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Reduction of Organic Compounds with Diborane CLINTON F. LANE Aldrich-Boranes, Inc., Milwaukee, Wisconsin 53233 Received July 25, 1975 (Revised Manuscript Received October 6, 1975) Contents I. II. III. IV. V. Vl. VII. VIII. Introduction The Reagent A. Preparation B. Physical and Chemical Properties C. Reaction with Acidic Hydrogens D. Borane-Lewis Base Complexes Reductive Cleavage A. Alkenes and Alkynes B. Cyclopropanes C. Organic Halides D. Alcohols E. Ethers F. Epoxides G. Miscellaneous Reduction of Organic Sulfur Compounds Reduction of Organic Nitrogen Compounds A. Imines B. Oximes C. Nitro Compounds and Related Derivatives D. Nitriles Reduction of Organic Oxygen Compounds A. Aldehydes and Ketones B. Quinones C. Carboxylic Acids D. Carboxylic Acid Anhydrides E. Esters and Lactones F. Amides Conclusions References and Notes 773 774 774 774 775 776 777 777 778 778 779 780 782 782 783 783 783 784 784 785 786 786 789 790 793 793 795 796 797 /. Introduction Diborane, B2H6, was first isolated and characterized by Stock in 1912.1 His process involved the preparation and hydrolysis of magnesium boride to give a mixture of higher boron hydrides. Thermal decomposition of the higher boron hydrides then gave B2H6 along with other boron hydrides. Although this pioneering work by Stock must be considered truly remarkable,23 the process developed by Stock was extremely tedious and gave exceedingly low yields of diborane. In 1931, Schlesinger and Burg reported an improved method for the preparation of diborane which involved passing hydrogen and boron trichloride through a silent electric discharge.4 The major product was chlorodiborane, B2H5CI, which dispropor- tionate upon fractional distillation to yield diborane and boron trichloride. This procedure was satisfactory for the preparation of micro quantities of diborane, which was all that was required for the studies carried out by Schlesinger and co-workers on the Stock high-vacuum apparatus. Schlesinger and his students were attracted to the boron hy- drides because their formulas, which had been established without a doubt by Stock, did not conform to the then accepted theories of valence and molecular structure. As part of their investigation, the reaction of diborane with the carbonyl group in certain organic compounds was studied by Brown as part of his Ph.D. research.5 This is the first report on the use of B2H6 for the reduction of an organic compound. The reactions were studied on the high-vacuum line in the absence of a solvent using micro amounts of reactants. Furthermore, relatively complex equipment was required to prepare the diborane. Consequently, although the results are now considered of fundamental im- portance, at the time the results of this study were of negligible importance to synthetic organic chemistry. Fortunately, in 1940 a National Defense Project, initiated at the University of Chicago under the direction of Schlesinger and Brown, ultimately resulted in the development of large-scale processes for the preparation of both sodium borohydride and diborane.6 With only minor modifications these processes are now used commercially in the United States to prepare both sodium borohydride and diborane. Unfortunately, the results of these projects carried out at the University of Chicago during the war were not made public until a series of eleven articles ap- peared in 1953.8 While preparing these articles for publication, Brown again became interested in the use of diborane as a re- ducing agent for organic compounds. Also, since sodium bor- ohydride had become commercially available and provided a ready source for diborane, it was apparent that diborane should be of utility as a reducing agent for applications in synthetic or- ganic chemistry. A preliminary communication appeared in 19579 which was followed by a full paper.10 This investigation of the reduction of organic compounds by Brown and Subba Rao was responsible for the discovery of the hydroboration reaction which kindled vigorous activity in the study of organoboranes as intermediates in organic syntheses. Both the hydroboration reaction and the chemistry of organo- boranes have been reviewed by Brown and others.711-18 In these reviews, the use of diborane for the reduction of organic compounds is either barely mentioned or only briefly dis- cussed.19,20 The molecular structure, molecular properties, physical properties, anci preparation of diborane are covered in a recent review.21 Also, the reaction chemistry of diborane has been reviewed with the emphasis being on the reaction of diborane with inorganic elements and inorganic compounds.22 However, a comprehensive review devoted exclusively to the selective reduction of organic compounds with diborane and related bo- rane complexes has not appeared.23 In view of the increasing