Methylcarbene

Spectator effects were examined computationally in methylcarbene derivatives, MeCX, where it was found that the activation energies for 1,2-H shifts from the Me group were linearly related to the electron donating power of spectator X, as represented by the (7r substituent constant of X.78 MeCX with X = MeO, HO, F, Cl, vinyl, and H were considered. Because electron donation from X modulates the a for the 1,2-H shift ( a increases as donation increases), the spectator also affects the lifetime of MeCX, which therefore increases with increasing electron donation by X.78... 

The simplest carbene capable of a 1,2-H shift, methylcarbene (78) is very difficult to study experimentally. Theoretical studies suggest that the carbene is a ground state triplet lying 5 kcal/mol below the singlet.97... 

Despite its fleetingness, 78 has been captured by CO in a matrix at 10 K, affording low yields of ketene." Modarelli and Platz were unable to observe the formation of a pyridine ylide upon LFP generation of 78 in pyridine/pentane at —40°C. However, with perdeuterated methylcarbene (78- 4) a weak pyridine ylide signal was detected, and the rate constant of the 1,2-D shift could be estimated as ito 2 x 109 s-1 (r 0.5 ns) using a Stem-Volmer analysis (see Eqs. 3-6 in Section II).89 The activation energy for this 1,2-D shift was estimated at 2.3 kcal/mol, assuming A 1011 s-1.89 However, it seems likely that both 2sa and A are somewhat lower, with A 108 to 109 s (AS = —17 e.u.).89... 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

Fig. 10. Calculated geometries and heats of formation of (a) methylcarbene (b) dimethylcarbene (c) formylcarbene (d) closed shell configuration of C3...

The mechanism for the ethylene to methylcarbene reaction has been calculated at the DFT (B3PW91) level with the model system RuHCl(PH3)2 [8, 21]. As in the case of the acetylene to vinylidene reaction, the starting complex was assumed to be the 14-electron complex RuHCl(PH3)2 generated in situ. The reaction path is very similar to that obtained with C2H2. They differ mainly in the overall direction of the energy pattern downhill for acetylene and uphill for ethylene. 

The same ethylidene ruthenium complex, as well as its iron congener, is alternatively obtained through direct protonation of the dimetallacycles 64a (M = Fe) and 64b (M = Ru) (64). In this case, the carbonyl alkyne carbon-carbon bond is broken irreversibly to give the cationic /x, 17s-vinyl complexes 65a and 65b, which undergo nucleophilic attack by hydride (NaBFLi) to produce complexes of methylcarbene (63a,b) (Scheme 21a). Deuterium-labeling experiments prove that the final compounds arise from initial hydride addition to the /3-vinylic carbon of 65. However, isolation of small amounts of the 7j2-ethylene complex 66 indicates that hydride attack can also occur at the a-vinylic carbon (64). 

Reaction of the [m-Ru20/-CMe)0/-COXCO)2(i C5H5)2]+ cation with sodium borohydride gives the //-methylcarbene complex m-Ru2(//-CHMe)(//-CO)(CO)2(/ -C5H5)2 (68). 

In contrast to ethene which is separated by a high potential barrier from the isomeric methylcarbene, silene 25 can undergo a 1,2 shift to either methylsilylene 276 or, less favourably, to silylmethylene 277 (equation 68). The thermochemistry and the kinetics of... 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

Finally, three papers are mentioned which have dealt with the potential energy surfaces for the dimerization of CH2 to ethylene, reaction (13), and the transformation of methylcarbene to ethylene, reaction (14). 

An alternative reaction which gives rise to C2H4 is the rearrangement of methylcarbene, MeCH. This reaction can occur in a variety of ways, and Altmann et al.217 carried out SCF calculations with a DZ basis set on two cross-sections of the potential hypersurface. The first cross-section was investigated in an MBS calculation of the variation of E with the CCH angle. 

Orpen, A. G. (1983). Structural chemistry of binuclear metal centres -crystal and molecular structures of the p,-vinyl and (ji-methylcarbene complexes [Fe2(CO)2((x-CO)(fx-CHCH2)(T1-C5H2)2][BF4] and... 

Despagnet E, Gornitzka H, Rozhenko AB, Schoeller WW, Bourissou D, Bertrand G (2002) Stable non-push-pull phosphanylcarbenes NMR spectroscopic characterization of a methylcarbene. Angew Chem Int Ed Engl 41 2835-2837... 

Alkoxy(l-alkynyl)carbene complexes also are obtained by condensation of (l-ethoxy)methylcarbene complexes with bulky acyl chlorides as side products (10-12% yield) together with [(2-acyloxy)ethenyl]carbene complexes (50-78% yield),36 e.g., (CO)5M=C(OEt)—CH3 + 3 RCOC1 + 3 Et3N — (CO)5M=C(OEt) - CR (M = Cr, W R = r-Bu, c-Pr). 

MetaIIa-l,3,5-trienes M = C—C=C—C=C play a pivotal role in many applications of carbene complexes to organic synthesis. These compounds are readily available from methylcarbene complexes (by condensation, e.g. with a,/3-unsaturated aldehydes), from 1-metalladienes (by insertion of an... 

Alkyloxy)ethenyl]carbene complexes (CO)5M=C(OEt) —CH=C (O-alkyl)R1151 (M = Cr, W) are accessible by several methods, including (i) the condensation of a methylcarbene complex (CO)5M=C(OEt)CH3 with dimethylformamide and other nonenolizable acid amides R1 — CONR2,174 (ii) the condensation of a methylcarbene complex (CO)5M =... 

In essence, Scheme 1 describes two pathways by which ethyne may be converted to y-methylcarbene at a dinuclear metal centre. 

In this connection it is interesting to note that studies of the chemisorption of ethyne on metal surfaces have led to suggestions that u-methylcarbene or u-methylcarbyne are formed as surface species, perhaps via y-vinylidene (]J 1 3). The sequence (la)->... 

The y-carbene complexes are structurally related to the carbonyl-bridged forms of the dimers [M2(C0)1((rrC5H5)2j and they react similarly with alkynes under u.v. irradiation. Reactions of the y-methylcarbene complexes (4,) are summarised in Scheme 4. The new complexes are derived by linking of the carbene and alkyne... 

Caibene reagents also functionalize alkanes. Triplet CH2 adds unselecdvely to alkane C—bonds. The product mixture obtained from n-pentane was found to be 48% n-hexane, 3S% 2-methylpentane and 17% 3-methylpentane, so that addition to a primary C—bond appears to be favored. Monochloro-methylcarbene, CHCl, is less reactive and more electrophilic and so the normal tertiary > secondary > primary selectivity pattern was observed. Ethoxycarbonylcarbene, formed on fdiotolysis of the corresponding diazo compound, inserts rather unselectively in to alkane C—H bonds to give the ethoxycarbo-nylmethyl derivatives in ca. 50% yield. Transition metals, such as copper(II) or rhodium(I). also usefully catalyze the insertion of carbenes into alkane C—bonds. 

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