Organoboranes in the synthesis and utility of organozinc compounds and six-membered aromatic heterocycles containing boron.

Abstract

Z-1-Iodo-1-hexen-1-ylpinacolboronate and two related compounds could be prepared by hydroboration of 1-iodo-1-alkynes. These pinacol $\alpha$-iodoalkenylboronates were converted to 1,1-organobimetallics of boron and zinc, and then treated with CuCN$\cdot$2LiCl to give 1,1-organobimetallics of boron and copper. These cuprates reacted well with electrophiles such as allylic halides, 1-iodo-1-alkynes, and $\alpha,\beta$-unsaturated ketones and esters, affording polyfunctional boronic esters. Oxidation of these adducts with alkaline hydrogen peroxide afforded polyfunctional ketones in good yield. 2-(1-Decyl)-1,3,2-dithiaborolane and three related compounds could be converted directly to the corresponding symmetrical dialkylzinc compounds upon boron-zinc exchange with diethylzinc. The functionalized dialkylzinc compounds thus obtained are not readily available by standard methods. These dialkylzinc compounds could be used in conjunction with CuCN$\cdot$2LiCl to form new carbon-carbon bonds upon treatment with the electrophiles mentioned above. The resulting adducts are formed in moderate to good yield. 1-Chloroboracyclohexa-3,5-diene could be treated with 3-(dimethylamino)propylmagnesium chloride followed by tert-butyllithium to afford lithium 1-(3-dimethylamino)propylboratabenzene. This lithium boratabenzene could be converted to $(\eta \sp 5$-1-(3-dimethylamino)propylboratabenzene)tricarbonylmanganese (I). The crystal structure of this manganese complex shows that it exists in the intramolecularly B-N-coordinated form with a B-N bond distance of 1.716(5) A. In toluene-d$\sb8$ solution however, the intramolecularly coordinated complex is in mobile equilibrium with its ring-opened isomer. Using $\sp{11}$B NMR spectroscopy, the equilibrium constants for this solution equilibrium have been measured over the temperature range $-$35 to 48$\sp\circ$C, allowing evaluation of $\rm\Delta H\sp\circ$ (6.0 kcal/mol) and $\rm\Delta S\sp\circ$ (23 cal/mol K). The base-catalyzed hydrostannation of diethynyl sulfide with dibutylstannane affords 4,4-dibutyl-1,4-thiastannin, which reacts with appropriate boron halides to give 4-methyl-1,4-thiaborin, 4-phenyl-1,4-thiaborin, and 4-chloro-1,4-thiaborin. Hartree-Fock-level ab initio calculations indicate a planar structure for 4-methyl-1,4-thiaborin, which is consistent with aromaticity. Low field $\sp1$H NMR signals for the thiaborins, as well as a red-shifted $\pi\to\pi\sp{\*}$ band in the ultraviolet spectrum of the methylthiaborin, also argue for some degree of aromatic character in these species. 4-Methyl-1,4-thiaborin was also converted to tricarbonyl($\eta\sp6$-4-methyl-1,4-thiaborin)molybdenum (0), which was characterized by single-crystal X-ray diffraction. Crystal data for this molybdenum complex show a striking similarity to the crystal data for borepinmolybdenum and thiophenechromium complexes, which indicates that thiaborin behaves as an arene ligand.