Bis-carbenoid

Igau A, Grutzmacher H, Baceiredo A, Bertrand G (1988) Analogous a,a -bis-carbenoid, triply bonded species synthesis of a stable 3-phosphino car-bcnc-75-phosphaacclylcnc. J Am Chem Soc 110 6463... 

Thus, in 1988, we published a full paper entitled Analogous a,a -bis-carbenoid triply bonded species Synthesis of a stable X -phosphinocarbene-X -phosphaacetylene . [21] Again, we were focussed on the multiple bond nature of our compound 17. However, in the last paragraph, we reported that pyrolysis of 17 at 300°C under vacuum, afforded the five-membered heterocycle 22 (as a mixture of four diastereomers), probably via the insertion of the singlet carbene... 

The Ru—Ca bonds of ruthenacyclopentatrienes are double bonds in character and therefore ruthenacyclopentatrienes are expected to behave as cyclic biscarbenoids. Indeed, the reaction of 1,6-diynes with norbornene in the presence of 6, 105, or 106 produced biscyclopropanation products 107 and [2 - - 2 + 2] cycloadducts 108 in various yields and ratios (Scheme 3.23) [78]. The former product is conjectured to be evidenee of the ruthenacycle intermediates acting as ruthenium bis(carbenoid) species. This carbenoid character of ruthenacyclopentatrienes also plays a critical role... 

As will be discussed in detail in the next section, when the reduction of the parent compound 6 was conducted at low temperature (—20 °C) with 2 equiv. of Na in tetrahydrofuran (THF) saturated with ethylene, complete salt removal was achieved, and the r 2-ethylene complex 20 was isolated.22 Upon irradiation, the latter released ethylene, behaving as a source of the d2 [W / -Bu -calix[4]-(0)4 ] carbenoid, which coupled to give a new W=W dimer [W=W, 2.582(1) A], isolated as the bis-Bu NC adduct 21. In H NMR, 21 exhibits a C5-symmetric pattern of signals for the calix[4]arene moiety. 

Simultaneous occurence of the pyrazoline and carbenoid route is observed in the presence of bis(campherquinone-a-dioximato)cobalt(II) 95), but the cyclopropanes derived from ethyl diazoacetate and H2C=CHX (X = COOMe, CN) were obtained only in low yield. 

Bis(pinacolato)diboron reacts with 1-halo-l-lithioalkenes, that is, alkylidene carbenoids, affording 1,1-diboryl-1-alkenes in good yields (Scheme 9).76 The reaction proceeds via formation of a borate intermediate, which is followed by 1,2-migration of the boryl group with elimination of the bromo group. 

To complete the range of geometric isomers of terminal and non-terminal dienes and trienes available, systems nominally derived from inaccessible (Z)-alkenylzirconocenes are desirable. Fortunately, insertion of the various carbenoids discussed above into mono- or bis(alkynyl) zirconocenes 64 and 65 affords dienyne products 66 [38], which are readily reduced to the desired ( ,Z,2)-trienes (Scheme 3.15) [45—47]. Insertion of the f5-alkynyl carbenoid 62 allows a convenient access to (Z)-enediynes 67. 

Scheme 3.32. Mono- and bis-insertion of benzyl carbenoids into saturated zirconacycles.

Insertion of phenyl, trimethylsilyl, and nitrile-stabilized metalated epoxides into zircona-cyclcs gives the product 160, generally in good yield (Scheme 3.37). With trimethylsilyl-substituted epoxides, the insertion/elimination has been shown to be stereospecific, whereas with nitrile-substituted epoxides it is not, presumably due to isomerization of the lithiated epoxide prior to insertion [86]. With lithiated trimethylsilyl-substituted epoxides, up to 25 % of a double insertion product, e. g. 161, is formed in the reaction with zirconacyclopentanes. Surprisingly, the ratio of mono- to bis-inserted products is little affected by the quantity of the carbenoid used. In the case of insertion of trimethylsilyl-substituted epoxides into zirconacydopentenes, no double insertion product is formed, but product 162, derived from elimination of Me3SiO , is formed to an extent of up to 26%. 

