ORGANOBORANES .53. A HIGH-FIELD VARIABLE-TEMPERATURE H-1 AND B-11 NMR-STUDY OF THE EFFECTS OF SOLVENT AND STRUCTURE ON REACTIVITY IN ALLYLBORATION

In order to evaluate the importance of solvent, temperature, and structural effects on the rates of allylboration, the reactions of benzaldehyde with structurally representative allylboron reagents were examined under a variety of conditions by high-field variable-temperature 1H and 11B NMR spectroscopy. In general, polar solvents which are poorly coordinating enhance the rate of allylboration while solvents capable of relatively stronger coordination retard the rate. ɑ-Trisubstituted aldehydes undergo allylboration relatively slowly compared to the less substituted aldehydes. Among cyclic allylboronates, the 1,3,2-dioxaborolane derivatives, 14-17, undergo allylboration more rapidly compared to the allyl-1,3,2-dioxaborinane (11). In contrast to the cyclic allylboronates 14-16 which exhibit resistance to allylboration at -78 ²C, the B-allyl-l,3,2-dioxabenzoborole (17) undergoes exceptionally rapid allylboration (100%, <30 s, -50 ²C). While the reactivity of B-allyl-1,3,2-oxazaborolidines 18 and 19 is comparable to that of fi-allyl-1,3,2-dioxaborolane (16), the B-allyl-3-(p-tolylsulfonyl)-1,3,2-oxazaborolidine (20) undergoes allylboration remarkably rapidly, even at -78 ²C. The acyclic allylboronates 21-25 undergo, without exception, more rapid allylboration at 0 ²C compared to allyl-l,3,2-dioxaborinane (11). Moreover, the B-allylbis(benzyl-oxy)borane (24) and allylboronic acid (25) undergo allylborations relatively rapidly, even at -50 ²C. While the tartramide reagent 26 undergoes allylboration at -78 ²C relatively slowly, the tartrate ester derivative 3 (Roush's reagent) undergoes effortless allylboration under identical conditions. These individual variations in the rates of allylboration of the boronate derivatives can be rationalized essentially in terms of the relative availability of the lone pairs of electrons on the atoms attached to boron. The B-allyldiisopinocampheylborane (7) and B-allyldi(4-isocaranyl)borane (8) undergo instantaneous allylborations at -78 ²C and have thus proven to be among the most reactive of the allylboron reagents presently known. © 1990, American Chemical Society. All rights reserved.