Aromatic boranes

Aromatic boranes are synthesized from diborane and arylmagnesium halides [i 5], arylmercury halides [iS5], or diarylmercury compounds [/S i]. Their oxidation furnishes phenols in variable yields. A large excess (up to 16 mol) of BH3 is necessary for high yields (equations 604 and 605). 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arac/tno-boranes tend to be more reactive (and less stable to thermal decomposition) than niiio-boranes and reactivity also diminishes with increasing mol wt. C/oio-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

The oxazaborolidines are easily prepared by heating ephedrine with borane dimethyl sulfide or the appropriate boronate ester. The aluminum reagent C is obtained by mixing ephedrine and trimethylaluminum. Borolidinc A is superior to its methyl derivative B and to the aluminum analog C. The diastereomeric borolidine obtained from borane and (S,S)-pseu-doephedrine failed to show any cnantioselectivity25. A variety of aromatic aldehydes can be enantioselectively alkylated in the presence of A, however, with heptanal the enantioselectivity is poor25. 

The tartrate ester modified allylboronates, the diisopropyl 2-allyl-l,3,2-dioxaborolane-4,5-di-carboxylates, are attractive reagents for organic synthesis owing to their ease of preparation and stability to storage71. In the best cases these reagents are about as enantioselective as the allyl(diisopinocampheyl)boranes (82-88% ee with unhindered aliphatic aldehydes), but with hindered aliphatic, aromatic, a,/l-unsaturated and many a- and /5-alkoxy-substituted aldehydes the enantioselectivity falls to 55-75% ee71a-b... 

Ketones can also be prepared by palladium-catalyzed reactions of boranes or boronic acids with acyl chlorides. Both saturated and aromatic acyl chlorides react with trialkylboranes in the presence of Pd(PPh3)4.233... 

The detection of the structural effects of B—C rr-bonding in unsaturated open-chain organoboranes should prove more difficult than in boracyclo-polyenes for several reasons. First, as already mentioned, there is the conformational freedom of the open-chain boranes, which reduces B(2pz)—C(2pz) overlap (cf. 17 vs. 19). Second, there is the linear overlap of 34 versus the cyclic, Htickel aromatic overlap of 35. 

Figure 20 The primary phosphine-borane adduct RPH2-BH3 (29) with the highly electron-withdrawing aromatic group R = p-CF3C6H4. (Adapted from ref. 49.)...

An aromatic C-H functionalization involving the m-addition of benzene to internal alkynes is mediated by a bimetallic palladium complex in the presence of catalytic amounts of a borane. The mechanism of process remains to be clarified (Equation (77)).73... 

New chiral oxazaborolidines that have been prepared from both enantiomers of optically active inexpensive a-pinene have also given quite good results in the asymmetric borane reduction of prochiral ketones.92 Borane and aromatic ketone coordinate to this structurally rigid oxazaborolidine (+)- or (—)-94, forming a six-membered cyclic chair-like transition state (Scheme 6-41). Following the mechanism shown in Scheme 6-37, intramolecular hydride transfer occurs to yield the product with high enantioselectivity. With aliphatic ketones, poor ee is normally obtained (see Table 6-9). 

RCOOH - RCHO (12,485). This borane (2 equiv.) reduces acids at room temperature to thexylboronic acid and aldehydes, which are best isolated as the sodium bisulfite adduct. Yields of aliphatic aldehydes are in the range 80-95%. Reduction of aromatic acids is slow, and yields are significantly lower. 

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