Jacobsen and co-workers (87) investigated the carbenoid-transfer reaction to inline acceptors. These workers found that bis(oxazoline)Cu(I) complexes are most effective among the catalysts screened, providing moderate yields and selec-tivities in this process. The reaction is complicated by the formation of pyrrolidines in racemic form as side products (Eq. 75). 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

Carbenoids generated from the catalytic decomposition of phenyliodon-ium bis(phenylsulfonyl)methylides (135) in the presence of thiobenzophe-nones lead to the formation of benzo[c]thiophenes 136 [94JCR(S)2] (Scheme 38). The earlier work [89JCS(P1)379] reporting the synthesis of isomeric benzo[fe]thiophenes from this reaction was found to be in error. 

Enantiomerically pure a-lithiated ethers have been prepared from stannanes and turned out to react with electrophiles under retention. The configurational stability of the hthium carbenoid 19 has been deduced from equation 10 . Lithiated benzyl methyl ether, chelated by a chiral bis(oxazoline) ligand, proved itself to be configurationally stable as welP . ... 

Functionalised 2,3-dihydro-l,4-dioxins can be synthesised in a three step-sequence from P-keto esters. The key step is the insertion of a Rh-carbenoid derived from an a-diazo-p-keto ester into an 0-H bond of a 13-diol <99H(51)1073>. The reaction of 2-(l,4-dioxenyl)alkanols with silyl enol ethers yields 23-disubstituted 1,4-dioxanes. When 13-bis(trimethylsilyloxy)-cyclobut-l-ene is used, the expected cyclobutanone products are accompanied by a spirocyclopropane derivative <99TL863>. 1,4-Dioxane-monochloroborane 57 is a highly reactive hydroborating reagent <990L315>. 

Saito et al. <1995S87> described a new method for the synthesis of heterocycle-fused[c]thiophenes via reaction of aryl heteroaryl thioketones with the carbene precursors. Heteroaromatic thioketones A react with carbenoids generated from bis(arylsulfonyl)diazomethanes or phenyliodonium bis(phenylsulfonyl)methylides to give heterocycle-fused[f]thiophenes B. The reaction involves the ring closure of the intermediary thiocarbonyl ylides, followed by restorative aromatization via the elimination of a sulfenic acid (Equation 11). 

For example, as shown in equation 43, Taguchi, Nozaki and coworkers reported in 1974 a one-carbon ring enlargement of cyclododecanone (187) to cyclotridecanone (190) with dibromomethyllithium through / -oxido carbenoid (188) . This reaction was expected to proceed via a one-carbon expanded enolate (189). Cohen and coworkers used the bis(phenylthio)methyllithium whereas Satoh and coworkers used a-sulfinyl lithium car-banion of 1-chloroalkyl aryl sulfoxides as the source of S-oxido carbenoids (equation 44) °. 

The bis(benzoyloxymethyl)zinc (71) was prepared either from zinc benzoate and diazomethane or from benzoyloxymethyl iodide and EtiZn under photolysis conditions (Scheme 9) . Such an acyloxymethylzinc compound appeared to be a reactive carbenoid capable of reacting with a variety of non-functionalized alkenes to afford cyclopropanes in excellent yields (Scheme 9). 

Functionalized zinc carbenoids have been prepared from carbonyl compounds by an indirect strategy. The deoxygenation of a carbonyl compound to an organozinc carbenoid can be induced by a reaction with zinc and TMSCl. Therefore, the aldehyde or ketone, when treated with TMSCl or l,2-bis(chlorodimethylsilyl)ethane in the presence of an alkene, generates the cyclopropanation product. This method is quite effective for the production of alkoxy-substituted cyclopropane derivatives. A 55% yield of the... 

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