Intramolecular reactions, chromium

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Photolysis of chromium alkoxycarbene complexes with aldehydes in the presence of Lewis acids produced /J-lactones [83]. Intermolecular reactions were slow, low-yielding, and nonstereoselective, while intramolecular reactions were more efficient (Eqs. 19 and 20). Subsequent studies showed that amines, particularly DMAP, could also catalyze this process (Table 13) [84], resulting in reasonable yields and diastereoselectivity in intermolecular cases. 

Another rather unusual case of rapid aquation is provided by those oxoanion ligands MO in which the M-0 bond is sufficiently weak for it to break rather than the Co-0 bond. This time we have an intramolecular reaction of a coordinated ligand. A recent example is provided by aquation of the [Co(NH3)5(OMo03)]+ cation (57). This aquation pathway is actually more commonly encountered for chromium(III) complexes. 

Chromium carbene complexes, 82 Methyl acrylate, 183 (2R,4R)-Pentanediol, 237 Titanium(IV) chloride, 304 Nitroaldols Nitromethane, 199 Intramolecular reactions Methyl acrylate, 183 Other aldol-type reactions Bis(2-pyridinethiolato)tin(II), 40 Alkoxycarbonylation (see Carboalkoxy-lation)... 

Several useful inter- and intramolecular reactions of chromium, molybdenum, tungsten, iron, or manganese Fischer carbenes with aUcenes to form cyclopropanated products have... 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

The intramolecular reaction of pentacarbonyl[phenyl(3-hexenyloxycarbene)]chromium(0), a chromium carbene complex bearing an alkenyloxy substituent, provides 6-ethyl-5-phenyl-2-oxabicyclo[3.1.0]hexane with moderate exo preference19. This example shows that carbene transfer to electronically neutral olefins can proceed nonstereospecifically. 

C)5W=C(0Et)CH=C(0Et)CH=CR(02CR)]. Aldehydes or ketones react with the [diosphine complexes, [(OC)4(PR3)Cr=C(OMe)Me] to give aldol condensation products. Reaction with the amino carbene complexes [(OC)sCr=C(NMe2)Me] results in aldol addition products. Aldol reactions of optically active aminocarbene complexes proceed with moderate to high diastereoselectivity. Photolysis of chromium alkoxycaibene complexes with aldehydes in the presence of Lewis acids produces -lactones in low yield. Intramolecular reaction by incorporation of the aldehyde into either side chain of the carbene complex is considerably more efficient. ... 

Chromium Fischer carbene complexes 409 and 410 underwent intermolecular or intramolecular cyclopropana-tion. The treatment of the carbene complex with lithium enolates resulted in the formation of a cyclopropane in good yield (Scheme 1.190) [265]. An intramolecular reaction for 410 was also reported [266]. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

An unexpected varying regiochemistry in intramolecular benzannulation has also been observed in the synthesis of cyclophanes. As mentioned above, there are only two possible regiochemical outcomes in the benzannulation reaction, which differ in the direction of alkyne incorporation. / -Tethered vinyl-carbene chromium complexes undergo an intramolecular benzannulation reaction with incorporation of the tethered alkyne with normal regioselectivity to give meta-cyclophanes [28]. 

Chromium aminocarbenes [39] are readily available from the reaction of K2Cr(CO)5 with iminium chlorides [40] or amides and trimethylsilyl chloride [41]. Those from formamides (H on carbene carbon) readily underwent photoreaction with a variety of imines to produce /J-lactams, while those having R-groups (e.g.,Me) on the carbene carbon produced little or no /J-lactam products [13]. The dibenzylaminocarbene complex underwent reaction with high diastereoselectivity (Table 4). As previously observed, cyclic, optically active imines produced /J-lactams with high enantioselectivity, while acyclic, optically active imines induced little asymmetry. An intramolecular version produced an unusual anti-Bredt lactam rather than the expected /J-lactam (Eq. 8) [44]. 

The intramolecular cyclization route to p-lactams still provides interest. P-Amino esters (obtained by a Reformatsky-type reaction of an imine and bromoacetates derived from chiral alcohols) are cyclized by the action Grignard reagents to 4-substituted P-lactams with impressive e.e. <96TL4095>. A similar approach through a Reformatsky-type reaction uses tricarbonyl(Ti -benzaldimine)chromium complexes and ultrasound <96T4849>. 3-Methyl-azetidin-2-ones (obtained from 3-amino-2-methylpropionates) have been resolved and their... 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

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