Carbene-like reactions

The only carbene-like reaction reported so far is the low-temperature addition of tert-butyl isocyanide to carbenes 2d and 2u.78 From the P-hydrogenophosphonio carbene 2d, the heterocycle 89 was isolated in high yield. It is believed that the initial coupling product 87d rapidly inserts a further equivalent of isocyanide into the P-H bond, leading to the intermediate 88, which then undergoes rapid elimination of diisopropylamine. When the same reaction was performed with the P-chloro(phosphonio)car-bene 2u, a 1/1 mixture of keteneimine 90 and phosphinonitrile was obtained. This result can be explained by the cleavage of the carbene-isocyanide coupling product 87u by residual HCN, inherently present in the f-BuNC. 

Carbene-like reactions, applications, and mechanistic insights of phosphinidene complexes (particularly, formation of P-heterocycles) 02EJO1127. 

Isonitriles also undergo carbene-like reactions with isocyanates and coreagents. For example, two isomeric [1+2+2] cycloadducts 50 and 51 are obtained from nitroso compounds, isonitriles and isocyanates. The reaction proceeds via addition across both the C=N and the C=0 bonds of the isocyanate . 

There are five distinct types of reactions that aluminum(I) compounds can undergo (1) substitutions, (2) disproportionations, (3) redox reactions (4), carbene-like reactions, and (5) Lewis acid base reactions. The latter will only be briefly mentioned as it is dealt with in detail in Chap. 2. 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Addition of Lewis acids under the form of monovalent or divalent metal ions produces similar effects,5 as shown in Figure 4.7. Analysis of the reactions metal ion and C02 reaction orders suggests the mechanism depicted in Scheme 4.7 for monovalent cations. The carbene-like adduct is stabilized, in this case by addition of one metal ion and one C02 molecule. 

This is the second type of stable phosphinocarbene to have been reported. In contrast to (phosphino)(silyl)carbenes, very little is known concerning their reactivity. No simple reactions typical of a carbene-like behavior have been reported. However, the reactivity at the periphery of the carbene center makes these derivatives powerful building blocks for novel compounds. 

The initial question was whether the active catalyst is copper metal, copper(I), or copper(II), because all metal precursors gave results. Without the proper control of the valence state and the ligand environment the selectivities for the copper catalysed cyclopropanations (or carbene insertion reactions) have remained low or inconsistent for a long period of time. It was only in the sixties that a more systematic study of these issues was started. Several divalent copper salts were successfully used, but Kochi and Salomon [1] showed with the use of Cu(I)OTf that most likely copper(I) was the actual species needed for this reaction. 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

Low-valent, 18-electron (Fischer-type) carbene complexes with strong n-acceptors usually are electrophilic at the carbene carbon atom (C ). These complexes can undergo reactions similar to those of free carbenes, e.g. cyclopropanation or C-H insertion reactions. The carbene-like character of these complexes becomes more pronounced when electron-accepting groups are directly bound to C (Chapter 4), whereas electron-donating groups strongly attenuate the reactivity (Chapter 2). 

Under some circumstances, the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. When a reaction involves a species that reacts as expected for a carbene, but must still be at least partially bound to other atoms, the term carbenoid is applied. Some reactions that proceed by carbene-like processes involve transition-metal ions. In many of these reactions, the divalent carbene is bound to the transition metal. Some compounds of this type are stable whereas others exist only as transient intermediates. 

Triplet carbenes have a singly occupied p orbital, as is the case for radicals, and hence react like those radicals. Hydrogen atom transfer reactions are fundamental reaction pathways of triplet carbenes. The reaction of a triplet carbene with a hydrocarbon is quite analogous to the free radical hydrogen atom transfer process (Scheme 9.6). 

The conversion of methanol to ethanol with carbon monoxide and hydrogen has attracted considerable attention. Further carbonylation to higher alcohols occurs much more slowly, but acetic acid formation is a competing reaction and this leads to ester formation. Using CoI2 in presence of PBu 3 as catalyst, the selectivity to ethanol was improved by addition of the borate ion B4072. 399 This was attributed to an enhanced carbene-like nature of an intermediate cobalt-acyl complex by formation of a borate ester (equation 76). This would favour hydrogenolysis to... 

The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial with singlet carbenes like dichlorocarbene (p. 28). This is clearly a forbidden pericyclic reaction, if it takes place in the sense 3.49 —> 3.50, This is known as the linear approach, in which the carbene, with its two substituents already lined up where they will be in the product, comes straight down into the middle of the double bond. The two reactions above, 3.47 and 3.48, are also linear approaches, but these are both allowed, the former because the total number of electrons (6)... 

Enamine formation occurs by the thermolysis of diazo compounds (Scheme 150)67 109,278 284 288,304 332 453 454 via a carbene-like intermediate.284 332 When R1 = Ph, it enters into competition with hydrogen migration,284,332 and the electrophilic character of the carbene enhances the migration of the dimethylaminophenyl more than the phenyl.332 When triazoline synthesis is carried out at temperatures higher than that at which thermolysis of diazo compounds occurs, enamines are obtained exclusively, as in the addition of phenyl azide to cinnamic nitriles and ketones, with phenyl migration dominating in the nitrile.284 Enamine is also formed quantitatively in the reaction of ethyl diazoacetate with benzylideneaniline at 110°C.455... 

An interesting Fe-catalyzed SN2 -like carbene insertion reaction using diazo compounds and allyl sulfides (the Doyle-Kirmse reaction) was reported by Carter and Van Vranken in 2000 [20], Various allyl thioethers were reacted with TMS-diazomethane in the presence of catalytic amounts of Fe(dppe)Cl2 to furnish the desired insertion products with moderate levels of stereocontrol [Equation (7.6), Scheme 7.14]. The products obtained serve as versatile synthons in organic chemistry, e.g. reductive desulfurization furnishes lithiated compounds that can be used in Peterson-type oleftnations to yield alkenes [Equation (7.7), Scheme 7.14] [21]. 

MNDO calculations on insertion reactions of Me2Sn into Cl—Sn and I—C bonds were reported by Dewar and coworkers381. Two alternative pathways including concerted carbene-like insertion (a) and nonconcerted radical two-step insertion (b) were investigated. 

FIGURE 5.11 Proposed mechanisms of participation of N heteroatoms in electrochemical processes on carbon surfaces (a) possible pseudo-Faradaic reaction of the pyridinic group in aqueous medium [95] (b) N-induced stabilization of carbene-like edge carbon atoms, sites responsible for the development of positive charge in acidic solution. 

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