Tricarbonylchromium complexes

Tricarbonylchromium complexes are useful for 1,4-addition of hydrogen to 1,3-dienes to afford monoenes selectively (40,42,43,44). With 1,4-dienes, isomerization into conjugation precedes hydrogenation. Isolated double... 

The tricarbonylchromium complex 6 of methyl 1/f-azepine-l-carboxylate undergoes photoin-duced [6 + 4] 7t-cycloadditions with dienes, e.g. 7, to give, after decomplexation with oxygen, azabicyclo[4.4.1]undecanes, e.g. 8, in high yields.276... 

It has been observed, however, that the enantioselectivity of reactions of tartrate ester modified allylboronates with metal carbonyl complexes of unsaturated aldehydes are significantly improved compared with the results with the metal-free, uncomplexed aldehydes72. Two such examples involve the (benzaldehyde)tricarbonylchromium complex and the hexacarbonyl(2-... 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

Scheme 17 Formation of the (tricarbonylchromium)-complexed fulvene 81 and the cyclo-penta[b]pyran 82 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne [76]...

Scheme Sml2-promoted reductive coupling of optically pure tricarbonylchromium-complexed benzaldimines and ferrocenyl diimines...

There are only a few examples of displacement of a fluorine atom in a cross-coupling reaction. With tricarbonylchromium complexes of fluoroarenes as substrates, cross-coupling with both boronic acids and vinylstannanes has been realized. Interestingly, only PMe3 is effective as ligand in this reaction (89).2" See Section 9.6.3.4.10 for an example of the involvement of unactivated fluoroarenes in cross-coupling reactions. 

The arene groups in (r/ -arcnc)tricarbonylchromium complexes are typically electron poor and display poor reactivity toward electrophiles. In the case of mercuration reactions, this lack of reactivity can be overcome by attachment of Lewis-basic substituents to the arene ring. For example, in the case of 75a-c, the presence of a pyridyl, oxazolyl, or methyl-A,A-dimcthylami no group promotes ortho-mercuration, leading to the formation of the bimetallic complexes 76a-c (Equation (28)). 07... 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

X-Ray data of tricarbonylchromium complex of 6-methylsulfanyl-5-phenyl-2,3-dihydro-177-pyrrolizine 20 showed that the benzene ring is inclined at about 40° to that of the dihydropyrrolizine. Moreover, the tricarbonylchromium group is positioned syn to the nitrogen atom <1998J(P1)1175>. 

The indanols 44 and 45 can only be the products of a formal [4 + 2] cycloaddition23 of the vinylketene complex 42.a with 1-pentyne. Note that upon reaction of 42.b with diethylpropynylamine a formal [2 + 2] cycloaddition65 is seen to take place, yielding the cyclobutenone 47 along with a tricarbonylchromium complex, tentatively identified as 48.66,67 As one would expect, the vinylketene complex 42.b underwent 1,2-additions with pyrrolidine and sodium methoxide in methanol, yielding 49 and 50, respectively. The CO-insertion step leading to vinylketene formation is reversible in some systems,51,68,69 but there is no evidence of this for complex 42.a. Heating a benzene solution of complex 42.a at 80°C under an atmosphere... 

Although slightly outside the scope of this review, an interesting case of stereoselection should be presented here. It has been observed by Gibson (nee Thomas) and coworkers during the deprotonation of tricarbonylchromium complexes of benzyl alkyl ethers by means of the chiral bis(lithiumamide) base 234 (equation 54) . The base removes the benzylic pro-R-H atom in 233 from the most reactive conformation to form the planary chiral intermediate 235. The attack of the electrophile forming 236 proceeds exclusively from the upper face in 235, because the bulky chromium moiety shields the lower face. Simpkins and coworkers extended the method to the enantioselective substitution of the chromium complexes of 1,3-dihydroisobenzofurans . 

An interesting result, contributed by Gibson (nee Thomas) and coworkers, should be added " The tricarbonylchromium complexes 258 are readily deprotonated by the chiral bis(lithium amide) 234 and alkylated to provide highly enantioenriched alkylation products 260 (equation 62). The decisive intermediate 259 presumably is formed by abstraction of the pro-S proton and is attacked from above. Related work has been published by Ewin and Simpkins . 

Tricarbonylchromium-complexed benzyl methyl ether was deprotonated using (eri-butvllithi-um and alkylated at the benzylic position without any Wittig rearrangement1. 

