Carbonylation mediated

Scheme 2.29 Addition to a, 3-unsaturated carbonyls mediated by Cu-NHC complexes...

Well-known double-addition product 58, obtained by an iron carbonyl-mediated reaction sequence from the corresponding cyclic diazenes and alkynes <1975JOM(101)231>, when treated with bromine was reported to give the... 

In acyclic secondary -allylic alcohols, epoxidation by the vanadium system shows opposite stereospecificity to that of peracid and molybdenum carbonyl-mediated epoxidation (see Scheme 6)22 The predominance of the erythro isomer in the former process is rationalized22 in terms of the energetically more favorable transition state (6, cf. 5) and in this context the mechanism has analogy in the epoxidation behavior of medium-ring cyclic allylic alcohols.23... 

The formation of l,3,4-triarylpyrroline-2,5-diones from the reaction of diphenylcyclopropenone and N-sulfinylarylamines with nickel carbonyl has been described earlier (see Scheme 36 in Section IV,A,2).64 In contrast with this result, 2,4,5-triphenyl-3-isothiazolone 1-oxide is produced in this type of process using iron pentacarbonyl, and the analogous cyclohexyl derivative is formed in a nickel carbonyl-mediated reaction (Scheme 115).64 It is probable that metallocyclic species (cf. 93) are intermediates in these transformations. 

Scheme 6.49 Cobalt carbonyl-mediated synthesis ofdiaryl ketones.

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

A viable iron carbonyl-mediated reduction process converts acid chlorides and bromoalkanes into aldehydes [3, 6]. Yields are high, with the exception of nitro-benzoyl chloride, and the procedure is generally applicable for the synthesis of alkyl, aryl and a,(i-unsaturated aldehydes from the acid chlorides. The reduction proceeds via the initial formation of the acyl iron complex, followed by hydride transfer and extrusion of the aldehyde (cf. Chapter 8). 

In many respects the apparently analogous reduction of nitroarenes with triruthenium dodecacarbonyl under basic phase-transfer conditions is superior to that of the iron carbonyl-mediated reductions. However, the difference in the dependence of the two processes on the concentration of the aqueous sodium hydroxide and the pressure of the carbon monoxide suggests that they may proceed by different mechanisms. Although the iron-based system is most effective under dilute alkaline conditions in the absence of carbon monoxide, the use of 5M sodium hydroxide is critical for the ruthenium-based system, which also requires an atmosphere of carbon monoxide [11]. The ruthenium-based reduction has been extended to the... 

A one-atom cobalt carbonyl-mediated ring expansion of the 3,6-dihydro-2//-1,2-oxazincs 56 provided access to the 4,7-dihydro-l,3-oxazepin-2(3//)-ones 57 in modest yields (Equation 11) <1996TL2713> however, these conditions were quite severe with carbon monoxide at 1000 psi being required for the cobalt carbonyl used. 

A noteworthy work on complexes of this type has been on the oxyallyl complexes (171) and their related dimers (172). Despite the fact that only electrophilic substitution is observed in the reactions of these complexes and their derived (oxyallyl)Fe(CO)4 cations, these compounds, or structurally very similar ones, have been proposed as intermediates in iron carbonyl mediated [4 + 3] cycloaddition reactions of a ,a -dibromoketones and dienes. " ... 

The iron carbonyl-mediated cycloaddition is also highly stereospecific despite a nonconcerted mechanism. In the reaction of dibromo ketone (57) with m-p-deuteriostyrene, only the cis stereoisomer of the adduct is detected at low conversion (equation 50). - This stereospecificity is attributed to the formation of a U-shaped intermediate, in which the bonds are fixed rigidly by charge-transfer or coulombic attraction between the iron enolate and the cationic center. The result can also be explained by rapid bond formation between the two termini before bond rotation can occur. Traru-anethole also couples stereos-pecifically with (57) (equation 51). ... 

Treatment of 5,6-dihydro-4/7-l,2,5-oxadiazines (38) with an iron carbonyl complex results in deoxygenation and ring contraction to the 2,4-disubstituted imidazoles (40) in 31-93% yield <83CL34l>. The effectiveness of the iron carbonyl decreases in the order Fe3(CO)i2> Fe2(CO)9 > Fe(CO)5. A reaction sequence involving base catalysed loss of A-methylphenylamine to give the nonisolable 4/7-1,2,5-oxadiazine intermediate (39) followed by an iron carbonyl-mediated deoxygenation, is proposed (Scheme 6). 

