Reactive Intermediate Chemistry

hand, it is certain that the C C double bond of {IIIa}2 is easily cleaved with electrophiles with liberation of IIIa.28 In any case, the carbene-stabilizing influence of the two amino groups was clearly recognized and interestingly Wanzlick also mentioned that an even larger stabilizing effect should result from the incorporation of a double bond in the ring, owing to ‘‘the participation of the aromatic resonance structures.’’4 Wanzlick et al.29 did demonstrate in 1970 that imidazolium salts such as 4a,b could be deprotonated by potassium tert-butoxide to afford the corresponding imidazol-2-ylidenes (IVa,b), which were trapped but not isolated. With phenylisothiocyanate and mercury acetate as trapping agents the corresponding zwitterion29a and carbene–mercury complex,29b respectively, were isolated (Scheme 8.4).

Some 30 years later, Wanzlick’s work was reinvestigated by Arduengo et al.,30 who were able to prepare and to isolate the imidazol-2-ylidene IVc (R ¼ adamantyl) in near quantitative yield (Scheme 8.4). The deprotonation of the 1,3- di-1-adamantylimidazolium chloride (4c) was carried out with sodium or potassium hydride, in the presence of catalytic amounts of either t-BuOK or the dimethyl sulfoxide (DMSO) anion. Carbene IVc is thermally stable in the solid state (colorless crystals, mp 240–241 C).

Very interestingly, in 1998, Arduengo published a paper31 entitled ‘‘1,3,4,5- Tetraphenylimidazol-2-ylidene: the realization of Wanzlick’s dream.’’ A modification of the procedure published by Wanzlick makes it possible to isolate the carbene IVb. In the conclusion, it is mentioned that ‘‘the inconvenient physical properties of the carbene, possible problems with respect to purity of the starting material, and the then widely accepted idea that imidazol-2-ylidenes are too labile to be isolated in pure form probably all contributed to the fact that Wanzlick et al. did not actually isolate IVb.’’ To be fair to Wanzlick, we have to realize that at that time the research facilities were quite different, otherwise this great scientist might have been the first to isolate a singlet carbene.

4. SYNTHESIS AND STRUCTURAL DATA FOR STABLE SINGLET CARBENES

This section will be divided into three parts following the chronological order of the discovery of the different types of stable carbenes. The first two parts deal with carbenes featuring two heteroatom substituents, first of type D-C-W, then of type D-C-D. Then, we will discuss stable carbenes, which have been discovered in the last 2 years in our group, and that feature only one electronically active heteroatom substituent.

4.1. Carbenes with a p-Electron-Donating and a p-Electron- Withdrawing Heteroatom Substituent (D-C-W)

Two families of carbenes which fall into this category are the (phosphino)(silyl)carbenes I and the (phosphino)(phosphonio)carbenes II.

Scheme 8.5

Since the discovery of the stable carbene Ia, many other (phosphino)(silyl) carbenes have been prepared, all of them using diazo precursors,32 but only a few of them are stable at room temperature. The silyl group at the carbene center can be replaced by an isoelectronic and isovalent phosphonio substituent without dramatic modification. Indeed, the stable (phosphino)(phosphonio)carbenes IIa33a and IIb33b were synthesized from the corresponding diazo precursors 2a and 2b in 76 and 85% yields, respectively (Scheme 8.5).

The (phosphino)(silyl)carbenes (I) are all characterized by high-field nuclear magnetic resonance NMR chemical shifts for phosphorus ( 24 to 50 ppm) and silicon ( 3 to 21 ppm), and low-field chemical shifts for carbon (78–143 ppm) with large coupling constants to phosphorus (147–203 Hz). Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron rich, whereas the carbon has a chemical shift in the range expected for a multiply bonded species. The values of the coupling constants are difficult to rationalize, but classical interpretation of the NMR data indicates that the P C bond of (phosphino)(silyl) carbenes has some multiple bond character. The (silyl)- and (phosphonio)(phosphino)carbenes I and II are very similar, as deduced from the similarity of their NMR spectroscopic features.

The exact nature of the bonding system in phosphinocarbenes was clarified to some extent by the X-ray analyses performed on the (phosphino)(phosphonio) carbene (IIa),33a and more recently on the (phosphino)(silyl)carbene Ib.34

The molecular structure of Ib (Fig. 8.3) shows that the ring and the PCSi frag-

˚

ment are coplanar (maximum deviation from the best plane: 0.03 A), the PC bond

˚

length [1.532(3) A] is short, and the PCSi framework is bent [152.6(3) ]. The presence of a strongly polarized PdþCd fragment is suggested by the short SiCcarbene

˚ ˚

[1.795 A compared with 1.86–1.88 A for Si Me] and PN bond distances [1.664(2)

˚

A]. These data indicate an interaction of the phosphorus lone pair with the formally vacant orbital of the carbene, and might suggest that the carbene lone pair interacts with the s orbitals of both the silyl and the phosphino groups (negative hyperconjugation). Therefore, at that stage, form A could be ruled out, but all the others were reasonable.

Dixon and Arduengo48 explained the extraordinary stability of the carbene IVc as mainly resulting from the inductive effect of the neighboring nitrogen atoms. The nitrogen lone pairs and the C C double bond were supposed to ensure sufficient kinetic stability because of their high electron density, which was supported by a variety of experimental techniques.49 A subsequent study by Cioslowski50 even came to the conclusion that the p-donation of the nitrogen lone pairs played only a minor role. However, in 1996, Apeloig,51a and Frenking51b and their co-workers independently investigated the importance of the aromaticity in carbenes (IV). According to structural, thermodynamic, and magnetic criteria, as well as the p populations and ionization potentials, cyclic electron delocalization does indeed occur in the imidazol-2-ylidenes (IV). This finding has been confirmed by inner-shell electron energy loss spectroscopy.52 Although this aromatic character is less pronounced than in the imidazolium salt precursors 4, it affords an additional stabilization of 25 kcal/mol.51a

Note that the other type of aromatic carbene isolated by Enders et al.,42 namely, the 1,2,4-triazol-5-ylidene (VIIa) is stable enough to be prepared by thermal elimination at 80 C, and it became the first carbene to be commercially available.

