Hydroxycarbene

We will use a similar procedure to investigate the second reaction, where formaldehyde transforms into the trans form of hydroxycarbene ... 

The zero-point corrected energy for the trans hydroxycarbene structure is -113.75709 hartrees at the RHF/6-31G(d) level of theory. 

These results predict that the hydroxycarbene to formaldehyde reaction will proceed significantly more easily than the fonvard reaction. However, for this problem, electron correlation is needed for good quantitative values. For example, the MP4/6-31G(d,p) level predicts a value of 86.6 kcal mol" for the activation energy of the forward reaction. 

We have already considered two reactions on the H2CO potential energy surface. In doing so, we studied five stationary points three minima—formaldehyde, trans hydroxycarbene, and carbon monoxide plus hydrogen molecule—and the two transition structures connecting formaldehyde with the two sets of products. One obvious remaining step is to find a path between the two sets of products. 

Determine the reaction path connecting trans hydroxycarbene and H2 + CO. Predict the activation energy, referring to the values for the SCF and zero-point energies for the products and reactants summarized at the conclusion of this problem. This reaction occurs via a two step process ... 

We can easily identify both structures by the value of the dihedral angle. In the one on the left, the dihedral angle has increased to 118.3°, indicating that this side of the path is leading to the trans form. Indeed, if we look at ail of the points in the reaction path, we see that the dihedral angle steadily increases on this side of the transition structure, and steadily decreases on the opposite side. From the latter, we can conclude that the right structure is tending toward the cis form. Thus, we have confirmed that this transition structure does in fact connect the cis and trans isomers of hydroxycarbene. 

Following a similar procedure, we locate and verify the transition structure connecting cis hydroxycarbene and the two dissociated species. Here is the transition structure and the two structures at the end of the reaction path computed by the IRC calculation ... 

Structure 1 is clearly tending toward cis hydroxycarbene. The other endpoint exhibits quite large bond distances between both hydrogens and the associated heavy atom. The hydrogens themselves are close enough to be bonded. This structure is a point on the path to H2 + CO. 

These values suggest that the two hydroxycarbene isomers convert into one another very easily. The barrier to molecular dissociation of the cis form is significant, however, and so this structure probably does not dissociate directly, but rather first converts to the trans isomer, which is subsequently transformed into formaldehyde, which dissociates to carbon monoxide and hydrogen gas. The article from which this study was drawn computes the activation energy for the trans to cis reaction as 28.6 kcal- moT at RMP4(SDQ)/6-31G(d,p) (it does not consider the other reactions). 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Table 15 Photo-driven reactions of hydroxycarbene complexes with alcohols...

The osmium hydroxycarbene complex 18 is formed in an acid-assisted migratory-insertion reaction of OsClEt(CO)2(PPh3)2. This alkyl compound results from reaction of the ethylene adduct 19 with one equivalent of acid 46). 

The hydroxycarbene isomer (H)Co(CO)3(CHOH) was also examined. It yielded a complex with molecular electronic energy more than 60 kcal/mole higher on the energy scale. The hydroxycarbene complex is not likely to play a significant role in the catalytic cycle. It is of some interest to inquire why the 18e hydroxycarbene complex (H)(CO) Co(=CH0H) is less stable than the 16e isomer (H)(CO)3C0(CH2O). The results suggest that the formation of the carbonyl double bond makes the critical difference. The electronically delocalized structure (H)(CO)3Co+5-CH2 0" may provide some extra stabilization for the formally unbonded formaldehyde moiety. The resonance form is dipolar and could be further stabilized by polar solvents. 

Figure 19. Detection of hydroxycarbene intermediate in CF3C02H induced disproportionation of 6...

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

An early approach to vinylidenes was by the formal dehydration of metal acyls, which is best achieved by treatment with an electrophile, often the proton in the form of a non- or weakly-coordinating strong acid. The reaction appears to proceed stepwise via a hydroxycarbene formed by protonation of the acyl, subsequent dehydration of which affords the vinylidene. Occasionally, mixtures of the two complexes are obtained, again suggesting the intermediacy of the carbene. 

Formation of metal acyl or carbonyl complexes from 1-alkynes in the presence of water is often assumed to proceed via attack on an intermediate vinylidene complex to give a hydroxycarbene complex (Equation 1.24) ... 

CO formation on copper electrodes appears to be accompanied by hydride formation as well [103]. In Sch. 3, the surface bound CO is reduced by a hydride transfer reaction to form a formyl species as shown in step 2. There are precedents in organometallic chemistry for late transition metal hydrides reducing bound CO [105-109]. Protonation of the adsorbed formyl in step 3 results in the formation of a hydroxy carbene species [110, 111]. This hydroxycarbene species could be considered to be an adsorbed and rearranged form of formaldehyde, and the reduction of formaldehyde at a copper electrode has been reported to form hydrocarbons [102]. However, reduction of... 

Although chain growth is not a feature relevant to methanation, the initiation and termination steps of the Anderson model for F-T synthesis are believed by at least some workers in the field to be applicable to the mechanism of the highly specific methanation reaction (71). The formation of methane is proposed to follow from the surface bound hydroxycarbene species by (19). 

In this regard, it is noteworthy that while surface bound hydroxycarbenes are postulated species, discrete complexes containing hydroxy- and alkoxy-carbenes have been known since E. O. Fischer s studies beginning in 1964 (56, 57). These complexes are possibly analogous to proposed surface intermediates, and their chemistry may model some of the heterogeneously catalyzed transformations. Coupling of alkoxy carbenes, for example, gives dialkoxy olefins as observed in (20). 

However, condensation of hydroxycarbene ligands, as in (15), has not yet been observed, and hydroxycarbene ligands in general show a propensity to eliminate with a hydrogen shift as aldehydes (57). 

A second postulated intermediate in the catalyzed reduction of CO by H2 is coordinated formaldehyde which is a tautomer of hydroxycarbene. Early in 1979 Roper (88f) reported the formation and structural characterization of a stable formaldehyde complex, thus providing support for the proposed intermediacy of this species in homogeneously catalyzed CO reduction schemes. The CH20 complex has the specific structure (17d), and eliminates H2 on heating to reform the starting Os(CO)3L2 complex. 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

Water also attacks the electrophilic a carbon of the ruthenium vi-nylidene complex 80. The reaction does not yield the ruthenium acyl complex, however, as is found for the reaction with the similar iron vinylidene complex [(i75-C5H5)(CO)2Fe=C=CHPh]+ (56), but rather 91 is the only isolated product (78). The mechanism for this transformation most reasonably involves rapid loss of H+ from the initially formed hydroxycarbene to generate an intermediate acyl complex (90). Reversible loss of triphenyl-phosphine relieves steric strain at the congested ruthenium center, and eventual irreversible migration of the benzyl fragment to the metal leads to formation of the more stable carbonyl complex (91) [Eq. (86)]. 

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