importance of selective reducing agents in synthetic organic chemistry, it was felt that such a review is both warranted and necessary. In this review the literature is covered through 1974 with several references from early in 1975. Originally, it was hoped that every reference which describes a reduction using a borane reagent could be included in this review. However, early in the literature searching it became apparent that such a compre- hensive coverage would not only be extremely difficult to obtain but would also probably not be appropriate for a journal review 773 774 Chemical Reviews, 1976, Vol. 76, No. 6 Clinton F. Lane article. Thus, a number of limiting factors have, by necessity, been introduced. The review deals exclusively with the use of diborane and borane-Lewis base complexes for the reduction of organic compounds. The substituted boranes, such as bis(3- methyl-2-butyl)borane,25 2,3-dimethyl-2-butylborane,26 and 9-borabicyclo[3.3.1]nonane,27 which are also useful reducing agents, are not covered in this review. To be consistent and objective it was decided that a publication must meet at least one of the following criteria before it would be cited in the review: (1) the reference must illustrate the selectivity of the reagent, (2) the reference must provide some insight into the mechanism by which the reagent operates, or (3) the reference must contain a detailed experimental section. A large number of borane reductions involve the use the bo- rane-tetrahydrofuran reagent, which will be abbreviated as BH3-THF. It should be understood that in all cases where BH3-THF is discussed, the reagent is actually a solution of the borane-tetrahydrofuran complex in tetrahydrofuran. Other ab- breviations used in this article are as follows. Ac Acyl Ar Aryl BMS Borane-methyl sulfide complex Diglyme Diethylene glycol dimethyl ether Et Ethyl Me Methyl N: Nucleophile Ts Tosyl //. The Reagent A. Preparation Diborane, BH3-THF, BMS, and various borane-amine com- plexes are all available commercially. A comprehensive cov- erage of the preparative chemistry of diborane is included in a recent review by Long.21 Thus, only a short discussion of the more convenient methods of preparation will be given here along with some recent results. Since sodium borohydride is available commercially at a reasonable price, this chemical is the starting material of choice for the preparation of diborane. For the vacuum-line preparation of small quantities of high-purity diborane, the Schlesinger-Burg process has been replaced by several more convenient pro- cedures. Diborane can be prepared in a vacuum line in good yield from the reaction of sodium borohydride and concentrated sul- furic acid.28 Sulfur dioxide, which is formed as a by-product, can be eliminated by the use of methanesulfonic acid in place of sulfuric acid.28 To obtain a purer sample, phosphoric acid is also recommended in place of sulfuric acid.29 A detailed literature procedure is available for the reaction of potassium borohydride with 85% orthophosphoric acid (eq 1).30 The yield is only 2KBH4 + 2H3PO4 —*• B2H6 + 2H2 + 2KH2PO4 (1) 40-50 %, but the purity of the diborane prepared by this method is excellent, only a trace amount of carbon dioxide (<0.1 %) is observed in some cases. Another potentially useful vacuum-line preparation of diborane involves the reaction of sodium borohydride in diglyme with either mercurous chloride (eq 2) or iodine (eq 3).31 The yield by either 2NaBH4 + Hg2CI2 diglyme> 2Hg + 2NaCI + B2H6 + H2 (2) 2NaBH4 + I2 d'glyme> 2NaI + B2H6 + H2 (3) of these methods is very good (88-90%) and the purity of the diborane is excellent (no detectable impurities). Unfortunately, a detailed experimental procedure is not available and there is no indication if these methods are useful on a preparative scale. Although diborane is available commercially in steel cylinders and can be handled safely by adequately trained personnel, it must be considered a very hazardous material. When large amounts of diborane are required for a preparative scale organic transformation, it is generally much safer to generate the dibo- rane as needed and pass it directly into a reaction mixture. Al- ternatively, the reagent can be purchased in the form of a bo- rane-Lewis base complex, such as BH3-THF or BMS. Both of these reagents are stable but still reactive and can be safely and easily handled. The reaction of sodium borohydride with boron trifluoride (eq 4), as developed by Brown and coworkers, is probably the most 3NaBH4 + 4Et2O-BF3 ^s™* 2 ^ H 6 ! + 3NaBF4 + 4Et2O (4) convenient method available for the preparative scale generation of diborane. The reaction was studied extensively by Brown and Tierney32 and experimental procedures for generating diborane have been reported.32'33 Further refinements in the process were later introduced,34 and a detailed experimental