Radical Carbonylations Mediated by Tin, Germanium, and Silicon Reagents... 

Ryu, Sonoda and coworkers reported that tris(trimethylsilyl)silane is a useful mediator for a three-component coupling reaction [45]. Table 4 summarizes examples of radical carbonylations mediated by (TMS)3SiH. The first example shows a three-component coupling reaction in which hexyl iodide, CO, and acrylonitrile combine to form a P-cymo ketone. The CO addition step is in competition with the addition to the alkene and the hydrogen abstraction from radical mediator. Thus, it is anticipated that a set of less efficient hydrogen donors, such as (TMS)3SiH, and the use of a smaller excess amount of an alkene is most favorable. Indeed, the reaction can be carried out at only 20 30 atm of CO pressure, substantially below the 80-90 atm which is used for carbonylative acyl radical reactions which are mediated by tin hydride, and a nearly stoichiometric amount (1.2 equiv) of acrylonitrile is sufficient. Some other examples, which include vinyl radical carbonylation, are also shown in Table 4. 

A safer alternative to toxic stannane derivatives is the use of nontoxic and inexpensive allyltrimethylsilane. The problem of the weaker nucleophilicity of this reagent was solved by an efficient in situ activation of the carbonyl mediated by the inexpensive titanium tetrafluoride as Lewis acid (Scheme 7.34). ° Good results in terms of yield and enantioselectivity were described using 10% of the complex preformed from titanium tetrafluoride... 

Suzuki and colleagues described the first palladium-catalyzed oxidative carbonylation of alkenylboranes as early as 1981. They prepared 1-alkenylboranes by hydroboration of alkynes and subsequent oxidative carbonylation mediated by a catalytic amount of PdCL, in the presence of NaOAc and BQ in methanol, which provided unsaturated esters in good yields (Scheme 8.27a) [106]. Later, a stereoselective synthesis of ]S-mono- and / , / -disubstituted a, ]S-unsaturated esters was established by a stepwise cross-coupling alkylation followed by an oxidative carbonylation of 2-bromo-1-alkenylboranes (Scheme 8.27b) [107]. Good yield and excellent stereoselectivity was achieved. 

The iron carbonyl mediated reactions of o,a -dibromoketones have been reviewed with particular emphasis on their use in natural product synthesis. A wide range of cyclic ketonic products is available from these reactions including cyclopentanone, cyclopentenone, 3-furanone and bicyclo[3,2,1]-octane derivatives. ... 

MacrocycHzation for the synthesis of macrolactones was also achieved by C-C bond formation using a variety of techniques (Scheme 6.5). Oxidative acetylene coupling for exaltolide 12, as reported by Bergelson et al. [61], nickel-carbonyl-mediated coupling of allyhc dibromides by Corey and Kirst [62], palladium-catalyzed... 

The iron-alkyne interaction is important because there are practical applications in organic synthesis, such as the iron carbonyl-mediated coupling of alkynes and CO. As a result, the reactions of alkynes with iron carbonyls have thus been the subject of many studies since the first report in 1953. Surprisingly, up until 1997, however, there was only one well-documented example of an Fe(CO)4(r7 -alkyne) species 3 (see Scheme 7). This is in part because complexes of this type are highly sensitive to the preparative conditions. Takats and co-workers have prepared a series of complexes of this type 3a-3e from Fe(CO)s (see Scheme 7) under carefully controlled photolysis conditions. The complexes Fe(CO)4(77 -alkyne) are shown to react with an excess of alkyne and CO, thus confirming early proposals that they are key intermediates in the coupling of alkynes with CO. ... 

In 2003, an extremely fast protocol for the cobalt carbonyl mediated formation of symmetric diaryl ketones from aryl halides was disclosed by Larhed and coworkers [38]. Microwave irradiation of aryl iodides and Co2(CO)g in acetonitrile for 10 s or less was enough to produce high yields (57-97%) of symmetric diaryhnethanones (Scheme 4.25). Please note that the Co2(CO)8-mediated chemistry was performed without an additional transition metal catalyst. 

In 2005, Larhed s group presented extremely fast cobalt carbonyl mediated syntheses of ureas from primary amines [39]. A protocol was developed where a primary amine, Co2(CO)8 (0.66 equiv.) and triethylamine (2 equiv.) in acetonitrile... 

Enquist, P.A., Nilsson, P. and Larhed, M. (2003) Ultrafast chemistry cobalt carbonyl-mediated synthesis of diaryl ketones under microwave irradiation. Organic Letters, 5, 4875-4878. 

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