Bromoboration of l-hexyne with BBr, followed by treatment with 2-propanol produced alkenyl boronic ester 3. Subsequent Pd(O)-catalyzed cross-coupling with acetylenic zinc chlorides and iodination furnished enyne iodides 6, which were then converted to enyne-allenes 8 by a second Pd(O)-catalyzed reaction with allenic zinc chlorides. We recently reported a new synthetic route to (Z)-1,2,4-heptatrien-6-ynes (enyne-allenes) and the use of these enyne-alIenes for synthetic applications through biical intermediates.1 The ea8e of initiating the Myers cyclization reaction under mild thermal conditions to produce the cz,3dehydrotoluenc biradicals2 coupled with the ability to move the radical center8 around within the molecule provide opportunitie8 for generating other reactive species for synthetic elaborations. We have since developed an alternative synthetic method for enyne-allenes by bmmoboration of a terminal allcyne with BBrs followed by consecutive Pd(O)-catalyzed coupling reactions with organozinc chloride8 derived from terminal alkynes and allenes. The simplicity of the procedure together with the ready availability of a variety of terminal alkyne8 and allenes make it possible to synthesize enyne-allenes with diveme chemical structunc8. Bromoboration of I-hexyne (1) with BBr3 3 followed by treatment with 2-propano14 furnished alkenylboronic ester 3 in 79% isolated yield (Scheme 1).
Conversion of the initially formed alkenyldibromoborane 2 to the chemically more stable 3 allowed the isolation of 3 by distillation. Because bromoboration of terminal alkynes with BBr3 proceeds with cis addition of the bromine-boron bond to the triple bond, 3 having predominantly the 2 geometry (Z:E = 96:4) was obtained. We also observed that 3 appeared to isomerize slowly to produce a substantial amount of the E isomer on storage. This isomerization process may he responsible for the presence of about 4% of the E isomer in the initially isolated material.
Conversions of 3 to enyne iodide8 6 were achieved in a one-pot operation by first using the Pd(O)-catalyzed cross-coupling reaction of 3 with alkynylzinc chloride8 4,35 prepared by treating the corresponding 1-lithio-1-alkyncs with aIihydroU8 zinc chloride, to product enyne boronic ester8 5. Alkenylboronic ester8 have been shown to be easily converted to the comsponding alkenyl iodide8 with retention of configuration by simply aating with iodine.4*6 Indeed, the transformation was also successful in the cases with 5 to yield 6 (R= n-Bu, 55% isolated yield, ZE = 964; R = Ph. 64% isolated yield, Z:E = 96~4).
The use of Pd(0) as a catalyst to promote cross-coupling of alkenyl iodides and allenic organomctallic8 to afford conjugated allenenes has been shown to be very efficient under mild reaction temperatums and with retention of configuration of the alkenyl group. ' The mildness of the reaction condition make8 this synthetic protocol especially attractive and suitable for the purpose of producing thermally labile enyne-allenes from 6. It was gratifying to observe that the Pd(O)-catalyzed cross-coupling hetween 6 and allenylzinc chlorides 7 proceeded smoothly to furnish the desired enyne-allencs 8 in good isolated yields and with high geometric purity (Table ). Enyne-allenes 8 were stable enough to allow purification by column chromatography at room temperature. However. cycloaromatization did appear to occur on prolonged storage (approximately 10 days) at 0' C.