Abstract:The preparation and reactions of several new
(η3-allyl)molybdenum complexes are
described. Functionalization of a carbon−carbon double bond
adjacent to the π-allyl unit
proceeds stereoselectively, and the stereochemical outcome of this
reaction is discussed in
light of the Curtin−Hammett principle. It is shown that carbonyl
groups can also be
functionalized with moderate stereoselectivity, and this behavior is
rationalized on the basis
of conformational analysis using molecular mechanics calculations.
“…Overall, these results appear to be consistent with our earlier proposition that the methyl group at C(2) of the allyl ligand will favor the s-trans conformer to some degree, while a C(1) methyl substituent causes a more even distribution between s-trans and s-cis arrangements. However, the observed stereoselectivities for complex 10 are no better than those observed in our earlier work for the unsubstituted π-allyl complex 1 , presumably as a result of the low steric demand from the aldehyde oxygen versus the hydrogen. We anticipated that the situation would be very different for the alkene side chain in complexes 11 and 14 , as indicated from the NOE studies outlined above.…”
Section: Resultscontrasting
confidence: 75%
“…Within the last three years, our group has been successful in developing efficient synthetic routes toward a variety of new acyclic π-allylmolybdenum complexes, , in exploring the stereodirecting effect of molybdenum during functionalization, and in developing reliable methods of demetalation . In the present work, we have shown that functionalization of carbon−carbon double bonds and carbonyl groups adjacent to π-allylmolybdenum units is the result of the interplay between the directing effect of the metal and conformer populations, resulting in a 25:1 selectivity during double bond functionalization of complex 11 .…”
Section: Discussionmentioning
confidence: 68%
“…We have recently shown that carbon−carbon double bonds and carbonyl groups adjacent to a π-allylmolybdenum unit in acyclic systems can be functionalized with modest stereocontrol, exemplified in eqs 1 and 2. Structural evidence for major diastereomeric products, correlated with molecular mechanics calculations, supported the hypothesis that the selectivities achieved result from an interplay between the directing effect of the metal and conformer populations.…”
Stereocontrol exerted by a pi-allyl-Mo(CO)(2)Tp system (Tp = hydrotris(1-pyrazolyl)borato), during addition of Grignard reagents to neighboring aldehyde functionality and during dihydroxylation of alkenes, is found to be dependent on conformational preferences of the substituent relative to the organometallic moiety. Incorporation of a methyl group at C(2) of the pi-allyl ligand leads to excellent conformational control when the lateral substituent is a vinyl group, and this results in a diastereomer ratio of 25:1 for the diol that is obtained from osmylation. Poorer conformational control is observed for an aldehyde, and nucleophile additions show correspondingly lower stereoselectivity. Introduction of a methyl substituent at C(1) of the pi-allyl ligand does not effect conformational control, as expected, and very poor lateral stereocontrol is observed during both alkene dihydroxylation and nucleophile additions to aldehyde.
“…Overall, these results appear to be consistent with our earlier proposition that the methyl group at C(2) of the allyl ligand will favor the s-trans conformer to some degree, while a C(1) methyl substituent causes a more even distribution between s-trans and s-cis arrangements. However, the observed stereoselectivities for complex 10 are no better than those observed in our earlier work for the unsubstituted π-allyl complex 1 , presumably as a result of the low steric demand from the aldehyde oxygen versus the hydrogen. We anticipated that the situation would be very different for the alkene side chain in complexes 11 and 14 , as indicated from the NOE studies outlined above.…”
Section: Resultscontrasting
confidence: 75%
“…Within the last three years, our group has been successful in developing efficient synthetic routes toward a variety of new acyclic π-allylmolybdenum complexes, , in exploring the stereodirecting effect of molybdenum during functionalization, and in developing reliable methods of demetalation . In the present work, we have shown that functionalization of carbon−carbon double bonds and carbonyl groups adjacent to π-allylmolybdenum units is the result of the interplay between the directing effect of the metal and conformer populations, resulting in a 25:1 selectivity during double bond functionalization of complex 11 .…”
Section: Discussionmentioning
confidence: 68%
“…We have recently shown that carbon−carbon double bonds and carbonyl groups adjacent to a π-allylmolybdenum unit in acyclic systems can be functionalized with modest stereocontrol, exemplified in eqs 1 and 2. Structural evidence for major diastereomeric products, correlated with molecular mechanics calculations, supported the hypothesis that the selectivities achieved result from an interplay between the directing effect of the metal and conformer populations.…”
Stereocontrol exerted by a pi-allyl-Mo(CO)(2)Tp system (Tp = hydrotris(1-pyrazolyl)borato), during addition of Grignard reagents to neighboring aldehyde functionality and during dihydroxylation of alkenes, is found to be dependent on conformational preferences of the substituent relative to the organometallic moiety. Incorporation of a methyl group at C(2) of the pi-allyl ligand leads to excellent conformational control when the lateral substituent is a vinyl group, and this results in a diastereomer ratio of 25:1 for the diol that is obtained from osmylation. Poorer conformational control is observed for an aldehyde, and nucleophile additions show correspondingly lower stereoselectivity. Introduction of a methyl substituent at C(1) of the pi-allyl ligand does not effect conformational control, as expected, and very poor lateral stereocontrol is observed during both alkene dihydroxylation and nucleophile additions to aldehyde.