A series of complexes with the ir-bonded aromatic and heteroaromatic frameworks 6 and 9 was prepared by ligand exchange. An example is the preparation of the r 6-tricarbonylchromium complex of indole (30) (68JOM359). Ligand exchange may lead to complexes 9. Thus, interaction of 2,4,6-triphenylpyridine with L3Cr(CO)3 yields the trinuclear jr-complex... 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

TABLE 6. X-ray crystallographic geometries of tricarbonylchromium complexes of cycloproparenes0... 

Early attempts to utilize I as a donor for a chromium sandwich complex did not meet with success268, but recent studies have now provided several /6-chromium(0) complexes 64- 99-200. Whereas unsubstituted cycloproparenes undergo oxidative addition of the ring to the metal followed by carbon monoxide insertion (Section V.B.4), the l,l-bis(trimethylsilyl) derivatives do not. Instead, reactivity is transferred to the arene and, with tris(acetoni-trile)tricarbonylchromium, -complexes are formed at the ring remote from the cyclo-proparene moiety (equation 28). However, the 1,1 -disilyl derivative of 1 does not react and... 

Bis(arene)hafnium complexes, characteristics, 4, 697 Bis(arene)iron dications, characteristics, 6, 173 Bis(arene)niobium complexes, characteristics, 5, 95 Bis(arene)titanium(0) complexes, characteristics, 4, 243-244 Bis(arene)tricarbonylchromium complex, synthesis, 5, 258... 

Bis-(tricarbonylchromium) complexes, preparation, 5, 258-259 3,5-Bis(trifluoromethyl)bromobenzene, in Grignard reagent preparation, 9, 36... 

Scheme S. Synthesis of the l,3-diaza-2-boroline-tricarbonylchromium complex 64.

In contrast, the nitrone 3, derived from tricarbonylchromium-complexed benz-aldehyde, undergoes these 1,3-dipolar cycloadditions to give essentially only the cis-disubstituted isoxazolidine (2) after decomplexation with CAN in methanol. How-... 

Scheme 4. Diastereoselective Baylis-Hillman reaction of planar chiral arylaldimine tricarbonylchromium complexes (Kundig et ai.).

Kundig and coworkers have reported the Baylis-Hillman-reaction of methyl acrylate and acrylonitrile with planar chiral arylaldimine tricarbonylchromium complexes, such as 19 (Scheme 4) [28]. These reactions proceeded by attack of the acrylate from the sterically less encumbered site of the metal complex and afforded the products 21 with very good diastereoselectivity. 

A Friedel-Crafts type alkylation of 1,1,2,3,3-pentamethylindane with (R)- or (.S )-ructhylcpoxide followed by acid-catalyzed condensation of the resulting alcohols with paraformaldehyde affords two diastereomeric pairs of isochro-mans 586, 587 (Scheme 127) <1999HCA1656>. Separation of the enantiomers of (4.S, 7/ /.S )-isochromans 586 is achieved by flash chromatography of their corresponding tricarbonylchromium complexes and subsequent oxidative demetallation the result of this synthetic work establishes that (4,y,71J)-isochroman is found to be responsible for the intense musk odour of the perfume galaxolide (Scheme 127) <1999HCA1656>. 

A mono- and a bis-tricarbonylchromium complex of a hexabenzocoronene derivative substituted with... 

Three different strategies have been envisaged. The chiral information can either be incorporated into the alkyne or linked to the heteroatom or to the a,/ -unsaturated substituent at the carbene complex carbene carbon. High diastereoselectivities (57a 57b >96 4) have been observed in reactions of vinyl carbene complex 55 with the chiral propargylic ether 56 bearing the bulky trityloxy substituent [57a]. A more general approach is based on chiral alcohols incorporated into the alkoxycarbene complex. Upon benzannulation with tert-butylethyne, the menthyloxy carbene complex 58 gave a diastereoselectivity of 10 1 in favor of the naphthalene tricarbonylchromium complex 59a [57c, 57d]. Finally, the tandem benzannulation-Mitsunobu reaction of optically active carbene complex 60 with 5-hexyn-l-ol afforded the anti-benzoxepine complex 61 as the only diastereomer (Scheme 23) [57b]. 

Scheme 25. Stereoselective haptotropic rearrangement in naphthalene tricarbonylchromium complexes.

Scheme 30. Synthesis and haptotropic rearrangement of meta-benzonaphthohydroquinonophane tricarbonylchromium complexes.

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