· Carbon Monoxide
· (MW 28.01)
(carbonylation of various organic compounds1)
Physical Data: mp -205.0 °C; bp -191.5 °C; d 1.250 g L-1 (d04 at 760 mmHg).
Solubility: appreciably sol some organic solvents, such as EtOAc, CHCl3, acetic acid.
Form Supplied in: cylinder types, valves, and pressure regulators.
Preparative Methods: may be generated by the dehydration of formic acid.
Purification: major impurities are CH4 (~0.2%), N2 (~0.5%), H2, and O2. O2 can be removed from CO by passing the gas through a short bed of reduced Cu or MgO2.
Handling, Storage, and Precautions: highly poisonous, odorless, colorless, and tasteless gas. All operations should be carried out in a fume hood.
Reactions with Carbocations.
The reaction of carbocations with CO leading to acylium cations is a key step in the Koch reaction. The Koch reaction produces tertiary carboxylic acids by treating alcohols with CO in a strong acid. It can be applied to alkanes, alkenes, and other compounds equivalent to alcohols under acidic conditions. A variety of acid systems, including Brønsted acids such as H2SO4, HF, and H3PO4, as well as in combination with Lewis acids such as BF3, AlCl3, SbCl5, and SbF5, are effective. The use of HCO2H as a CO source is a practical method for laboratory scale reactions and this method is often used as an in situ preparation of CO. The treatment of alcohols, including secondary ones, with HCO2H in 96% H2SO4
gives tertiary carboxylic acids (eq 1).2 Adamantane is also carbonylated to 1-adamantanecarboxylic acid (eq 2).3 Besides the HCO2H method, the Koch synthesis can also be conducted at normal pressure of CO in the presence of copper or silver salts. Cyclohexene, for example, yields only 1-methylcyclopentanecarboxylic acid via ring contraction of the first-formed carbocation under 1 atm of CO (eq 3).4 The Gatterman-Koch reaction5 is the formylation of aromatic hydrocarbons with CO and HCl in the presence of AlCl3 (eq 4).6 Other catalytic systems such as HF/BF3, HF/SbF5, HF/CF3SO3H/BF3, and CF3SO3H have been investigated and found to be effective for this reaction. Formylation and
sulfonation take place when aromatic compounds are exposed to an atmosphere of CO at 0 °C in a HSO3F/SbF5 system (eq 5).7
Reactions with Carbanions.
The reaction of organolithium reagents with CO gives carbonyl lithium reagents (acyllithium or aroyllithium), which have not been utilized in practical synthetic reactions until recently. Difficulties in controlling the reaction of carbonyl lithium reagents are attributed to their extremely high reactivity. Acyllithium can be trapped by Chlorotrimethylsilane under extremely careful reaction conditions (eq 6).8 A controlled, slow-rate addition of alkyllithium to a solution of Me3SiCl saturated with CO at -110 °C is recommended. Direct nucleophilic acylation of ketones by in situ generated acyllithium reagents leading to a-hydroxy ketones has been described (eq 7).9 The use
of aldehydes,10 lactones,11 CS2,12 isocyanate13 disulfide,14 and carbodiimide15 as trapping electrophiles has also been reported. Acylcuprate reagents successfully undergo nucleophilic 1,4-addition to a,b-unsaturated ketones and aldehydes (eq 8).16
The reaction of an a-silylalkyllithium with CO is a convenient access to acylsilane enolates, which can be trapped by electrophiles such as H+, Me3SiCl, and benzaldehyde (eq 9).17 The formation of the enolates takes place in a highly stereoselective manner to give (E) enolates.
Reactions with Radicals.
The reaction of alkyl halides with Tri-n-butylstannane in the presence of Azobisisobutyronitrile (0.1-0.2 equiv) under CO pressure (65-80 atm) leading to aldehydes proceeds via the reaction of alkyl radicals with CO to give acyl radicals followed by abstraction of a hydrogen atom from HSnBu3 (eq 10).18 Aromatic halides can also be converted to aromatic aldehydes.19 The use of Tris(trimethylsilyl)silane (TTMSS) in place of HSnBu3 as a radical mediator permits the carbonylation under a lower pressure of CO (20 atm).20 The radical carbonylation is applied to carbonylative cyclization of 4-alkenyl halides to cyclopentenes (eq 11).21 The intramolecular trapping of acyl radicals by alkenes is utilized for the synthesis of functionalized unsymmetrical ketones.22
The carbonylation of alkynes in the presence of thiols initiated by AIBN gives b-alkylthio-a,b-unsaturated aldehydes (eq 12).23
Transition Metal-Catalyzed Reactions.