“…Cationic molybdenum allyl nitrosyl complexes of the form [Mo(NO)(CO)(η 3 -allyl){HB(pz) 3 }] + are readily obtained via the reactions of allyl complexes [Mo(CO) 2 (η 3 -allyl){HB(pz) 3 }] with nitrosonium salts and have enjoyed considerable attention as intermediates in stoichiometric organic synthesis. , Accordingly, the reaction of the allyl complex [Mo(CO) 2 (η 3 -C 3 H 5 ){HB(mt) 3 }] ( 4 ) with [NO]BF 4 in 1,2-dimethoxyethane (−78 to 0 °C) was explored and found to provide the nitrosyl complex 3 (52%) rather than the anticipated salt [Mo(NO)(CO)(η 3 -C 3 H 5 ){HB(mt) 3 }]BF 4 . While this result was unexpected, it may be noted that nucleophilic (i.e., reductive) displacement of allyl ligands from molybdenum centers has precedent: e.g., the synthesis of [Mo(NCMe) 2 (CO) 2 (PPh 3 ) 2 ] from [MoCl(η 3 -C 3 H 5 )(NCMe) 2 (CO) 2 ] and PPh 3 .…”
The syntheses of a range of poly(methimazolyl)borate -ligated nitrosyl complexes of tungsten and molybdenum are reported, the characterization of which includes the crystal structure determinations of the compounds [W(NO)(CO) 2 {κ 3 -S,S′,S′′-HB(mt3 }], [W(NO)(CO) 2 (PPh 3 ){κ 2 -S,S′-H 2 B(mt) 2 }], and [Mo(NO-)(CO) 2 {κ 3 -H,S,S′-H 2 B(mt) 2 }] (mt ) methimazoyl). The reaction of [W(NO)(CO) 3 (PPh 3 ) 2 ]PF 6 with Na[HB(mt) 3 ] and Na[H 2 B(mt) 2 ] provides respectively [W(NO)(CO) 2 {HB(mt) 3 }] and [W(NO)(CO) 2 (PPh 3 ){κ 2 -S,S′-H 2 B(mt) 2 }]. The complexes [M(NO)(CO) 2 {HB(mt) 3 }] and [M(NO)(CO) 2 {κ 3 -H,S,S′-H 2 B(mt) 2 }] (M ) W, Mo) arise from the reactions of Na[M(CO) 3 {H 2 B(mt) 2 }] with N-methyl-N-nitrosotoluenesulfonamide. The molybdenum complex [Mo(NO)(CO) 2 {HB(mt) 3 }] is also obtained unexpectedly from the reaction of [Mo(η 3 -C 3 H 5 )(CO) 2 {HB(mt) 3 }] with [NO]BF 4 .
“…Murai et al recently reported that upon reaction of 1-(trimethylsilyloxy)-1,3-butadiene ( 4a ) with bis(acetonitrile)palladium dichloride (PdCl 2 (MeCN) 2 ), the η 3 -allylpalladium complex 5a was isolated as an 87:13 syn / anti mixture in almost quantitative yield (Scheme ). , In addition to this single example employing palladium, some related molybdenum and ruthenium complexes (e.g., 6a 11c and 6b ) have been isolated from similar reactions of 1-(trimethylsilyloxy)-1,3-butadienes with [(η 5 -C 5 Me 5 )(MeCN) 2 (CO)Mo] + BF 4 - or [Cp(MeCN) 2 (CO)Ru] + BF 4 - , respectively (Scheme ). Pearson et al recently reported the synthesis of an additional member of this family by a reduction−oxidation sequence (Scheme , Tp = hydridotris(1-pyrazolyl)borato) 4 5 …”
Reaction of acyclic 1-(trimethylsilyloxy)-1,3-dienes with
bis(acetonitrile)palladium dichloride, in benzene, affords formyl-substituted
η3-allylpalladium complexes in good to
excellent yield. In contrast, using palladium diacetate as the
palladium(II) source produces
4-acetyloxy-substituted 2-alkenals. Excellent stereoselection in
favor of the E-alkenal is
usually observed. The corresponding benzoic acid esters can be
prepared, with similar
stereoselection, using 1-(trimethylsilyloxy)-1,3-dienes and palladium
diacetate in the presence
of an excess sodium benzoate.
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