Molybdenum 2-acyl complexes

Related, achiral cc,/ -unsaturated molybdenum-( 2-acyl) complexes, such as 8, have been shown to undergo nucleophilic 1,4-conjugatc addition upon treatment with sodium borohy-dride or methyllithium to generate enolate species, such as 9 (produced by addition of hydride). Subsequent alkylation by iodomethane provides the a-alkylated product 1088. Extension of this tandem Michael addition-alkylation sequence to nonracemic molybdenum species has not yet been reported. 

In the case of manganese(I), with an electronic configuration of 3d , the four-electron donor group RCO— would meet the EAN rule. Indirect support to the suggestion of a small steady state concentration of this type of intermediate along the reaction profile comes from isolation of stable rj -coordinated acyl complexes of ruthenium(II) , of group IVA transition elements and of thorium , all in the highest oxidation state. Also > -acyl complexes of vanadium(I) ° and niobium(V) have been reported. Most of the molybdenum acyl complexes are of the type . 

The achiral molybdenum enolate 2 reacts with benzophenone and benzaldehyde at —78 °C to yield the x,/i-unsaturated molybdenum-acyl complexes34, which presumably arise via elimination of hydroxide from unobserved aldolate intermediates such as 3. No examples of such aldol reactions with complexes that are chiral at molybdenum have been reported. 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

The molybdenum-acyl enolate 4 has been characterized spectroscopically by NMR and has been reported to exist as a single (observable) isomer88. Extended Hiickel calculations on model complexes suggest that a conformation similar to A is most favorable for enolates such as 488. The deprotonated di-hapto acyl ligand may also be described as a 2-(C,0)-ketene ligand both the ketene and enolate terminology appear in the literature. 

Ring-closing metathesis is well suited for the preparation of five- or six-membered heterocycles, and has also been successfully used to prepare tetrahydropyridines on insoluble supports (Entries 1 and 2, Table 15.23). Because metathesis catalysts (ruthenium or molybdenum carbene complexes) are electrophilic, reactions should be conducted with acylated amines to avoid poisoning of the catalyst. 

An efficient method for the formal abstraction of oxide from acyl ligands was developed in our laboratory 44). Oxalyl halides react directly with the pentacarbonylmetal acyl complexes of chromium, molybdenum, and tungsten to form the fra ,s-alkylidyne(halo)tetracarbonyl complexes [Eq. (5)]. Other suitable Lewis acids are COCI2, CI3COCI, CI3COCOCI, and... 

Protonation of several molybdenum and tungsten alkylidyne complexes of the type M(CR)(f) -C5H5)(CO)L with acids such as HCI or CFjCOjH was reported by KreissI et al. to afford the / -acyl complexes 121 [Eq. (104)] (134). The formation of the acyl complexes I2I presumably in-... 

Other mechanistic studies on carbonyl insertions of molybdenum(II) complexes, RMo(f7 -C5H5)(CO)3, postulated formation of an acyl intermediate stabilized by interaction of the hydrocarbon portion of the acyl group with the metal, similar to the well-established agostic interaction). 

Treatment of the carbyne complexes (// -cyclopentadienyl)(CO)[P(OMe)3]M = CR 1, where M = molybdenum or tungsten and R = cyclopropyl, with hydrogen chloride in diethyl ether results in a rearrangement reaction to give 7 -acyl complexes 2 in quantitative yield. 

Relative susceptibility of isocyanide and carbonyl ligands to insertion depends on reaction systems. The greater thermodynamic stability of the iminoacyl complexes than the acyl complexes was demonstrated by thermal isomerization of acyl(isocyanide)molybdenum to iminoacyl(carbonyl)molybdenum complexes (Eq. 7.13) [94]. 

Carmona and co-workers (943-945) detailed the synthesis and strucmre of molybdenum(II) complexes bearing an agostic acyl ligand. Thus, addition of dithiocarbamate salts to [MoCl(CO)(PMe3)3 t -C(0)CH2X ] (X = H, SiMe3)... 

The 3,4-dimethyl-l-phenylphosphole (the so called Mathey-phosphole ) entered into Friedel-Crafts acylation only through its molybdenum complex (20). Elimination of the Mo(CO)5 moiety from product 21 furnished 2-acylphosphole 22 (.Scheme 5) [42],... 

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

For most methoxycarbene complexes a one pot modification of the above method is utilized. This involves direct alkylation of the initially formed lithium acylate carbene complex with trifluoromethanesulfonate or with methyl fluorosulfonate. The method is successfully employed for preparation of chromium140 as well as molybdenum and tungsten monocyclic uation 65), bicyclic (equation 66) and tricyclic carbene complexes... 

CpCl2[P(OMe)3]WCOCH2(c-Pr) in 90% yield (equation 100)158. Molybdenum complexes are more vulnerable to ring-opening reactions with HC1, particularly the acyl-substituted complexes (see below). 

Thermolysis of an allyl-cyclopropyl carbyne complex under the mild conditions used for the acyl carbynes did not result in an oxymetallacycle or a cyclopentenone. Instead, 2-cyclohexenone-5-spirocyclopropane was obtained in low yields (5%) together with moderate yields (43%) of the open-chain (3-allyl V-pentadienal) molybdenum complex (equation 104)163. 

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