Ring-opening siloxymethylation to 1,3-diol derivatives takes place in the reaction of cyclopentene oxide with HSiEt2Me and CO in the presence of Co2(CO)8 (eq 13).24 The reagents ML
n/HSiR3/CO provide several important synthetic methods, which are covered under the reagent headings of Octacarbonyldicobalt-Diethyl(methyl)silane-Carbon Monoxide, Dodecacarbonyltetrarhodium-Dimethyl(phenyl)silane-Carbon Monoxide, and Tricarbonylchloroiridium-Diethyl(methyl)silane-Carbon Monoxide.
Hydroformylation has been an extremely important industrial process, and consequently has been the most extensively studied of all carbonylation reactions.25 A wide variety of metals such as Pt, Co, Rh, Ir, and Ru exhibit catalytic activity for hydroformylation of alkenes. Among them, Rh has been preferred for laboratory use because of its higher activity (eq 14).26 Various phosphine-modified catalysts such as Co2(CO)8/phosphine, (phosphine)PtCl2/SnCl2, and phosphine-modified Rh have been examined extensively to obtain high selectivity for the more desirable linear aldehydes. Practical, regioselective hydroformylation of functionalized terminal alkenes can be achieved by using a bis-organophosphite ligand (1).27 Catalytic systems
using diphosphines with natural bite angles near 120° increase regioselectivity for straight-chain aldehydes.28 Highly selective formation of branched-chain aldehydes is attained by use of a zwitterionic Rh complex (eq 15).29 Hydroformylation using PtCl2/SnCl2 in the presence of chiral ligands such as (-)-DIPHOS, (-)-DIOP, and chiraphos is reported to produce moderate enantioselectivity.30 The highest level of enantioselective discrimination is realized with PtCl2/SnCl2 and (2S,4S)-4-(diphenylphosphino)-2-[(diphenylphosphino)methyl]pyrrolidine ((-)-BPPM) in the presence of Triethyl Orthoformate, which converts the product aldehyde to its diethyl acetal, although the branched/linear ratios are low.31
Hydrocarboxylation or hydroesterification is also an important process of CO. The asymmetric synthesis of (+)-ibuprofen and (+)-naproxen is attained based on asymmetric hydrocarboxylation of vinylarenes using (S)-(+)-1,1´-binaphtyl-2,2´-diyl hydrogen phosphate ((S)-BNPPA) (eq 16).32
Terminal alkynes may be carbonylated to acetylenecarboxylic esters in the presence of PdCl2 as catalyst and CuCl2 as reoxidant in the presence of NaOAc (eq 17).33 Hydroesterification of alkynes to a,b-unsaturated esters is catalyzed by [P(p-Tol)3]2PdCl2/SnCl2,34 PdCl2/CuCl/HCl/O2,35 and (PPh3)2PtCl2/SnCl2.36 Pd-catalyzed intramolecular carbonylation of alkynyl alcohols is an attractive route to a-methylene-g-lactones (eq 18).37
Ketones and aldehydes are formed using CO. Pd-catalyzed carbonylation of aryl or vinyl halides (or triflates) is an attractive method for a wide variety of carbonyl compounds. Carbonylation of aryl halides in the presence of hydrogen and a tertiary amine leads to the formation of aldehydes (eq 19).38 Use of hydrogen donor such as HCO2Na,39 poly(methylhydrosiloxane) (PHMS),40 HSiEt3,41 and HSnBu342 also gives aldehydes. The Pd-catalyzed carbonylative coupling of aryl triflates with organostannanes is a potentially valuable route to unsymmetrical ketones (eq 20).43 Organozinc reagents may be used in ketone syntheses.44 Esters are obtained from aryl, vinyl, and heteroaryl halides (triflates) via the Pd-catalyzed carbonylation in
the presence of alcohols.45 The intramolecular version has been utilized for the preparation of a-methylene-g-lactones from vinyl halides (eq 21).46 The synthesis of lactams is also achieved in the presence of Pd(OAc)2/PPh3.47 Aryl halides may be doubly carbonylated to a-keto amides in the Pd-catalyzed carbonylation of aryl halides with amines (eq 22).48
