Acyclic diaminocarbenes

Since this discovery, a few types of other stable singlet carbenes have been described and reviewed5 imidazol-2-ylidenes,6 l,2,4-triazol-3-yli-denes,7 imidazolidin-2-ylidenes,8 acyclic diaminocarbenes,9 thiazol-2-yli-denes,10 and acyclic aminooxy- and aminothiocarbenes.11... 

Scheme 4. Synthetic routes to an iron complex containing an acyclic diaminocarbene.

Much less coordination chemistry of acyclic diaminocarbenes 4 as compared to that of cyclic ones has been described (Scheme 6). Interestingly, in the case of... 

Scheme 11 Suzuki reaction catalyzed by acyclic diaminocarbenes-Pd complexes...

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 7i-donating a-attracting amino substituents. This is the reason that in recent years other diami-nocarbenes were isolated, including acyclic diaminocarbenes of type IX." " ... 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Summary Bis(trimethylsilyl)mercury cleanly reacts at low temperature with chloroiminium chlorides to form stable metal-free cyclic and acyclic diaminocarbenes as well as aryl-, oxy-, chloro-, hydrogeno- and alkyl-amino carbenes. The aryl-, chloro-and hydrogeno-amino carbenes were formed as transient intermediates that undergo dimeri2ation into the corresponding alkenes, which were isolated in good yields. Alkyl-and oxy-amino carbenes were observed at low temperature by C NMR spectroscopy. 

Figure 11 Cross metathesis of allylbenzene and CDAB with acyclic diaminocarbene-based catalysts...

Bicyclic bisamidines can be prepared from tra i-A, A( -dunethyl-l,2-diaminocyclohexane and a palladium bis(arylisocyanide) complex [38] (Scheme 3.22). Oxidation of the initially-formed palladium bis(acyclic diaminocarbene) complex with air or idosobenzene (Phi = O) followed by treatment with excess amounts of methylisonitrile (MeNC) yields bicyclic bisamidines. The structure of the product obtained in each step is unequivocally determined by X-ray crystallographic analysis. 

Wanniarachchi, YA. and Slaughter, L.M. (2007) One-step assembly of a chiral palladium bis (acyclic diaminocarbene) complex and its unexpected oxidation to a bis(amidine) complex. 

In early studies of stable diaminocarbenes, it was suggested that ir-donation from the two nitrogen lone pairs into the empty p orbital on the carbene centre might not be important. [53,54] This view was challenged from a theoretical standpoint by Apeloig [55] and Frenking, [56] but it was not until acyclic diaminocarbenes were prepared that a fuller experiment assessment could be made. 

A striking feature of the imidazol-2-ylidenes is the small N-C-N bond angles (around 100-102°). These angles are smaller than in the imidazolium ion precursors and this trend, which is reproduced by good quality calculations, continues with the saturated imidazolidin-2-ylidenes right through to the comparison of acyclic diaminocarbene 6 (121°) [57] and its precursor 22 (133°). [58]... 

Acyclic diaminocarbenes (ADCs) as a promising alternative to N-hetero-cyclic carbenes (NHCs) in transition metal catalyzed organic transformations 12CCR2029. 

An enantioselective version of this reaction has been described recently by Handa and Slaughter using gold(I) catalysts of the type K bearing chiral acyclic diaminocarbene ligands (ADCs Scheme 4-85). Compared with NHCs, their N-C-N angles are wider so that chiral substituents are closer to the metal center. LiNTf2... 

Dhudshia, B. and Thadani, A.N. (2006) Acyclic diaminocarbenes simple, versatile ligands for... 

Scheme 16.2 Alternative routes to acyclic diaminocarbene (ADC) complexes.

PALLADIUM-(ACYCLIC DIAMINOCARBENE) SPECIES AS ALTERNATIVE TO PALLADIUM-(NITROGEN HETEROCYCLIC CARBENES) IN CROSS-COUPLING CATALYSIS... 

Figure 11.1 Palladium complexes with N-heterocyclic carbenes (NHCs) and acyclic diaminocarbenes (ADCs).

Acyclic diaminocarbene when complexed to palladium has been shown to preclude the homocoupling of alkynes in the Sonogashira coupling with acyl chloride inducing a complete chemoselectivity for the desired product. ... 

Barriers to rotation around the Cca —N bonds have been determined experimentally for diaminocarbenes (3) and (4) and their protonated and lithiated counterparts the possible involvement of lithium or a proton in the dimerization of these acyclic diaminocarbenes was also reported. A computational study of the dimerization of diaminocarbenes has been performed via rate constant calculations using general transition-state theory calculations. Such a dimerization has been shown to be a rapid equilibrium between the carbenes and the tetra-A-alkyl-substituted enetetramines (5), by characterization of metathesis products when two different tetramines were mixed. The thermodynamic parameters of this Wanzlick equilibrium have been determined for the A-ethyl-substituted compound the enthalpy of dissociation has been evaluated at 13.7kcalmol and the entropy at 30.4calmor K . Complex-ation of diaminocarbenes by alkali metals has been clearly established by a shift of the C NMR signal from the carbene carbon of more than 5 ppm. ... 

Wang YM, Kuzniewski CN, Rauniyar V, Hoong C, Toste FD. Chiral (acyclic diaminocarbene)gold(I)-catalyzed dynamic kinetic asymmetric transformation of propargyl esters. Chem Soc. 2011 133 12972-12975. 

Slaughter recently reported a modular entry into chiral bis(ADC) complexes (ADC = acyclic diaminocarbene) starting from the corresponding... 

The development of acyclic diaminocarbenes in transition metal-catalysed reactions has been covered with a fruitful effort to rationalize the previous studies and observations to highlight the pros of these alternative carbenes. ... 

Calculations have been carried out in order to assess the reactivity of cyclic and acyclic diaminocarbenes with CO2 in the presence of alcohols These calculations indicate that the formation of carbonate salts is favoured compared to the direct interaction with CO2. Because of their high values, these carbenes represent efficient systems for the capture and sequestration of carbon. Computations on the stereoselective transformation of 2-benzyloxyphenyl trimethylsilylketone to substituted benzofuran (Scheme 1) have been carried out. They revealed an exergonic 1,2-Brook rearrangement to an oxycar-bene, exhibiting two unpaired antiparallel electrons that feature in H migration followed by diradical coupling. 

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