Carbene adducts

An additional amount of material ca. 0.7 g) crystallizes from the mother liquors on standing. Recrystallization from acetone gives monodibromo-carbene adduct 17-ketone (43a) as prisms mp 154-155°. 

However, the presence of a fused benzene ring was found to hmit the scope of this type of ring expansion severely. Thus the dichloro-carbene adducts of both 2H- (33) and 4H-chromen (34) failed to rearrange to 2,3-benzoxepines (35) on heating. Although some... 

Dimethyl-1,2,4-triazolium iodide with palladium acetate yields the carbene adduct 182 (97JOM(530)259). Under water it undergoes cis-trans isomerization to 183. Some other derivatives were reported in 1981 (81BCSJ800). 1,1 -Methylenebis(4-alkyl-l,2,4-triazolium)diiodides (alkyl = /-Pr, n-Bu, octyl) with palladium(II) acetate give the mononuclear complexes [L Pdl ] (99EJIC1965), where L2= l,l -methylenebis(4-R-l,2,4-triazol-2-ylidene) (R = /-Pr, n-Bu, octyl). Thermolysis of the products in THF gives the rran -dinuclear complexes 184... 

Efficient stirring is required. A solution of 225 g. (5.6 moles) of sodium hydroxide in 225 ml. of water can be added to the stirred mixture of the organic substrates in dichloromethane if a more efficient mechanical stirrer is used. In the original procedure, the submitters noted an induction period of about 20 minutes which was stated to vary somewhat with the stirring rate, stirring-bar size, and relative amount of phase-transfer catalyst. Three moles of base are required for the reaction one to generate the carbene and two to react with the additional two moles of hydrochloric acid lost by the amine-carbene adduct in the isonitrile formation step. If less base is used, the excess hydrochloric acid reacts with the isonitrile by a-addition, and the yield is substantially reduced. 

Treatment of aromatic aldehydes such as p-anisaldehyde with Zn-powder and l,2-bis(chlorodimethylsilyl)ethane 45 give Zn-carbene adducts such as 2096 which add readily to olefins such as cyclohexene [22, 26] or styrene [26] to give high yields of cyclopropanes such as 2097 and the oxide 47 [26]. Acetals such as 2098 react analogously with cyclohexene to afford the endo and exo cyclopropanes 2099 and 2100 [22, 27] (Scheme 13.11). 

The exact nature of the catalytically active Ni species in these reactions is yet to be conclusively established. Hydrodechlorination proves optimal with a NHC Ni ratio of 2 1 suggesting that 14-electron Ni(NHC)2 is involved, whereas the 1 1 NHC Ni ratio necessary for hydrodefluorination implies that it is the 12-electron mono-carbene adduct Ni(NHC) which is catalytically active [10]. Smdies by Matsubara et al. revealed that treatment of NKacac) with either one or two equivalents of IMes HCl 1 or SlMes HCl 2 in the presence of NaOHu formed the mono-NHC complex Ni(NHC)(acac)j which, upon reduction with NaH in the presence or absence of carbene, formed Ni(NHC)2 [11]. Density functional theory (DFT) calculations suggest that the strength of the Ni-NHC bond (ca. 50 kcal/mol) makes... 

Push-spectator carbenes of the type 31 (R, R = alkyl) were synthesized and reacted with various Lewis Acids to compare the reactivity of the phosphorus and carbene centers. Two such reactions are shown in Scheme 7.11. From an X-ray structural analysis, the phosphorus substituent was shown to act as a spectator, leaving its lone pair available to react in a Lewis basic manner. When carbene 31 was reacted with BF3, only the carbene adduct 32 was formed. By contrast, when 31 was reacted with the softer Lewis Acid BH3, it was the phosphorus that reacted to yield adduct 33. These types of carbenes exhibited C-NMR shifts in the range of 320-348 ppm, a P-C-N angle of 116.5° a short C-N distance of 1.296 A, and a long C-P distance of 1.856 A. The latter is very similar to that of a typical C-P single bond. 

Carbene adducts of dimethylcadmium 188-190 have been obtained by simple addition of CdMe2 to the corresponding carbene at room temperature.2 9 All the three adducts have similar structures. In the case of 190, the Ccarbene-Cd bond of 2.33 A is 0.15 A longer than the Cmethyi-Cd bonds of 2.18 A (Figure 28). As a result, the cadmium atom is in a distorted trigonal-planar environment with a Cmethy -Cd-Cmethyl angle of 136°. 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

When tetrabromide 41 was treated with an excess of MeLi/LiBr in cyclohexene as a trap for carbenoids and carbenes, adduct 43 was obtained in 4% yield. In addition, 44 was isolated in 3% yield. The main product, however, was tetrabro-mide 45, whose yield amounted to 32%. The structure of 45 was established by single crystal X-ray crystallography.18... 

