Carbene sources

Rh(Por)l (Por = OEP. TPP, TMP) also acts as a catalyst for the insertion of carbene fragments into the O—H bonds of alcohols, again using ethyl diazoacetate as the carbene source. A rhodium porphyrin carbene intermediate was proposed in the reaction, which is more effective for primary than secondary or tertiary alcohols, and with the bulky TMP ligand providing the most selectivity. ... 

Some other groups have studied the opportimity to enhance the diastere-oselectivity of the transformation using the usual copper-bis(oxazohne) catalysts but modifying the carbene source. France et al. [25] observed that the use of (trimethylsilyl)diazomethane associated with a bis(oxazoline) and [Cu(CH3CN)4]PF6 as catalyst precursor allowed the formation of the trans isomer with high yield and selectivity, probably due to the steric bulk of the trimethylsilyl group. 

Aryldiazomethane can also be used for iron porphyrin-catalyzed alkene cyclopropanation [55]. For example, the treatment of p-tolyldiazomethane with styrene in the presence of [Fe(TTP)] afforded the corresponding arylcyclopropapane in 79% yield with a high transicis ratio of 14 1 (eq. 1 in Scheme 11). Interestingly, when bulkier mesityldiazomethane was used as carbene source, ds-selectivity was observed (cisitrans = 2.0 1). Additionally, mesityldiazomethane was found to react with frans-p-styrene, the latter was found not to react with EDA or trimethyl-silyldiazomethane under the similar reaction conditions, to give l-mesityl-2-methyl-3-phenylcyclopropane in 35% yield. Trimethylsilyldiazomethane is also an active carbene source for [Fe(TTP)]-catalyzed cyclopropanation of styrene, affording l-phenyl-2-trimethylsilylcyclopropane in 89% yield with transicis ratio of 10 1 (eq. 2 in Scheme 11). 

Interestingly, the cyclopropanation of styrenes with EDA catalyzed by the half sandwich iron complex [CpFe(CO)2(THF)] BF4 afforded cyclopropanes in good yields and with ds-selectivity cisitrans = 80 20) [62]. With phenyldiazomethane as a carbene source, excellent cA-selectivity (92-100%) was achieved (Scheme 15) [63]. 

Carbenoid N-H insertion of amines with diazoacetates provides a useful means for the synthesis of ot-amino esters. Fe(III) porphyrins [64] and Fe(III/IV) corroles [65] are efficient catalysts for N-H carbenoid insertion of various aromatic and aliphatic amines using EDA as a carbene source (Scheme 16). The insertion reactions occur at room temperature and can be completed in short reaction times and with high product yields. It is performed in a one-pot fashion without the need for slow... 

In 1986, Pfaltz et al. introduced a new type of pseudo C2-symmetrical copper-semicorrin complex (68) as the catalyst (Scheme 60).227 228 The complexes (68) are reduced in situ by the diazo compound or by pretreatment with phenylhydrazine to give monomeric Cu1 species (69), which catalyze cyclopropanation. Of the semicorrin complexes, complex (68a) (R = CMe2OH) showed the best enantioselectivity in the cyclopropanation of terminal and 1,2-disubstituted olefins.227,228,17 It is noteworthy that complex (68a) catalyzes cyclopropanation, using diazomethane as a carbene source, with good enantioselectivity (70-75% ee).17... 

The detailed mechanism of transition metal-catalyzed cyclopropanation using diazo compounds as a carbene source is still covered by clouds of controversy, but it is generally accepted that the reaction proceeds through metal-carbenoid complexes,17-21 and the valency of the metal ions (M) changes with carbenoid formation (Scheme 85). 

The use of an excess (3 equivalents) of the Simmons-Smith reagent gave exclusively the spiropentane derivatives 618 in 60-70% yield [163a], In contrast, not even trace amounts of these products were observed by using samar-ium-dihalomethane as the carbene source [4b],... 

Independently Volpin17 synthesized diphenyl cyclopropenone from diphenyl-acetylene and dibromo carbene (CHBr3/K-tert.-butoxide). This reaction principle of (2 + 1) cycloaddition of dihalocarbenes or appropriate carbene sources ( caibenoids ) to acetylenic triple bonds followed by hydrolysis was developed to a general synthesis... 

Trichloromethyl lithium (generated from BrCCl3 and CH3Li at —100 °C) adds to dialkyl acetylenes and to monoalkyl acetylenes23, thus monoalkyl cyclopropenones became accessible which could not be obtained from terminal acetylenes by reaction with the above carbene sources. The 3,3-dihaIogeno-A1,2-cycIopropenes formed as primary products in the dihalocarbene reactions are usually not isolated, but are hydrolyzed directly to cyclopropenones. 

Another unique synthetic methodology for the carbene phosphine bidentate ligand is the utilization of destabilized C-amino phosphorus ylides 25 as carbene source [66]. Due to the electrostatic repulsion between the two lone pairs on the nitrogen and on the ylidic carbon, the yUdic bond is very labile. Thus, these C-amino phosphorus ylides readily act as 1,6-bidentate ligand precursors by insertion of the metal fragment between the phosphine and carbene centers. 

