Boron selected methods

Tab. 2.2-1. Selected methods for boron detection and quantification in tissue. 

As with the similarly polarized boron and aluminium hydrides, hydrido-silanes can transfer formal hydride ions to electropositive carbon centres. Unlike the first two reducing agents, hydridosilanes require additional activation of the carbon centre by Lewis or protic acids before such hydride transfer can take place. This overall process is known as ionic hydrogenation (4). The reagent system of triethylsilane and boron trifluoride etherate has provided an extremely selective method for the reductive deoxygenation of lactols (5), derived in turn from DIBAL reduction (6) of the corresponding y- or 6-lactones ... 

Ethers can be cleaved rapidly at room temperature by diborane and halogen 0-Demethylation with retention of O-tosyl groups can be achieved with boron trichloride Alcohols can be obtained by degradation of carboxylic acids through carboxy-inversion of acyl peroxides . Aldehydes can be efficiently decarbonylated under mild conditions in the presence of a rhodium complex . A new mild and potentially selective method for the prepn. of aliphatic... 

The fast rate of reduction of ArCHO by 3 as compared with simple B-alkyl-9-BBN derivatives is because of added stability imparted to the transition state of the empty p orbital on the boron atom in the additional 9-BBN substituent [10, 13]. The stabilization also leads the reaction to occur at lower temperature than for example, with B-s-Bu-9-BBN, a feature that favors a more selective process. As noted above 1,3-repulsions for the Me-C-C-(9-BBN) array are greater than for Me-C-C-Me, and this aspect disfavors the alternating transition state, leading to cis-vinylborane. Consequently, the combination of stereo- and electronic factors favor the transition state, and the whole process makes the preparation of trans 2 from 3 a highly efficient and selective method [8]. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

Amine boranes have been examined by a variety of spectroscopic methods (24—29). The boron-substituted alpha-amino acids have been utilized in animal model studies. These compounds along with their precursors and selected derivatives have been shown to possess antineoplastic, antiarthritic, and hypolipidemic activity (30—32). The boron amino acid analogues are also being evaluated for possible utility in boron neutron capture therapy (BNCT) (33). 

Scheme 5 details the asymmetric synthesis of dimethylhydrazone 14. The synthesis of this fragment commences with an Evans asymmetric aldol condensation between the boron enolate derived from 21 and trans-2-pentenal (20). Syn aldol adduct 29 is obtained in diastereomerically pure form through a process which defines both the relative and absolute stereochemistry of the newly generated stereogenic centers at carbons 29 and 30 (92 % yield). After reductive removal of the chiral auxiliary, selective silylation of the primary alcohol furnishes 30 in 71 % overall yield. The method employed to achieve the reduction of the C-28 carbonyl is interesting and worthy of comment. The reaction between tri-n-butylbor-... 

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

Scheme 2.7 gives some examples of the control of stereoselectivity by use of additional Lewis acid and related methods. Entry 1 shows the effect of the use of excess TiCl4. Entry 2 demonstrates the ability of (C2H5)2A1C1 to shift the boron enolate toward formation of the 2,3-anti diastereomer. Entries 3 and 4 compare the use of one versus two equivalents of TiCl4 with an oxazoldine-2-thione auxiliary. There is a nearly complete shift of facial selectivity. Entry 5 shows a subsequent application of this methodology. Entries 6 and 7 show the effect of complexation of the aldehyde... 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Several methods for the anti-selective, asymmetric aldol reaction recorded in the literature include (i) the use of boron, titanium, or tin(ll) enolate carrying chiral ligands, (ii) Lewis acid-catalyzed aldol reactions of a metal enolate of chiral carbonyl compounds, and (iii) the use of the metal enolate derived from a chiral carbonyl compound. Although many of these methods provide anti-aldols with high enantioselectivities, these methods are not as convenient or widely applicable as the method reported here, because of problems associated with the availability of reagents, the generality of reactions, or the required reaction conditions. 

---