Haloallenes

Some prapargylic halides can be converted into haloallenes by treating them with copper[I) halide and lithium halide, preferably in THF as solvent. A catalytic amount of the copper salt, which forms a soluble complex with lithium halide, is... 

The reaction of 3-substituted 3-haloallenes with various cuprates, the converse reaction of propargyl derivatives, proceeds in an SN2 manner to form alkynes 69b. Very high anti stereoselectivity is achieved693. 

The solvolysis of a number of alkyl substituted haloallenes, 244, also has... 

Scheme XVIIl. Mechanism of Solvolysis of Trisubstituted Haloallenes...

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

Scheme 4.15 A highly flnt/-selective synthesis of haloallenes and prediction of stereochemistry of haloallenes by optical rotations with revised parameters.

For syntheses of optically active allenes by the reaction of haloallenes in the presence of a palladium catalyst, see (a) C. J. Elsevier,... 

The aryl bromide 40, prepared from cross-coupling between 1,2-dibromobenzene and (trimethylsilyl)acetylene, was converted to the corresponding arylzinc halide 41a and arylboronic acid 41b for subsequent coupling with the haloallenes 42 to produce the benzannulated enyne-allene 43 in -40% yield (Scheme 20.10) [38]. Desilylation with tetrabutylammonium fluoride (TBAF) then afforded 44 in 67% yield. 

Table 2. Haloallenes from (2-Propynyl) Methanesulfonates by Halocuprate(i)-Mediated 1,3-Sub-stitution with Chirality Transfer R2...

Reaction of trialkyl-substituted haloallenes with organocopper(II) reagents generally leads to chiral a-alkynylalkanes via 1,3-substitution79-82 83 85. 

Numerous haloallenes are known, but the perhaloallenes of the type XjC - C CXj were unknown until a study at the Univ of Calif, LA (Ref 5) was undertaken as part of the US Dept of the Army Contract DA-04-495 ORD-527. It is expected that compds such as tetrafluorocdlene, F,C C CF2 will be of interest as monomers for both homopolymerization and copolymerization studies and for the prepn of materials similar to Teflon which is the homopolymer of tetra-fluoroethylene F2C CF2. This study is being continued as of 1958 Refs l)Beil 1, 248,(107),[223] 19221... 

The trimethylsilyl group is easily replaced by a halogen to yield the corresponding halide. Propargyltrimethylsilanes react with bromine or iodine to yield 3-haloallenes (equation 1 90. This method has been applied in eicosanoid synthesis907,908. 

The situation is somewhat more complex for 1-haloallenes. With chlorides or bromides, Pd(PPh3)4 and PhZnCl or Ph2Zn, predominant inversion of configuration is observed with the corresponding iodides, variable levels of retention are realized132. 

Numerous haloallenes are known, but the perhaloallenes of the type X,C C = CX, were unknown until a study at the Univ of Calif, La (Ref 5) aras undertaken as part of the US Dept of the Army Contract DA-... 

Lithium alkynylcuprates react with haloallenes to give similar skipped diacetylenes (see below). The related skipped enynes can be prepared by treatment of (pentadienyl)iron(tricarbonyl) halide complexes with dilithium trialkynylcuprates, the compounds being isolated as the iron(tricarbonyl)(diene) complexes (Scheme 4). Further examples of alkylation reactions of copper alkynides are illustrated in Scheme 5. Reaction between a lithium cyanoaikynecuprate and an iodoallene leads to a skipped diacetylene. This useful reaction has been used by Corey in his synthesis of hybridalactone (Scheme 6). °... 

Saalfrank, R.W., Welch, A., Haubner, M. and Bauer, U., l-Halo-l-acceptor-/l,l-diacceptor-substituted allenes. Part 9. Functionalized allenes, haloallenes, and bwallenes via [2,3]/[3,3]-sigmatropic rearrangements and their reactivity, Liebigs Ann., 171, 1996. 

---