Diazo compounds, carbenes

T rifluoromethyl-3-(m-iodophenyl) diazirine N=N 353 nm/266 24,000 Carbene (diazo compound is inert) Brunner (1981), Brunner and Semenza (1981), Spiess et al. (1982)... 

Keywords Copper catalysts. Metal carbenes, Diazo compounds. Cyclopropanes, C-H insertion, Ylides... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Diazirines are in most cases more easily available than linear diazo compounds. Moreover, their decomposition via true carbenes is free of side reactions, whereas linear diazo compounds in presence of H-donors may react by a cationic pathway. Only where reactions of linear diazo compounds are optimized for carbene formation do they give the same products as do decomposing diazirines. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

There are several examples of intramolecular reactions of monocyclic /3-lactams with carbenes or carbenoids most of these involve formation of olivanic acid or clavulanic acid derivatives. Thus treatment of the diazo compound (106) with rhodium(II) acetate in benzene under reflux gives (107), an intermediate in the synthesis of thienamycin (80H(14)1305, 80TL2783). 

A final method of /3-lactam 3,4-bond formation which has found fairly wide application is based on carbenlc insertion (78T1731 p. 1739). The carbenic centre can be generated by photolysis of a diazo compound as in the case of (158) (72JA1629, 79CC846) or from organometalllc precursors, for example (159) (71ACS1927). 

Ring enlargement of benzene derivatives by carbenes generated from diazo compounds (better in the presence of a Rh catalyst) Conversion of aldehydes to ketones by diazo compounds (Schlotterbeck) see also Ptau Planer... 

The Bamford-Stevens decomposition of tosylhydrazones by base has been applied to steroids, although not extensively. It has been demonstrated that the reaction proceeds via a diazo compound which undergoes rapid decomposition. The course of this decomposition depends upon the conditions in proton-donating solvents the reaction has the characteristics of a process involving carbonium ions, and olefins are formed, often accompanied by Wagner-Meerwein-type rearrangement. In aprotic solvents the diazo compound appears to give carbene intermediates which form olefins and insertion products ... 

In this review an attempt is made to discuss all the important interactions of highly reactive divalent carbon derivatives (carbenes, methylenes) and heterocyclic compounds and the accompanying molecular rearrangements. The most widely studied reactions have been those of dihalocarbenes, particularly dichlorocarbene, and the a-ketocarbenes obtained by photolytic or copper-catalyzed decomposition of diazo compounds such as diazoacetic ester or diazoacetone. The reactions of diazomethane with heterocyclic compounds have already been reviewed in this series. ... 

When an aprotic solvent is used, the reaction proceeds via an intermediate carbene 6. In the absence of a proton donor, a diazonium ion cannot be formed and the diazo compound 3 loses nitrogen to give the carbene 6 ... 

The strained bicyclic carbapenem framework of thienamycin is the host of three contiguous stereocenters and several heteroatoms (Scheme 1). Removal of the cysteamine side chain affixed to C-2 furnishes /J-keto ester 2 as a possible precursor. The intermolecular attack upon the keto function in 2 by a suitable thiol nucleophile could result in the formation of the natural product after dehydration of the initial tetrahedral adduct. In a most interesting and productive retrosynthetic maneuver, intermediate 2 could be traced in one step to a-diazo keto ester 4. It is important to recognize that diazo compounds, such as 4, are viable precursors to electron-deficient carbenes. In the synthetic direction, transition metal catalyzed decomposition of diazo keto ester 4 could conceivably furnish electron-deficient carbene 3 the intermediacy of 3 is expected to be brief, for it should readily insert into the proximal N-H bond to... 

Dibenzo[/>,rf]thiopyrylium tetrafluoroborate (1) reacts with ethyl lithiodiazoacetate at — 120 C to form the diazo compound 2 which, with dimeric ( 3-allyl)chloropalIadium at 20 C, gives ethyl dibenzo[ ,d]thiepin-6-carboxylate (3), via a carbene intermediate.5 Compound 3 is quite stable the ethoxycarbonyl group can be hydrolyzed by alkali and decarboxylated to give the corresponding parent compound 4 in good yield. 

The mechanism through which catalytic metal carbene reactions occur is outlined in Scheme 2. With dirhodium(II) catalysts the open axial coordination site on each rhodium serves as the Lewis acid center that undergoes electrophilic addition to the diazo compound. Lewis bases that can occupy the axial coor-... 

