i.e., nonmacromolecular) organic Ziegler-Natta catalyst systems are potent alkylating agents toward olefinic and acetylenic linkages as evidenced from their widespread use in synthesizing linear stereospecific polyhydrocarbon~.~ However, single nonrepetitive alkylations of isolated carbon-carbon multiple bonds useful for extending the carbon framework in organic systems are not common. The paucity of research reported in this area is somewhat surprising if one accepts the idea that organic synthesis can be divided into the broad areas of (1) formation of carbon skeletons and (2) introduction, modification, andlor removal of functionalit? and that within these areas the first is generally the more diffi~ult.~ Recently, we have had some success in utilizing organoaluminum-titanium systems capable of polymerizing ethylene to alkylate alkynols to give olefinic Specifically, an alkynol such as 3-butyn-1-01 was incorporated into the complex (3-butyn-l-oxy)chlorobis(2,4-pentanedionato)titanium(IV), [Ti(OR)Cl(acac)2], and allowed to react with diethylaluminum chloride at -78 "C. After hydrolysis a -%"o yield of the terminally alkylated cis-addition product trarzs-3hexen-1-01 was obtained. Several additional alkynols were alkylated, and in all cases the alkyl group added to the carbon furthest from the hydroxyl functionality and gave the cisaddition product.In this paper we wish to report a new alkylation system which gives significant improvements in yields and synthetic convenience and also demonstrates the potential for control of regiospecificity through variation of ligand environments. In previous work it was necessary to first synthesize and isolate a titanium-alkynoxy complex, [Ti(OR)Cl(acac)2], by the re-action of [TiCl2(acac)z], alkynol, and pyridine. The pyridinium chloride was filtered from the reaction mixture, and the complex was then isolated from the filtrate. A much simpler procedure would be the addition of an alkynol to a solution of the organoaluminum reagent liberating an alkane and generating an [A12(0R)R,Cl+x] species which subsequently could be added to a solution of [TiCl2(acac)z]. Through ligand exchange this system might become equivalent to the initial one and thus eliminate the inconvenience of preparing the titanium-alkynoxy complexes. Attempts to effect alkylation by this latter route were unsuccessful. However, upon replacing [TiCl2(acac)z] with titanocene dichloride, CpzTiClz, in the 3-butyn-1-01-diethylaluminum chloride system a 55% yield of the ethylated products trans-3-hexen-1-01 (I) and 3-ethyl-3-buten-1-01 (11) (ca. 5050) was obtained as represented in reactions 1 and 2. On lowering the quantity of CpzTiCl2 relative to 3-butyn-1-01 yields improved significantly (see Table I). Using 10 mol % of CpzTiClz, 80-90% yields of ethylated products were obtained with the r...