Divalent Carbon Compounds Carbenes

When ethene gas is passed into an aqueous solution containing bromine and sodium chloride, the products of the reaction are the following  

Write mechanisms showing how each product is formed. 

The simplest carbene is the compound called methylene ( CH2). Methylene can be prepared by the decomposition of diazomethane (CH2N2), a very poisonous yellow gas. This decomposition can be accomplished by heating diazomethane (thermolysis) or by irradiating it with light of a wavelength that it can absorb (photolysis)  

The structure of diazomethane is actually a resonance hybrid of three structures CH2—r = N — CH2=r =N — tCHg-I II III 

We have chosen resonance structure I to illustrate the decomposition of diazomethane because with I it is readily apparent that heterolytic cleavage of the carbon-nitrogen bond results in the formation of methylene and molecular nitrogen. 

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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. ... 

Carbenes are divalent carbon compounds, also known as methylene. They have neutral carbons with a lone pair of electrons, and are highly reactive. Methylene can be prepared by heat or light initiated decomposition of diazomethane (explosive and toxic gas). 

Divalent Carbon Compounds. There are a number of CRR species, generally called carbenes, which play a role in many reactions even though they are short-lived,5 A general means of generating carbenes is by photolysis of diazoalkanes this is done in the presence of the substrate with which the carbene is intended to react, such as an olefin ... 

The diazo reactions in this chapter are characterized by processes run either in the gas phase, in relatively inert matrices, or in — typically, but not exclusively — aprotic and comparatively apolar solvents, either thermally or photolytically or with transition metal catalysis of various types. The metastable intermediates are carbenes (RR C ), i. e., neutral, apparently divalent, carbon compounds, or their transition metal complexes (coined carbenoids, see later in this section). It is interesting to recall that the synthesis of a compound that we now call a carbene, namely methylene (H2C ), was already attempted in the early 19th century, i.e., before the tetravalency of carbon was established. Dumas (1835) and Regnault (1839) thought then that it should be possible to obtain a compound consisting of one carbon and two hydrogen atoms by dehydration of methanol (a compound of which only the atomic ratio 1C 4H lO was then known). ... 

Carbenes can be classed with carbanions, carbocations, and carbon-centered radicals as among the fundamental intermediates in the reactions of carbon compounds. Carbenes are neutral, divalent derivatives of carbon. Depending on whether... 

M-heterocyclic carbenes are neutral compounds with a divalent carbon atom located between the two nitrogens. The four types of stable diaminocarbenes used for the synthesis of chiral complexes are listed below (Fig. 2) ... 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

A relatively unique type of reactive metabolite is carbene, i.e., a divalent carbon, which is a proposed intermediate in the oxidation of methylene dioxy-containing compounds. A methylenedioxy group in aromatic compounds is subject to O-dealkylation, e.g., 3,4-methylenedioxyamphetamine, as shown in Figure 8.20. The process generates formic acid and the catechol metabolite as final products. However, in the course of the reaction, a... 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

The theoretical section introduced divalent carbon(O) compounds as molecules CL2 where the ligand L is a ct donor. In principle this task can be fulfilled by various neutral group 15 compounds (N2, nitriles, amines, phosphanes, arsanes, etc.), neutral group 16 compounds (sulfides, selenides, etc.) as well as by divalent C(II) with a free pair of electrons at a carbon atom, such as isonitriles, NHCs, carbenes, CO, ylides, etc. The neutral and isolable compound C2PPh3 [14,15] may also serve as a donor L to stabilize a carbon atom. 

Theoretical studies on carbodiphosphoranes and related divalent carbon(O) compounds (carbones) of the type CL2 and the collection of forgotten CL2 compounds show that there is a class of molecules which has a unique bonding situation that is clearly different from divalent carbon(II) compounds (carbenes). 

A highly reactive compound containing a neutral divalent carbon with two nonbondmg electrons (ie.,. CR2 or a substitution derivative). The nonbonding electrons can have parallel spins (triplet state) or antiparallel spins (singlet state). The parent species, iCRz, is also known as methylene. A number of carbene derivatives have been used as photoaffinity labels of proteins. Irradiation of 3 -0-(4-benzoyl)benzoyl-ATP will cause 70% inactivation of mitochondrial Fi-ATPase. ... 

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

Consider the methane molecule, CH . The ground stale of a carbon atom is iP corresponding to the electron configuration of Is Zs lp. Carbon in (his state would be divalent because Only two unpaired electrons are available for bonding in the and py orbitals. Although divalent carbon is well known in methylene and carbene intermediates in organic chemistry, stable carbon compounds are tetravalent. fn order for four bonds to form, the carbon atom must be raised to its valence state. This requires... 

Carbenes, which may be formed photochemically from precursors such as diazo compounds and diazirines, are highly reactive entities containing divalent carbon. The first photogenerated reagent was a carbene, formed by irradiation of diazoacetyl chymotrypsin (Singh et al., 1962). 

For a long time, carbenes, neutral carbon species with a divalent carbon atom bearing six valence electrons, were considered to be too reactive to be isolated [1]. As a consequence, many chemists hesitated to make use of these compounds, especially as spectator ligands for transition metal chemistry. However, whereas the majority of carbenes are short-lived reactive intermediates, this picture does not hold for N-heterocyclic carbenes [2],... 

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