Chem. Rev. 1988. 86. 763-780 763 Synthetically Useful Reactions wlfh Metal Boride and Aluminide Catalysts BRUCE GANEM' and JOHN 0. OSBY Lbpam"ef of Chemistry, BBker Laboratory, Cornell University. Ithaca. New York 14853 Received February 27. 1986 (Revised Manuscript Received April 23. 1986) Contents 1. Introduction 763 A. Background 763 E. Catalyst Preparation 764 1. Borides 764 2. Aluminides 764 C. Catalyst Composition and Properties 764 1. Borides 764 2. Aluminides 765 D. Useful Reducing Systems 765 11. Reactions Invokina Borides and Aluminides 766 A. Hydrogenation of Alkenes and Alkynes 766 1. Borides 766 2. Aluminides 768 B. Reduction of Arenes 769 C. Reduction of Halies 770 Bruce Ganem was born in Boston. MA, in 1948 and attended 1. Borides 770 HaNard College. After graduate study with Gilbert Stork at C* 2. Aluminides 770 iumbia University and a National Institutes of Health postdoctoral D. Reduction of Nitriles 771 fellowship with W. S. Johnson at Stanford. he joined the Cornell E. Reduction of Nitro Compounds 774 faculty in 1974 where he became Professor of Chemistry in 1980. Dr. Ganem's early scientific interests in natural products synthesis. 1. Nitroaranes 774 synthetic methodology. and bioorganic chemistry have led recently 2. Nitroalkanes 774 to new interdisciplinary forays into biochemistry. enzymology. and F. Reduction of Other Nitrogenous Functional 774 immunology. Current interests include studies on the shikimic acid Groups biosynthetic pathway, the mechanism and function of carbohy- drate-processing enzymes. new chemistry of naturally occurring 1. Amides 774 polyamines, and the design and assembly of well-characterized 2. Oximes 774 synthetic vaccines. 3. Azoxy, Azo. Nitroso Compounds, and 775 Hydroxylamines 4. lsoxazoliiinas 775 G. Deoxygenation Reactions 776 1. Sulfoxides 776 2. Phosphine Oxides 776 3. Ethers and Esters 776 4. Aryl Ketones 776 H. Desulfurization Reactions 177 I,Miscellaneous Reactions 778 1. Formation of Amine Boranes 778 2. Hydroboration 778 3. Epoxide Opening 778 4. Deselenation 779 J. Conclusion 779 John 0. Osby was born in Denver. CO. in 1958 He received his I. Introductlon B.A. degree in Chemistry in 1980 from The Johns Hopkins Univ- ersity. After a one-year industrial position in inorganic analytical A. Background chemisby. he entered Corneli University where he joined Professor Bruce Ganem's research group. Dr. Osby's graduate work focused Since the pioneering discovery of nickel-catalyzed on the mechanism of coban(llt and nickel(1Ibpromoted sodium hydrogenation by Paul Sabatier, for which he won the borohydride and lihium aluminum hydrae reductions. He received Nobel Prize in 1912, organic chemists have been fas- his h.D. degree in 1985. and has since joined the Lbw Chemical cinated with transition metals and their compounds as Company as a research chemist in Midland, Michigan. promoters for other synthetically important reductions. In the past 40 years, metal hydrides, particularly so- reagents capable of reducing most functional groups. dium borohydride and lithium aluminum hydride, have Moreover by attaching organic ligands at boron or emerged as preeminent reducing agents in modern or- aluminum or changing the metal counterion, one can ganic chemistry.l.* These are extraordinarily versatile modulate the scope, regia-, and stereoselectivity of such 0009-2665/86/0766-0763$06.50/0 0 1986 American Chemical Societv 7G4 Chemical Reviews, 1986, Vol. 86, No. 5 Ganem and Osby reductions. Literally hundreds of substituted boron and TABLE I. NAB& Reduction of Transition-Metal Cations aluminum hydrides have been described in the chemical metal element boride metal element boride literature and dozens are now commercially a~ailable.~?~ Fe(II1) Fe(0) Fe(B) Pd(I1) Pd(0) Pd(B) More recently, transition metal salts have been used Co(I1) CozB Ad11 Ado) as catalysts or additives in conjunction with N&H4 and Ni(I1) Ni2B Os(VII1) Os(0) Os(B) LiAlH, to modify or enhance the properties of these Cu(1I) Cu(0) Cu(B) Ir(1V) Ir(0) Ir(B) reagents. Nearly every conceivable combination of salt Ru(II1) Ru(0) Ru(B) Pt(I1) Pt(0) Pt(B) and hydride has been investigated with the concomitant Rh(II1) Rh(0) Rh(B) development of many useful new synthetic method^.