Confirmation was provided by the observation that the species produced by the photolysis of two different carbene sources (88 and 89) in acetonitrile and by photolysis of the azirine 92 all had the same strong absorption band at 390 nm and all reacted with acrylonitrile at the same rate (fc=4.6 x 10 Af s" ). Rate constants were also measured for its reaction with a range of substituted alkenes, methanol and ferf-butanol. Laser flash photolysis work on the photolysis of 9-diazothioxan-threne in acetonitrile also produced a new band attributed the nitrile ylide 87 (47). The first alkyl-substituted example, acetonitrilio methylide (95), was produced in a similar way by the photolysis of diazomethane or diazirine in acetonitrile (20,21). This species showed a strong absorption at 280 nm and was trapped with a variety of electron-deficient olefinic and acetylenic dipolarophiles to give the expected cycloadducts (e.g., 96 and 97) in high yields. When diazomethane was used as the precursor, the reaction was carried out at —40 °C to minimize the rate of its cycloaddition to the dipolarophile. In the reactions with unsymmetrical dipolarophiles such as acrylonitrile, methyl acrylate, or methyl propiolate, the ratio of regioisomers was found to be 1 1. 

Diazo compounds have been extensively used in the preparation of three-membered carbocycles either as carbene sources or as precursors for 1-pyrazolines or 3//-pyrazoles. Nitrogen extrusion from pyrazolines is particularly valuable for the synthesis of alkylcy-clopropanes, since the direct carbene route is impractical, as a matter of fact, owing to rapid intramolecular processes in alkylcarbenes. The cycloaddition of diazo compounds to unsaturated bonds to give 1-pyrazolines and 3/f-pyrazoles usually proceed in a concerted manner, and hence is stereospecific. In the subsequent nitrogen extrusion from the adducts,... 

Carbene precursors are compounds that have or acquire good leaving groups (e.g., halide ions). Thus, halogen compounds frequently are carbene sources. Trihalomethanes are the oldest known sources of dihalocarbenes but there are other methods for generating carbenes, and some of these are listed for reference in Table 14-2 (see also Section 14-10C). There is a question as to whether a free carbene actually is formed in some of these reactions, particularly those involving metals, but for our purposes we will classify them as routes to carbenes or carbenelike species. 

In a long-term research project, Hossain and coworkers investigated the usefulness of the CpFe(CO)2+ fragment [35-38] in the cydopropanation reaction of alkenes by a carbene transfer utilizing diazo esters as the carbene source (Scheme 9.17). The cydopropanation products of styrene derivatives could be obtained in good yields of up to 80% and excellent cis selectivity by using an excess of the alkene, whereas the cydopropanation of aliphatic alkenes was less effective, yielding the desired cyclopropane derivative in up to 51% yield. 

Catalysis via Ylide Formation from a Carbene Source... 

An alternative for carbene-ruthenium bond formation is the use of a car-bene transfer agent. Silver carbenes have demonstrated their utility in serving as valuable carbene sources for the preparation of various transition metal... 

As in organic chemistry, diazoalkanes have found wide application as ultimate carbene sources. In contrast to the organic chemistry of diazoalkanes, however, the reactions do not proceed via free carbenes but rather via metal-mediated transformations of coordinated diazoalkanes. In some cases, diazoalkane complexes may be isolated (Figure 5.8). The parent diazoalkane, H2C=N2, has found somewhat less success in the synthesis of terminal methylene complexes LJM=CH2 however, this is primarily due to the lack of any kinetic (steric) or thermodynamic (71-dative) stabilization by carbene substituents. Thus methylene ligands... 

There appears to be no report of an attempted preparation of a per-chloroalkylcopper compound, despite the interest in polychloroalkyl-lithium reagents as carbene sources (176). Addition of chloroform to lithium dimethylcuprate which contained a stoichiometric excess of methyllithium in THF-ether at - 80°C gave a methylcopper-like precipitate and then an intense red color but no isolable ketones subsequent to the addition of acetyl chloride (f44). 

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

The key synthetic routes for Fischer carbenes fall into one of three general categories, illustrated by equations (1-3). In equation (2) an acyl or similar species (often but not always formed by a nucleophilic attack on a CO or a similar ligand) is treated with an electrophile to give a Fischer carbene. In equation (3), an H (Fischer case) or an H+ (Schrock case) is abstracted from the a-position of an alkyl and in equation (5) a traditional carbene source is used. 

Alternatively, one can add a carbene source to some form of a C=N bond. This will generate bonds c and b . This model is limited by the availability of cyclic imine or imine-like structures. Several methods are available to prepare aziridines through the functionalization of a C=N bond, although formation of the fused rings will require an intramolecular addition to a pendent imido-bond. The Darzens process and ylide addition to C=N bond are generally not applicable in most cases owing to the structural requirements of the cyclic system. This requirement renders this process less useful overall than the addition to a C=C bond. 

---