The best known of metal carbene reactions, cydopropanation reactions, have been used since the earliest days of diazo chemistry for addition reactions to the carbon-carbon double bond. Electron-donating groups (EDG) on the carbon-carbon double bond facilitate this catalytic reaction [37], whereas electron-withdrawing groups (EWG) inhibit addition while facilitating noncatalytic dipolar cycloaddition of the diazo compound [39] (Scheme 5). There are several reviews that describe the earlier synthetic approaches [1, 2,4, 5,40-43], and these will not be duplicated here. Focus will be given in this review to control of stereoselectivity. 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

In all cases the formation of carbenes was accompanied by partial photoisomerization of the precursor diazirines [53] into the corresponding diazo compounds [55] which possess a higher photochemical stability than the diazirines. 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

Decomposition of diazo compounds by iron porphyrins is a convenient method for the synthesis of non-heteroatom carbene-iron porphyrins [22]. Reaction of [Fe(F2o-TPP)] [F20-TPP = meso-tetrakis(pentafluorophenyl)porphyrinato dianion] with diazo compounds N2C(Ph)R (R = Ph, C02Et, C02CH2CH=CH2) under an inert atmosphere afforded complexes [Fe(F2o-TPP)C(Ph)R] in 65-70% yields (Scheme 2). Like the halocarbene complex [Fe(TPP)(CCl2)], [Fe(F2o-TPP)CPh2] reacted with Melm to afford six-coordinate species [(MeIm)Fe(F2o-TPP)CPh2] in 65% isolated yield. 

Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

Figure 7.16. (a) ORTEP representation of the diazo compound 35a (b) disordered structure of both 35a and 35c and (c) ORTEP representation of the carbene 35c after removing that of... 

Laser flash irradiation of diazofluorene in perdeuterated matrices, in contrast, gave severely nonexponential decay of the carbene spectra. Analyses of the products formed in the low-temperature matrices showed that, as with the EPR studies, the carbene was not undergoing D-abstraction. LFP of the diazo compound 36 in CFCl3-CF2BrCF2Br glasses gave linear first-order decays, and linear Arrhenius plots, which were attributed to classical Cl and Br abstractions. 

On the contrary. Chapman and McMahon found that H-transfer in o-tolylcar-bene can be directly observed at very low temperatures, through several different spectroscopic methods. Irradiation of Ar matrix isolated diazo compound 50 at 4.2 to 10 K gave triplet carbene 51, which could be characterized by EPR, IR, and UV/VIS spectroscopy. The various spectra of 51 slowly decayed at temperatures as low as 4.2 K (tin ca. 64 h), and o-xylylene (52) could be observed to grow correspondingly... 

The related [1,2]-H shift in the isomeric triplet 1-phenylethylidene (53) was also investigated. The triplet carbene, generated from irradiation of the corresponding diazo compound, was characterized in low temperature inert matrices by EPR, IR, and UV/VIS spectroscopy. In this case, the carbene was stable in Ar up to the temperature limits of the matrix (36 K). Irradiation, however, readily converted the carbene to styrene. 

Carbenes from Diazo Compounds. Decomposition of diazo compounds to form carbenes is a quite general reaction that is applicable to diazomethane and other diazoalkanes, diazoalkenes, and diazo compounds with aryl and acyl substituents. The main restrictions on this method are the limitations on synthesis and limited stability of the diazo compounds. The smaller diazoalkanes are toxic and potentially explosive, and they are usually prepared immediately before use. The most general synthetic routes involve base-catalyzed decomposition of V-nitroso derivatives of amides, ureas, or sulfonamides, as illustrated by several reactions used for the preparation of diazomethane. 

Carbenes from Sulfonylhydrazones. The second method listed in Scheme 10.8, thermal or photochemical decomposition of salts of arenesulfonylhy-drazones, is actually a variation of the diazoalkane method, since diazo compounds are intermediates. The conditions of the decomposition are usually such that the diazo compound reacts immediately on formation.147 The nature of the solvent plays an important role in the outcome of sulfonylhydrazone decompositions. In protic solvents, the diazoalkane can be diverted to a carbocation by protonation.148 Aprotic solvents favor decomposition via the carbene pathway. 

Metal-Catalyzed. Cyclopropanation. Carbene addition reactions can be catalyzed by several transition metal complexes. Most of the synthetic work has been done using copper or rhodium complexes and we focus on these. The copper-catalyzed decomposition of diazo compounds is a useful reaction for formation of substituted cyclopropanes.188 The reaction has been carried out with several copper salts,189 and both Cu(I) and Cu(II) triflate are useful.190 Several Cu(II)salen complexes, such as the (V-f-butyl derivative, which is called Cu(TBS)2, have become popular catalysts.191... 

---