procedure is now available.35 Ih addition to the use of externally generated diborane or the use of preformed borane-Lewis base complexes for organic reactions, these reducing agents can also be prepared In situ in the presence of a reactive compound. A variety of procedures for the in situ generation of diborane were developed for use in the hydroboration reaction,36 and many of these procedures can be used for organic reductions. However, the presence of strong Lewis acids, such as boron trifluoride or aluminum chloride, can sometimes result In the formation of unexpected by-products (vide infra). Also, the starting material may react with the basic alkali metal borohydride (vide Infra). Consequently, for explor- atory work on a given reduction, It Is usually wise to first use either externally generated diborane or preformed BH3-THF. Almost invariably this results in fewer side reactions and a higher purity product. A recent report of a simple in situ hydroboration procedure is interesting and deserves mention. By this procedure a mixture of an alkene and sodium borohydride in THF is treated with glacial acetic acid to give an organoborane.37 Alkaline peroxide oxi- dation then gives a good yield of the corresponding alcohol (eq 5). It is unlikely that diborane is involved in this reaction. The in H-C4H9CH=CH2 ^3OK1H20O2" ' "-C4H9CH2CH2OH (5) 82% situ presence of alkene is probably required. In the absence of alkene, sodium borohydride reacts with acetic acid and forms the relatively unreactive complex 1 (eq 6).38 This mixture of ? ! CH3COH + NaBH4 —»> Na[CH3COBH3] (6) 1 sodium borohydride plus acetic acid is currently under investi- gation as an interesting new reducing agent.39,40 B. Physical and Chemical Properties The physical, chemical, and molecular properties of diborane were summarized in two recent reviews.21,22 Also, a complete compilation of the major physical and thermodynamic properties of diborane is available in a concise graphical format.41 The property data covered include critical constants, vapor pressure, heat of vaporization, heat capacity, density, viscosity, surface tension, thermal conductivity, heat of formation, and free energy of formation. The properties of BH3-THF and BMS are discussed in the present review in the section dealing with borane-Lewis base complexes. Sodium borohydride42 and LiAIH443 are both widely utilized Reduction of Organic Compounds with Dlborane Chemical Reviews, 1976, Vol. 76, No. 6 775 for the selective reductions of organic compounds. These re- agents react principally by nucleophilic attack on an electron- deficient center. Conversely, diborane, which is already elec- tron-deficient, is believed to function through attack on an electron-rich center in the functional group.5 4 4 Thus, diborane is an acidic-type reducing agent which exhibits markedly different selectivity than the basic-type reducing agents, sodium bor- ohydride and LiAIH4.9 This interesting difference in the reducing activity of diborane and sodium borohydride prompted an ex- tensive study of the reduction of organic compounds with dibo- 4CH,OH + B2H6 2(CH3O)2BH + 4H2 (8) rane 10,24 In addition to the Lewis acid character of borane, other im- portant chemical properties have enhanced the utility of borane complexes as reducing agents. Many reactions involving borane complexes have unusually low activation energies. Conse- quently, most reactions occur readily at room temperature or below. These low temperatures favor clean reaction mixtures with a minimum of side products. The solubility of diborane in ether solvents means that the reactions are usually homoge- neous, proceed without induction periods, and are easily con- trolled. Finally, the inorganic by-product of a borane reduction is usually an inert, water-soluble borate salt, which can be washed away over a broad pH range. All of these chemical and physical properties combine to make diborane one of the most chemically versatile compounds known. C. Reaction with Acidic Hydrogens Boron hydrides and other metal hydrides react rapidly and quantitatively with various acidic hydrogens (H-Y), liberating one mole of hydrogen per equivalent of hydride (eq 7). Both the ^ > B - H + H-Y — • ^ > B - Y + H2 (7) Y = O, S, or N acidity of the hydrogen and the ability of the donor atom Y to share a pair of electrons influences the rate of these reac- tions.24 The direct measurement of the volume of hydrogen gas pro- duced upon hydrolysis of a boron hydride provides a convenient and accurate method for the determination of either the purity of a boron hydride or the concentration of a boron hydride in an appropriate solvent.45 Also, a simple, rapid, and quantitative procedure for determining acidic hydrogens in organic materials has been developed based upon hydrogen evolution from a large excess of BH3-THF.46 The method is especially valuable for