Oxacyclopentylidene complexes

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

Most efforts to explore the reactivity of ruthenium carbene complexes have employed the alkoxycarbene species so readily synthesized from the inter- or intramolecular reaction of vinylidene complexes with alcohols. These electrophilic alkoxycarbene complexes exhibit only limited reactivity at Ca, primarily with hydride reagents. For example, treatment of the 2-oxacyclopentylidene complex 97 with NaAlH2(OCH2CH2OMe)2 affords the neutral 2-tetrahydrofuranyl complex (98) [Eq. (89)] (55), as was anticipated from similar reductions of iron carbene complexes (87). 

Weyershausen, B, Nieger, M, Doetz, K H, Chiral 2-oxacyclopentylidene complexes via metal-assisted cycloisomerization of carbohydrate-derived butynols, Organometallics, 17, 1602-1707, 1998. 

The oxacyclopentylidene complexes 101 react with N,N-dimethylformamide dimethyl acetal (DMF-DMA) to form complexes 102 and 103. Related results are seen for acyclic carbene systems. 

Carbene anions react with ethylene oxide to give 2-oxacyclopentylidene complexes (Anderson and Casey, 1974b). The reaction can be envisioned as nucleophilic attack of the carbene anion on the epoxide, followed by intramolecular displacement of methoxide from the intermediate adduct. As expected for a nucleophilic attack, propylene oxide is attacked by carbene anions at the least hindered carbon atom. 

To test for the intermediacy of free carbenes in the decomposition of metal-carbene complexes, the thermolysis of the 2-oxacyclopentylidene complex XVII was studied (Anderson and Casey, 1975). 2-Oxacyclopentylidene has been generated by pyrolysis of the sodium salt of butyrolactone tosylhy-drazone and shown to rearrange to give both dihydrofuran and cyclobutanone (Agosta and Foster, 1972). The thermolysis of XVII in decalin at 180° for 15... 

Scheme 11.58 Diastereoselective [4-r2] and [2-i-2]cycloaddi-tion reactions of a-exo-methylene oxacyclopentylidene complexes.

Scheme 11.59 Diastereoselective [4+2]cycloaddition reactions to trisspiro oxacyclopentylidene complexes ( Yield referring to complex 76a because of its low stability complex 77b was prepared in situ and transformed without previous separation).

In 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

The photochemical protocol may be the method of choice in cases where the thermal reaction fails. This is true for exo-alkylidene oxacyclopentylidene chromium complexes such as 62, which are inert under thermal conditions but undergo a photoinduced benzannulation with, for example, 3-hexyne to give benzofuran 63 (Scheme 24) [61]. 

Scheme 24. Photochemical benzannulation of an exo-alkylidene-oxacyclopentylidene chromium complex.

C. P. Casey, and R. L. Anderson, Thermolysis of (2-Oxacyclopentylidene)-pentacarbo-nylchromium(O) Evidence Against Free Carbenes in Thermal Decomposition of Metal-Carbene Complexes, J. Chem Soc., Chem. Comm. 1975, 895-896. 

FIGURE 10.10 Face differentiation on methylene-2-oxacyclopentylidene chromium complexes 119 (Scheme 10.39, Ref. [91]). 

Metal carbene centers may be coimected via either the carbon or the heteroatom carbene chain [47]. The first example of a homodimetallic biscarbene complex has been synthesized by trapping a metal carbene anion - generated in situ by a-de-protonation of chromium oxacyclopentylidene 73 - by its exo-methylene congener 74 formed upon the addition of the same chromium carbene anion to formaldehyde in a Michael-type addition reaction [48]. A more satisfactory yield of bischro-mium biscarbene 75 has been obtained by using the formaldehyde equivalent CICH2OCH3 as source of the Cj-bridge and Lil (Scheme 11.23). 

Aminolysis of alkoxycarbene complexes with primary or secondary amines may generate /Z-mixtures of aminolysis products the configuration of which may be controlled within certain limits by the reaction conditions. For example, the reaction of tris(2-aminoethyl)amine with chromium methoxy(phenyl)carbene 130 carried out at -30 °C in ethereal solution gives a 91% yield of a Id-mixture of the E/E/E- and the / /Z-isomers oftripodal trischromium aminocarbene 131, whereas at ambient temperature the pure / /Z-isomer is formed in 74%. The ring-opening aminolysis of chromium oxacyclopentylidene 73 carried out at -78 °C in a 1 d -mixture of DMF and CHjClj affords a nearly quantitative yield of the tripodal E/E/E-amino(hydroxypropyl)carbene complex 132 (Scheme 11.31) [56]. 

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