Boranes alkoxy

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... 

Results of the asymmetric 2-propenylborations of several chiral a- and /i-alkoxy aldehydes are presented in Table 11 74a-82 84. These data show that diisopinocampheyl(2-propenyl)borane A and l,3-bis(4-methylphenylsulfonyl)-4,5-diphenyl-2-propenyl-l,3,2-diazaborolidine C exhibit excellent diastereoselectivity in reactions with chiral aldehydes. These results are in complete agreement with the enantioselectivity of these reagents in reactions with achiral aldehydes (Section 1.3.3.3.3.1.4.). In contrast, however, the enantioselectivity of reactions of the tartrate 2-propenylboronate B (and to a lesser extent the tartrate (/i)-2-butenylhoronate)53b is highly... 

AuBerst selektive 1,2-Reduktionsmittel sind Bis-[2-methyl-propyl]-aluminiumhydrid1, Tris-[2-methyl-propyl]-aluminium2, Natrium-bis-[2-methyl-propyl]-dihydrido-aluminat3 und besonders 9-Bora-bicyclo[3.3. ljnonan4 (9-BBN, S. 7). Letzteres ergibt zuerst ein Alkoxy-boran, das zweckmaRig mit 2-Amino-athanol gespalten wird. 

There is no systematic study of the effect of borane reagents, and the best choice would be highly dependent on the catalysts and substrates. A series of di(alkoxy)bo-ranes have recently been synthesized and subjected to hydroboration of cyclopen-tene at ambient temperature in the presence of RhCljPPhjjj (Scheme 1-2) [18]. The... 

Alkoxy groups can be displaced from boron by alkyl- or aryllithium reagents. The reaction of diisopropoxy boranes with an organolithium reagent, for example, provides good yields of unsymmetrically disubstituted isopropoxyboranes.3... 

To improve the levels of selectivity in additions to chiral aldehydes, it is possible to resort to the tactic of double diastereoselection with the use of chiral allylic boranes and boronates (see section Double Diastereoselection ). Bis(isopinocampheyl) allylic boranes and the tartrate allylic boronates (see following section), in particular, are very useful in the synthesis of polypropionate natmal products by reaction with a-methyl and a-alkoxy functionalized aldehydes. 

Boron trifluoride etherate was used in conjunction with the reducing agent borane to rearrange aromatic O-triisopropylsilyl ketoximes to cyclic and acyclic aniline derivatives. The steric hindrance of the substituents on the silicon atom, the size of the aliphatic ring and the presence of alkoxy substituents on the aryl group played important roles in the aniline formation. 

The residue contains (R)-p-methylbenzenepropanol and an alkoxy borane species that undergoes quantitative hydrolysis to (R)-p-methylbenzenepropanol during the subsequent chromatography step. As an alternative to flash column chromatography, the alkoxy borane species can be cleaved by treatment of the residue with 1 N aqueous sodium hydroxide solution. 

The stability of the coordinative bond in the borane complexes is determined by the electrophilicity of the borane, by the nucleophilicity of the ligand and by steric factors. The electrophilicity of the borane, in turn, depends on the electronegativity of the groups bonded to the boron and on the overlap between the unoccupied p orbital of the boron and the occupied p orbitals of the atoms directly bonded to it (p -p interaction). The predominance of the latter is reflected by the fact that the relative acidities of halogenoboranes increase in a sense opposite to that expected from electronegativity considerations and is supported by the very weak Lewis acidity of the alkoxy- and amino-boranes. The importance of steric factors is pointed out by Brown and co-workers.13-15... 

Boron Hydride Derivatives. Alkyl boranes can be prepd by alkylation of key intermediates, of diborane or of higher boranes. Alkyiation of a borane will proceed more readily if a functional group, such as a halogen or an active merai arom, is attached to the borane. For example, a halo-borane may react with a metal aikyl to produce an alkyl borane, or a metal polyhydropolyborate may react with an alkyl halide to produce an alkyl borane(Ref 35). Boranes may be alkylated or ary-iated with an unsaturated hydrocarbon(Ref 13). Alkoxy derivs of boranes can be prepd by allowing a borane to react with an appropriate alccho (Ref... 

Fu and co-workers have shown that the l-chloro-boracyclohexa-2,4-diene 97 reacts with 4-phenylpyridine at room temperature to provide the />-terphenyl analog 26 in 76% yield as an early example of a borabenzene-pyridine-type complex (Equation 6). The X-ray structure of 26, discussed in Section 7.14.3, shows the three rings not to be coplanar <1997OM1501>. Exposure of the l-chloro-boracyclohexa-2,4-diene 98 to a stabilized carbene results in formation of the complex 29 in 83% yield as an air- and moisture-sensitive colorless powder (Equation 7), crystals of which were grown from toluene the X-ray structure of 29 has been discussed in Section 7.14.3. The use of bulky lithium bases to deprotonate the bis(amido) or bis(alkoxy)boranes 19 allows for the formation of the dianionic 2,2 -diboratabiphenyl derivatives 20 (X = NHPh, OBn), the solid-state structures of which have been discussed (Equation 8) <2006CJC81>. 

Migration of tertiary alkyl groups in the dichloromethyl methyl ether reaction does however mean that ketones cannot be synthesized using thexylborane and the approach adopted in the carbonylation and cyanidation processes. In order to obtain ketones by this route, boranes bearing chloro or alkoxy groups, i.e. groups of very low migratory aptitude, have to be used as substrates (Equation B3.17). 

Reaction of a 5-alkoxy derivative with NaBD4 gave the 5,5-dideutero derivative exclusively <87JHC25l>. Reduction of the compounds (79) with borane in THF gave the amines (81), whose oxidation with DDQ in AcOH afforded the derivatives (80) <92JMC320>. 

In contrast to the dihydroxylations, the hydroboration of allylic alcohols takes place without an inside alkoxy effect in the sense 5.132 with the oxygen atom treated as the medium-sized group. It is probably significant that the inside alkoxy effect is most noticeable with reagents which are relatively electrophilic in nature, and not with boranes, which are only mildly electrophilic. 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

---