hydroxyl group determinations, and a precision of about 1 % is possible. In reactions of diborane with compounds containing acidic hydrogens, hydrogenolysis of the C-Y bond is usually not ob- served. Upon hydrolysis the alcohol, amine, thiol, or related functional group is regenerated unchanged. However, in a few specialized cases those benzylic alcohols which can readily form carbonium ions are transformed by diborane into the corre- sponding hydrocarbons (vide infra; section III.D). Even though the alcohol, thiol, and amine groups are normally recovered unchanged following a diborane reduction, their presence and reactivity must be considered when carrying out a diborane re- duction; i.e., sufficient diborane must be added to compensate for loss of hydride activity upon reaction with acidic hydrogens. Consequently, an understanding of the reactivity of diborane toward alcohols, thiols, and amines is important. 1. Alcohols The hydrolysis of borane with simple alcohols proceeds in stages. The first two hydrides react rapidly, but the third is so slowly hydrolyzed that the intermediate dialkoxyborane can be isolated. Using this reaction, dimethoxyborane (2) was first isolated and characterized by Burg and Schlesinger (eq 8).47 No evidence was found for dimerization of 2. Even in the presence of excess diborane, there was no indication of the formation of monomethoxyborane. Later investigations by Shapiro and co- workers, on the preparation of dimethoxyborane,48 diethoxy- borane,49 and diisopropoxyborane,50 substantiated the earlier results. For example, Shapiro found that when ethanol is added to a large excess of diborane, there is no detectable formation of EtOBH2 by ir analysis.49 Also, the diborane is quantitatively converted into diethoxyborane before there is any detectable formation of triethoxyborane. Even with excess ethanol, the rate of formation of triethoxyborane (triethyl borate) from diethoxy- borane is slow at room temperature.51 Stoichiometric evidence is also available which indicates that HB(OH)2 is formed as an intermediate in the hydrolysis of diborane in aqueous solutions at temperatures around —70 0 C. 5 2 Upon warming to room temperature, the remaining hydrogen is rapidly evolved. In the presence of excess BH3-THF, the rate of hydrogen evolution for alcohols decreases in the order: primary > sec- ondary > tertiary.24 The acidity of the hydroxylic hydrogen also decreases in this order. A factor, in addition to the acidity of the hydrogen, must be involved in these reactions because diborane reacts relatively slowly with phenol. The results can be ration- alized by prior coordination of BH3 with the alkoxy oxygen to give the intermediate 3 which decomposes with evolution of hydrogen (eq 9). Mass spectrometric evidence is now available for the H ROBH, + Ho (9) ROH + BH3 — - R O — B H 3 3 existence of an intermediate donor-acceptor adduct 3 in the reaction of borane with 2-propanol.53 Also, stoichiometric evi- dence points to the formation of a dihydrate of diborane (em- pirical formula B2H6-2H20) in the reaction of diborane with water a t - 1 3 0 0 C. 5 4 Cyclic dialkoxyboranes are formed by the reaction of diborane with 1,2- and 1,3-diols. For example, 1,3,2-dioxaborolane (4) . 0 2CH2—CHo + OH AH B2H6 EUO \ BH + 2H2 / 2 Me1N \ (10) BHiNMe-, / can be prepared through the reaction of ethylene glycol with diborane in diethyl ether and can be isolated as the trimethyl- amine adduct (eq 1O).55 The corresponding 1,3,2-dioxaborinane (5) can also be prepared from 1,3-propanediol.56 r°\ ( BH 5 The reaction of borane with polyalcohols apparently results in a chelation which must enhance the reactivity of the boron hydride. Thus, hydrolysis of all three hydrides occurs very rapidly in the presence of a polyglycol, such as glycerol or manni- tol.45 2. Thiols The ability of sulfur to bring d orbitals into hybridization with 776 Chemical Reviews, 1976, Vol. 76, No. 6 Clinton F. Lane s and p orbitals to form multiple bonds apparently exerts a pro- nounced effect on the products obtained through the reaction of diborane with alkanethiols. Thus, the first reported alkylthio derivative of diborane, methylthiodiborane,57 has the structure 6 based on NMR data.58 In this and other alkylthio derivatives of diborane, the alkylthio group occupies exclusively a bridging position, e.g., structure 7.58 H, CH, H S H B B C2H5 S S C2H5 H2BN^ / B H 2 C2H5 as a strong electron pair acceptor (Lewis acid) forming coordi- nation complexes with suitable electron donors (Lewis bases). Of the various known complexes, the borane-amine, borane- ether, and borane-alkyl sulfide complexes are all particularly interesting because of their wide range of physical and chemical properties. These borane-Lewis base complexes provide a convenient source of borane for use as a reducing agent. Another reason for discussing these complexes is