Alkyl derivatives boron reagents

Alkyl derivatives of boron and alane can function as reducing reagents in a similar fashion. Two reagents of this type, disiamylborane and diisobutylaluminum hydride (DiBAlH) are included in Table 5.3. The latter is an especially useful reagent. 

The synthesis in Scheme 13.49 features use of an enantioselective allylic boronate reagent derived from diisopropyl tartrate to establish the C(4) and C(5) stereochemistry. The ring is closed by an olefin metathesis reaction. The C(2) methyl group was introduced by alkylation of the lactone enolate. The alkylation is not stereoselective, but base-catalyzed epimerization favors the desired stereoisomer by 4 1. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

The generality of the carbon monoxide insertion reaction is clear from reports that methylcyclopentadienyliron dicarbonyl (16), ethylcyclopentadienylmolylbde-num tricarbonyl (66), alkylrhenium pentacarbonyls (50), alkylrhodium dihalo carbonyl bisphosphines (34), allylnickel dicarbonyl halides (35), and mono-and di-alkyl derivatives of the nickel, palladium, and platinum bisphosphine halides (P), also undergo the reaction. The reaction of Grignard reagents (24), and of boron alkyls (51) with carbon monoxide probably takes place by the same mechanism. 

The first organic compound of boron was isolated by Frankland in 1862, by the interaction of zinc methyl and triethyl borate. This method and the use of zinc alkyls with boron trichloride was continued until the discovery of the Grignard reagent. In 1900 boron tri-methyl was prepared by the action of magnesium methyl iodide on trimethyl borate, and in 1921 a number of boron trialkyls were obtained by the interaction of magnesium alkyl halides and boron trifluoride. Of the derivatives of the type RgB, boron trimethyl is a gas, spontaneously inflammable in air boron triethyl, a spontaneously inflammable liquid whilst the higher members of the series are liquids, readily oxidised when exposed to air. The metl yL fpd ethyl derivatives combine with ammonia to foriii IliS type I BOH only... 

Derivatives of 9-BBN. Like other boron hydrides, a variety of proton sources (ROH, RCO2H, RSO3H, HX (X = Cl, Br, OH, SH, 02P(0H)2, NHR)) as well as boron halides can be effectively employed to prepare useful derivatives from 9-BBN-H. The synthetic value of B-MeO-9-BBN lies principally in the preparation of B-alkyl derivatives through organometallic reagents as described above. Asa byproduct in other processes, it is also easily converted to 9-BBN-H with BMS (eq 17). ... 

The order of reactivity of halides and triflates for the cross-coupling reaction of boron reagents is 1 > Br > OTf Cl. Thus, the sequential cross-coupling reactions of 4-bromophenyl triflate with two 9-alkyl-9-BBN derivatives obtained from two different alkenes furnishes the unsymmetrically disubstituted benzene derivative (Scheme 18). ... 

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. 

Alkyl, aryl, and allyl derivatives of boron can be prepared directly from the corresponding halides, BF3, and magnesium metal. This process presumably involves in situ generation of a Grignard reagent, which then displaces fluoride from boron.2... 

In a parallel study, Wipf and Fritch11041 have shown that also urethane-protected (Boc), and even amino acid segments, are tolerated as acyl compounds on the aziridine nitrogen. The best results were obtained with alkylcopper reagents derived from CuCN and an alkyl-lithium in the presence of boron trifluoride-diethyl ether complex. Some 6-alkylated compounds (11-15%) were isolated as well. This work was extended to a solid-phase procedure that resulted in resin-bound alkene isosteres that could immediately be used in further peptide synthesis.11051 For this purpose, the 2-nitrophenylsulfonyl (oNbs) group was used for nitrogen protection and aziridine activation. It could be readily cleaved with benzenethio-late, which was compatible with the acid-sensitive Wang linker used. 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

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