^ like nitrogen, or under hydrogen pressure.17 Borides The resulting systems are complex, however, and in have been deposited in the presence of a second (pro- most cases virtually nothing is known about mechanism moter) metal,18 on inert solid supports,lgor as colloidal or reactive intermediates. Boron and aluminum hy- suspensions on solvent-swollen polymers.20 As will be drides may combine with metal halides in several dif- seen, small variations in the method of preparat'Ion can ferent ways: (1) simple metathesis (e.g., LiCl + N&H4, dramatically affect the activity, selectivity, physical, and LiBH, + NaC1),6 (2) reduction of the metal halide to chemical properties of the boride.21 Cobalt(II), nick- the metal,7 (3) conversion of metal halide to metal hy- el(II), and copper(I1) salts uniformly produce borides dride: (4) some combination of (2) and (3), viz., FeC1, when treated with NaBH, in protic solvents. Iron(III), + LiBH, - Fe(BH4)2,9or (5) formation of a boride'O ruthenium(III), rhodium(III), palladium(II), osmium- or aluminide.ll Furthermore, it is often unclear whether (VIII), iridium(IV), and platinum(1V) salts afford black the metal salt serves a true catalytic function or whether precipitates which catalyze NaBH, decomposition. some transient, metalloidal complex formed in situ is They may be borides, zerovalent metals, or a mixture the actual reducing agent. Recently we had occasion (see Table I).5122 to probe the mechanism of several transition-metal- assisted hydride reductions which are of particular in- 2, Aluminides terest to synthetic organic chemists.12-14 The unam- biguous involvement of metal borides and aluminides The first aluminides of iron and cobalt were reported in case after case we studied prompted us to organize 30 years ago by Schaeffer and Ste~art.~~llReaction of the present review. CoBr2with LiAlH, in ether gave two mol of hydrogen and a black, pyrophoric precipitate of CoAl,. Likewise B. Catalyst Preparation LiA1H4 reacted with FeC13 to give as the ultimate products aluminum, FeAl,, LiC1, and H2 (eq 8). A series 1. Borides of five discrete processes was proposed to account for Historically, borides were first produced by the com- the overall stoichiometry in the latter process (eq 3-7). bination of boron with metallic or metalloidal elements less electronegative than itself. For the most part, FeC13 + LiA1H4 - FeC12 + AIHB + LiCl + 1/2H2(3) borides are very hard, high-melting, refractory sub- stances whose structures and stoichiometries do not FeC1, + 2LiA1H4 - LiCl + FeA12H, (4) conform to the ordinary concepts of valence. Borides FeA12H8 FeA12H6+ H2 (5) with low boron-to-metal ratios (M4B,M3B, M2B) con- - tain isolated boron atoms, however as the proportion FeAl2H6 FeA1, + 3H2 (6) of boron increases (M3B2, M4B3,M3B4), borides with - single and double chains of borons appear. Borides with AlH3 - A1 + 1.5H2 (7) formulae like MB4, MB6, and MB12 exist in three-di- mensional arrays with open networks of boron atoms overall: FeC1, + 3LiA1H4 - interpenetrating a regular metal atom lattice.15J6 3LiC1+ FeA12 +A1 + 6H2 (8) The industrial synthesis of borides usually involves (1) reduction of metal oxides using a mixture of boron C. Catalyst Composition and Properties carbide and carbon, (2) electrolysis, or (3) direct reaction of the elements. Some borides prepared in this fashion I. Borides possess good electrical and thermal conductivity prop- erties while others show promise as high-temperature The actual composition of borides prepared from semiconductors. A much simpler synthesis was dis- inorganic salts depends to a great extent on the specific covered by H. I. Schlessinger in his pioneering work on mode of preparation. Maybury, Mitchell, and Haw- borohydrides.1° Combinations of cobalt or nickel (or thorne analyzed nickel and cobalt borides prepared in other metal salts) with aqueous NaBH4 deposit finely ethanol under N2 using excess NaBH, and concluded divided black precipitates of Co2B and Ni2B (eq 1). that the stoichiometries Ni2B and Co2B inadequately represented their constitution. Besides containing 4NaBH4 + 2NiC12 + 9H20 - solvent and adsorbed hydrogen, which was released Ni2B + 3H3BO3+ 4NaC1 + 12.5H2 (1) upon heating, the solids were contaminated with tightly Because they actively catalyzed the decomposition of trapped NaC1. Freshly precipitated cobalt boride in borohydride,1° these borides have been commonly used water slowly liberated hydrogen with formation of boric as a practical, controlled source of hydrogen (eq 2). acid. On the basis of the meta1:boron ratio and the NaBH, 2H20 NaB02 4H2 amounts of H2 evolved, the formulae (Ni2B)2H3and + - + (2) (CO~B)~H~were suggested.23 Other versions of this synthesis have been conducted The effect of solvent on boride activity and selectivity in alcoholic or ether solvents, under an inert atmosphere was first demonstrated by C. A. Brown and H. C. Brown Boride and Aluminide Catalysts Chemical Reviews, 1986, Voi. 86, No. 5 705 who prepared a series of hydrogenation catalysts by the TABLE 11. Surface Characterization of Metal Borides reaction of NaBH, with Ni(OA& "P-1" nickel boride B/M B-H/ prepared in water was considerably more active than catalvst mecursor metal %" ratiob %B-I %B-I1 Mc Raney nickel and exhibited a markedly lower tendency Ni2B Ni(OAc), 61 0.54 51 49 0.45 to isomerize olefins. "P-2" nickel boride prepared in NiS04 86 0.55 58 42 0.37 ethanol could selectively hydrogenate olefins and dienes NiClz 83 0.47 56 44 0.32 of different substitution patterns, and converted alk- NiBr, 77 0.38 56 44 0.27 Ni(HC02)2 ca. 15 ynes stereoselectively into &-alkenes.% The cis-trans Ni(N03)* ca.