Carbenes resonance-stabilized

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

The term metalla-/3-diketonate refers to an anionic diacyl complex, such as L M(RCO)(R CO) , in which the two acyl ligands occupy adjacent coordination sites of the metal atom. Resonance stabilization of the negative charge by 7r-delocalization onto the two acyl ligands gives Lewis structures 1 and 2. The two acyl ligands acquire an acyl/carbene... 

In contrast to methylene, carbenes which contain an oxygen atom bonded to the carbenic center are known to exist as singlet ground states. The rational for this is that the lone pair electrons on the adjacent oxygen atom have a resonance stabilizing effect on the electron deficient carbenic center ... 

If the resonance stabilization of singlet carbenes is increased even more, for example, by replacing the CH3O substituents of (CH30)2C by amino groups [as in (R2N)2C] or phosphorus substiments (13), stable, isolable, nucleophilic carbenes can be obtained. These species are considered in Chapter 8 in this volume. 

It seemed possible that treatment of cation 233a with a strong proton abstracting base could led to a resonance stabilized carben 234a (R = H). Pohl by... 

The effect of fluorination on the structure, stability, and reactivity of carbenes has been the subject of several reviews (also sec Houben-Weyl. Vol. E19b, p 1461).13,119 121 a-Fluorinated carbenes are ground-state singlet rather than triplet species as a consequence of resonance stabilization.122,123... 

This resonance stabilization should be even more pronounced in the cyclic dithiocarbene (274), as indicated by the neutral carbene structure (274) and the ylid structures (275-280), in which the potential aromatic sextet of the 1,3-dithiolium ion is retained. Consequently, H-2 in 269 should be acidic. The analogy with the thiazolium and imidazolium ions, which have been intensively studied by Bres-... 

The pJCa values of Fischer carbene complexes such as 3.24 are comparable to those of active methylene compounds such as diethylmalonate. This is due to the resonance stability of the conjugate base of 3.24. 

The coenzyme thiamine pyrophosphate (1) plays a central role in many parts of metabolism (Fig. 1). Its mechanism of action involves the formation of a thiazolium zwitterion 2 that was stabilized by a carbene resonance form 3 (10). This discovery opened up studies of the chemistry that such stabilized carbenes could catalyze, as chemists realized that the otherwise impossible chemistry that thiamine pyrophosphate catalyzes in nature could be generalized and adapted for useful synthetic processes. 

Bromodifluoromethylation of C-nucleophiles which are not resonance stabilized is supposed to proceed via the carbene mechanism (Scheme 2.170). A typical by-product of this reaction is the brominated nucleophile [13]. 

A more reasonable hypothesis is that the transition state is imbalanced, as shown in equation (102), but that there is a structural feature characteristic of carbene complexes, absent from other carbon acids, that masks the imbalance by reducing a. The most likely candidate is the 7r-donor effect of the methoxy group. Inasmuch as the contribution of 151 leads to resonance stabilization of the carbene complexes, this resonance is expected to add to the intrinsic barrier of proton transfer. This is because, as is true for resonance effects in general, its loss at the transition state should be ahead of the proton transfer. As Z becomes more electron... 

With phthalic anhydride and trialkyl phosphites, the reaction takes V an entirely different course, the main products being biphthalyl (70%) and trialkyl phosphate. It was suggested that the phosphorus atom of a phosphite ester attacks the oxygen of the anhydride carbonyl, forming an intermediate which undergoes valency expansion to generate in this unique instance a carbene and a phosphate ester (276). Dimerization of this resonance-stabilized carbene w ould furnish the product. 

The reduction of perchlorofluorene with an excess of triphenylphosphine, followed by hydrolytic treatment, gives 9//,9//-octachlorofluorene (Camps, 1979). This reduction is interpreted as in (52). However, the postulated intermediate carbene, perchlorofluorenylidene, because of resonance stabilization, would rather react with triphenylphosphine to give the Wittig compound [66] than abstract a hydrogen atom (53). Accordingly, when the... 

The results with dimethoxyearbene highlight an inherent deficiency of Eq. 4 it is an empirical correlation of parameters normalized to the electrophilic car-bene, CCI2. Its electrophilic heritage means that although the equation can predict values for highly resonance stabilized, nucleophilic carbenes such as (MeO)2C or Me2NCOMe, these are virtual selectivity indexes. The nucleophilic carbenes simply do not add to the aUcenes of the standard set. However, the equation helps us define the Wcxy regions in which electrophilic and nucleophilic carbenes reside. The electrophilic species, which react appropriately with the standard alkenes of Table 1, exhibit w xy values between 0.29 (BrCCOOEt) and... 

When di-isopropylcarbodi-imide was tried in place of CSa, a (2 -F 2] cycloaddition followed by a prototropic shift occurred, giving (192). All of the CSa reactions could be satisfactorily interpreted on the basis of intermediate formation of a resonance-stabilized 1,3-dithiolium carbene (193), potentially a non-benzenoid aromatic. 

In a similar way, resonance stabilization of dissociated species can also be used to make C=C double bonds weaker. Instead of radicals formed from a single bond, dissociation of a double bond produces carbenes. Approaches to carbene stabilization can involve a variety of resonance patterns. For example, the C=C... 

The reactivity of most singlet carbenes with alkenes is dominated by the electrophilic character of the carbene (the empty p orbital). Thus, the more electron rich the alkene, the faster the carbene addition. Increasing alkyl group substitution on alkenes increases the rate of addition. This trend parallels the reactivity for the addition of other electrophiles with alkenes, such as acids, Xj, and borane. Dialkylcarbenes are less selective than dihalocarbenes, whereas carbenes with neighboring O or N atoms are resonance stabilized (see margin) and are highly selective. This trend tracks the reactivity-selectivity principle (see Chapter 7), where the more stable carbenes are the more selective. 

In contrast to the electrophilic nature of most carbenes, highly resonance-stabilized carbenes can be nucleophilic, as suggested by the right-hand resonance structure in the margin. Dimethoxycarbene is a good example of a nucleophilic carbene. Intermediate cases such as methoxychlorocarbene can be ambiphilic, showing increased reactivity to both electron rich and electron deficient olefins. 

Our earliest studies concerned the mechanism by which thiamine pyrophosphate (1) acts as a coenzyme (Fig. 1.1). We discovered that the C-2 hydrogen on the thiazolium ring of thiamine was able to ionize to form a thiazolium ylide (2) that has the important resonance structure (3), which can be called a stabilized carbene . We pointed out this carbene contribution to its structure, related to the well-known hybrid structure of carbon monoxide, and also saw that an imidazolium ion (4) and an oxazoKum ion (5) could form such yhde/carbene resonance structures. Recently such structures, usually referred to as stabilized carbenes , have proven to be useful ligands for catalytic metal ions. The carbenes are of course stabilized by electron donation from the heteroatoms, forming the ylide resonance forms. 

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