Hydroboration of 3-Hexyne: Free Radicals With Catecholborane but Stereospecific Addition of Dichloroborane

Abstract

The reaction of catecholborane with 3-hexyne was found to produce more of the frans-addition product, catechol (£)-3-hexenyl-3-boronate, than the (2)-isomer, a result totally inconsistent with the stereospecific c/s-addition of catecholborane to internal alkynes previously reported. Reaction in the presence of azobis(isobutyronitrile), a free radical initiator, increased the proportion of (£)-isomer to >90%. Reaction in the presence of galvinoxyl, a free radical inhibitor, led to some increase in the amount of c/s-addition product, but the frans-isomer still predominated. Attempted addition of dichloroborane to 3-hexyne using boron trichloride and triethylsilane was apparently superseded by boron trichloride addition, but the use of diethylsilane with boron trichloride resulted in efficient stereospecific ris-hydroboration. The resulting (Z)-3-hexenyl-3-boron dichloride was easily converted to the methyl ester with methanol. It was found that heat or radical catalysts would isomerize the (Z)-3-hexenyl-3-boronic ester to the (£)-isomer.

Asymmetric Synthesis of Serricornin via Boronic Esters

“…At the outset of this work, it was expected that hydroboration of 3-hexyne with catecholborane would produce a single regio- and stereoisomer, which could be converted to a single geometric isomer of 3-bromo-3-hexene, then to 3-lithio-3-hexene, for reaction with ( R )-DICHED ( S )-(1-chloroethyl)boronate ( 9 ) to form the propylidene analogue of (α-methylenealkyl)boronic ester 10 . However, commercial catecholborane with 3-hexyne yielded gross ( E , Z )-mixtures as a result of free radical side reactions, and ( Z )-3-hexenyl-3-boronic esters isomerize thermally to ( E , Z )-mixtures, reported in detail elsewhere . Such mixtures would have unduly complicated the characterization of a series of intermediates, hence, the use of 2-bromo-1-butene ( 8 ) instead.…”

Boronic Esters in Asymmetric Synthesis

“…A chemoenzymatic synthesis of pure 26 and 34, perhaps useful toward that end, has been reported recently. 31 Japonilure (35), the pheromone of the Japanese beetle Popillia japonica, is only attractive to the insects if its enantiopurity is very high. Attempts to homologate an alkynylboronic ester have not been successful, but the less demanding alkynylation of an (α-chloroalkyl)boronic ester provided the key step (Scheme 4).…”

“…34 Hydroboration of 3-hexyne with catecholborane yielded a boronic ester that was unexpectedly labile to free radical Z/E isomerization and thus could not lead to a useful bromoalkene intermediate. 35 Pure 2-bromo-1-butene was obtained via bromoboration of 1-butyne followed by protodeboronation, then converted to the Grignard reagent 36. 34 Reaction with chloro boronic ester 37 set up the remainder of the synthesis with the correct chiral director.…”

“…Instead, the masked ketone function was provided by an alkene that was subsequently oxidized (Scheme ) . Hydroboration of 3-hexyne with catecholborane yielded a boronic ester that was unexpectedly labile to free radical Z / E isomerization and thus could not lead to a useful bromoalkene intermediate . Pure 2-bromo-1-butene was obtained via bromoboration of 1-butyne followed by protodeboronation, then converted to the Grignard reagent 36 .…”