However, aromaticity is not the major stabilizing factor for carbenes of types IV and VII. More important is the interaction of the carbene center with the p-donating s-attracting amino substituents. This is the reason that in recent years other diaminocarbenes were isolated, including acyclic diaminocarbenes of type IX.44

With an amino group present, other p-electron-donor groups can be used as the second substituent, as shown by the preparation of 1,3-thiazol-2-ylidenes (VIII),43

340 STABLE SINGLET CARBENES

acyclic aminooxy- X,45 and aminothio-carbenes XI.45 However, the superior p- donor ability and therefore the superior stabilizing effect of amino versus alkoxy groups (and thio groups) has been evidenced experimentally. Indeed, the bis- (dimethylamino)carbene Me2N C NMe2 can be observed by NMR spectroscopy at room temperature,53 while the dimethoxycarbene MeO C OMe has only been characterized in matrices at low temperature (lifetime in solution at room temperature: 2 ms).18

The carbene carbon of III–XI resonates at rather low field in 13C NMR spectra (d ¼ 205–300 ppm) compared to the corresponding carbon atom of the cationic precursors (d ¼ 135–180 ppm).

The solid-state structures of derivatives of type III, IV, and VI–X have been elucidated by single-crystal X-ray diffraction studies. The bond angles observed at the carbene centers (100–110 ) are in good agreement with those expected for singlet carbenes of this type. The larger value observed in the acyclic bis(diisopropylamino)carbene of type IX (121.0 ) probably results from severe steric effects.

˚

N C bond lengths being only a little bit shorter (1.28–1.33 A). These data as a whole indicate that the N Ccarbene bonds have some multiple-bond character, which results from the donation of the nitrogen lone pairs to the carbene vacant orbital. This finding is confirmed by the large barriers to rotation about the

N Ccarbene bond determined for IX and X (13 and at least 21 kcal/mol, respectively) by variable temperature solution NMR experiments.44,45,54 Therefore, di-

aminocarbenes are best described by resonance forms B and C, which may be summarized by structure D (Fig. 8.6). For aminothio and aminooxycarbenes the S Ccarbene and O Ccarbene bonds have less p character, and therefore, the best representation for these monoaminocarbenes is provided by resonance form B, in which oxygen and sulfur act as s-electron-withdrawing moieties.

Figure 8.6. Resonance structures for D-C-D0 carbenes.

4.3. Carbenes with One Electronically Active Heteroatom Substituent

The isolation of stable singlet diheteroatom-substituted carbenes represents a spectacular synthetic achievement. However, their reactivity (see Section 5) is strongly influenced by the interaction of the two heteroatom substituents with the carbene center and therefore is somewhat different from that of their transient cousins. Thus it was tempting to try preparing carbenes with only one heteroatom substituent.

with aromatics, but not with simple olefins. For example, this interaction led to an

extension of the benzylchlorocarbene lifetime from 23 ns (isooctane) to 285 ns (benzene).58c Similarly, we found that photolysis of 12a in hex-1-ene at 60 C

led to the carbene dimer {XIIa}2, while in toluene we obtained the carbene XIIa, which is stable up to 30 C (Scheme 8.7).

Calculations59 carried out on the simplified model compound XIIb (R ¼ NH2), predict the molecule to be significantly bent (P C C bond angle: 126.4 ), the

˚

phosphorus planar, and the P C bond length short (1.584 A) as expected for this type of singlet carbene. The singlet–triplet energy gap was predicted to be small (12 kcal/mol), but in favor of the singlet state, and the energy barrier for the dimerization negligible, in perfect agreement with the experimental results.

We then investigated the possibility of replacing the s-attracting CF3 group (-I) by the bulky 2,6-bis(trifluoromethyl)phenyl group, which is both a s and a p attractor ( I, M). For all solvents used, photolysis of the diazo precursor at 10 C afforded the corresponding carbene XIIc, which is stable for weeks at room temperature both in solution and in the solid state (melting point 68–70 C).56 The molecular structure of XIIc (Fig. 8.8) shows that the phosphorus atom is in a planar

˚

environment and the P1 C1 bond length [1.544(3) A] is short, as expected because of the donation of the phosphorus lone pair into the carbene vacant orbital. The aromatic ring is perpendicular to the CcarbenePNN plane allowing the delocalization of the carbene lone pair into the ring, which is apparent from the very large angle at the carbene center (162 ). Clearly, the aryl group acts as an electron-withdrawing group and helps in the stabilization of the carbene center. In other words, carbene XIIc has to be considered a push–pull carbene.

The isolation of XIIc was of importance since it demonstrated for the first time that monoheteroatom-substituted carbenes could be stable, indefinitely, at room temperature. The next step was to investigate whether a single electronically active substituent is sufficient to isolate a carbene.

Since the carbene XIIc is stable thanks to the presence of two substituents of opposite electronic properties, which preserve the electroneutrality of the carbene center, it was tempting to use a single substituent, which on its own would be both an electron donor and an electron acceptor; the amino group was the obvious choice.

The iminium salt 13a, bearing a bulky tert-butyl group at nitrogen and a 2,4,6- tri-tert-butylphenyl group at carbon was deprotonated with potassium hydride at