The catalytic carbonylation of benzyl chloride to arylacetic acid derivatives is shown to occur with Pd complexes as catalysts in the presence of a base under mild reaction conditions (eq 23).49 Co2(CO)8-catalyzed carbonylation under phase transfer conditions (organic solvent/aq NaOH system containing catalytic amounts of quaternary ammonium salt) is also effective for the transformation of benzyl chlorides to arylacetic acids.50
Pd-catalyzed cyclocarbonylation of aryl- or heteroaryl-substituted allyl acetates with Ac2O gives bicyclic aromatic systems (eq 24).51 Allylic phosphonates are cleanly carbonylated to b,g-unsaturated amides in the presence of a catalytic amount of Rh6(CO)16/NBu4Cl (eq 25).52 Use of NH4Cl/Et3N in place of PhCH2NH2 is found to be effective for the preparation of primary amides. Under more harsh reaction conditions (50 atm, 110 °C), allylamines may be carbonylated directly to b,g-unsaturated amides in the presence of Pd(OAc)2/dppp.53 The Pd-catalyzed carbonylation of propargyl carbonates in MeOH affords allenic esters (eq 26).54
Exposure of an azirine to CO in the presence of Pd(PPh3)4 affords bicyclic b-lactams (eq 27).55 2-Arylaziridines are carbonylated with retention of configuration in the presence of [RhCl(CO)2]2 to give monocyclic b-lactams (eq 28).56 Similar carbonylative ring-expansion reactions are observed in the Rh-catalyzed reaction of azetidine-2,4-diones,57 Co-catalyzed reaction of azetidines,58 and Co/Ru-catalyzed reaction of oxetanes.59
Cyclic ethers, including oxiranes, oxetane, and tetrahydrofuran, react with CO and N-(trimethylsilyl)amine in the presence of Co2(CO)8 to form siloxy amides (eq 29).60
N-acyl-a-amino acids can be synthesized by amidocarbonylation of aldehydes with acetamide and synthesis gas (eq 30).61 It is possible to achieve a direct, one-step synthesis of N-acyl-a-amino acids from precursors other than aldehydes themselves. For example, allyl alcohols can be converted to N-acyl-a-amino acids via isomerization of allyl alcohols to aldehydes followed by amidocarbonylation (eq 31).62
Heterocyclic rings can be constructed by oxidative carbonylation under mild reaction conditions. The formation of a pyran ring is achieved by oxidative carbonylation of hydroxy alkenes (eq 32).63 Transformation of allenic amines to a-heterocyclic acrylic acid derivatives is also carried out under ambient conditions (eq 33).64
CO acts as reducing agent in the water gas shift reaction (WGSR).65 Application of WGSR in organic synthesis includes hydrogenation, related reductions, and carbonylation. Ru-catalyzed reduction of aromatic nitro compounds to aromatic amines under mild reaction conditions (at rt under 1 atm of CO) has been achieved (eq 34).66 Montmorillonite-bipyridine-Pd(OAc)2/Ru3(CO)12 is an active catalyst for the reductive carbonylation of aromatic nitro compounds to urethanes (eq 35).67 The reaction is highly selective and no side products such as azo, azoxytoluene, or N-methylanilines are detected.
Selenium effectively catalyzes the carbonylation of various amines with CO and O2 to give urea derivatives quantitatively (eq 36).68 Turnover numbers of Se reach 104 by using 0.8 mg of Se (0.01 mmol) for the carbonylation of benzylamine (200 mmol). The reaction can be applied to the synthesis of cyclic ureas.69
CO forms Lewis acid-base complexes with organoboranes. When the organoborane is heated with CO to 100-125 °C, a tertiary alcohol is obtained after workup by oxidation (eq 37).70
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Naoto Chatani & Shinji Murai
Osaka University, Japan
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