Air-sensitive carbene adducts of chlorine (92-94) have been iso-... 

When triethylborane is used, the borane-carbene adduct 44 is stable in solution for several weeks at -20°C, but only for a day at room temperature.25 Elimination of diethyl(dicyclohexylamino)borane occurs leading to... 

Alternatively, NHC ligands can also be transferred from triethylborane-carbene adducts [103] or complexes of the type [M(NHC)(CO)5] (M = Cr, Mo, W) [104], but these procedures are limited to special cases and are of less importance. The coordination chemistry of silver NHC complexes [105] and the advantages and applications of the Ag20 method [106] have recently been reviewed. 

Apparently the fusion of a benzocyclopropene to two cyclobutanes represents the upper limit for isolable benzocyclopropenes. Several attempts to synthesize dicyclopropabenzenes failed. Thus, attempted aromatization of the carbene adducts of the isomeric cyclohexadienes, 99 and 101 afforded none of the expected 100 and 102, respectively, and attempted aromatization of 103 resulted only in uncharac-terizable decomposition products without any evidence for the intermediacy of 104. ... 

Other possible carbene precursors (vicinal dihalides, peresters, ketenes, carbene adducts with stable hydrocarbons, etc.) cannot generally be used for in situ generation of carbenes, because the fragments are likely to recombine. However, they can be used, for example, in experiments involving pyrolysis or other forms of external carbene generation where the fragments get a chance to separate in the gas phase and become trapped in distant matrix sites. AU conceivable halo- and dihalocar-benes were made and smdied in this way (see, e.g., the 1993 review by Sander et al." ) However, such methods can only be applied to carbenes which resist thermal rearrangement to more stable products. 

The method has been used for y-lactone cis annulations to A 2- and A 3-cholestene via dibromo-carbene adducts.81... 

The alkyl shifts between metal and nitrogen during redox reactions are mechanistically interesting (equation 26). The question as to whether the alkyl migration is intramolecular or intermolecular, or a dual process, does not seem to have a clear-cut answer as yet.150 Reactions of the carbene adducts are summarized in Scheme 37. 

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

The 3-phospholene 1-oxide derivative is a potential heterocycle for easily producing chemically modified phosphorus heterocycles because the compound possesses a reactive C=C double bond, allylic methylene, an electron-deficient methylene group a-positioned to phosphorus, etc. 4-Chloro-l,6-dihydrophosphinie derivatives 16A and 16B are prepared from dichloro-carbene adducts 15 with l-(R)-3-ethylphospholene 1-oxide (Scheme 5) [4]. 

Two diasteromers of phosphorus heterocycles 33A and 33B are prepared from 3,4-dimethyl-3-phospholene 32 via the carbene adduct forma-... 

ADF-DFT calculations indicated the presence of a-type orbital interactions mainly generated by the U(/z3) orbital with a minor contribution from the C—H (.v// )-type of orbitals. Incidentally, the same complex reacts with an /V-hetero-cyclic carbene to form the corresponding U(III)-carbene adduct (20). 

Cl mass spectroscopic studies at temperatures <100°C show the parent molecular ions, but sublimation does not proceed as smoothly as with solvent free Ln(btsa)3. Application of higher temperatures (ca. 150 °C) results in the release of THF followed by the detection of dinuclear species in the mass spectra. The THF ligands in Y(bdsa)3(THF)2 could be replaced by stronger donor molecules as the carbene ligand l,4-dimethylimidazol-2-ylidene [140]. The bdsa ligand is rather flexible in these exchange reactions to afford both mono, Y(bdsaXL), and bis (carbene) adducts Y(bdsa)(L2), respectively. 

Reaction of (180, X = Cl) with methyl lithium in the presence of alkenes at ambient temperature leads to apparent carbene adducts (183), in this case derived from ring opening of (181) to the highly functionalised isoprenoid carbene (182). Surprisingly, the bromide (180, X = Br) reacts by a different course, leading to (184), apparently through initial lithium-bromine exchange followed by 1,3- rather than 1,2-elimination of LiCl 130>. 

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