Aluminum carbonyl complexation

Examination of electronic and thermodynamic factors in the aforementioned conventional enolate formation revealed that steric factors were of fundamental importance in fhe reaction. One alternative is to complex a carbonyl compound with a bulky Lewis acid (Fig. 6.13). Bulky aluminum reagents usually form relatively stable 1 1 complexes irreversibly wifh carbonyl compounds. We first hypothesized that even in the presence of a strong base (LDA or LTMP), a steric environment applied in the aluminum-carbonyl complex would kinetically adjust site-selective deprotonation of carbonyl compounds which offer multiple sites for enohzation and kinetically stabilize fhe resulting bulky enolates by retarding the rate of proton transfer or other undesirable side reactions. These fundamental considerations found particular application in fhe formation and reaction of novel aluminum enolates. 

Aluminum-Carbonyl Complexation, Activation, and Nucleophilic Reaction Pericyclic Reaction and Asymmetric Reaction... 

Homogeneous wetting, 22 111 Homogenization, 26 699 aluminum alloys, 2 329 Homogenizers, 8 703 10 127 Homoglycans, 4 697, 701 23 62-64 classification by structure, 4 723t Homo-interface, 24 71 Homo-ionic interactions, 8 77 Homojunction BJTs, 22 166 Homojunction devices, LEDs asm 173 Homojunction diode arrays, 29 163 Homojunction laser diode, 24 699 Homoleptic tetranuclear carbonyl complexes, 26 63... 

Infrared spectra, see also specific compounds aluminum hydrides, 41 223 of bipy and phen complexes, 12 159-162 of borates, 25 200-201, 203, 205-206, 211 of carbonyl complexes, satellite bands, 12 ... 

Meerwein-Ponndorf-Verley-Type Reduction Reduction of ketones by 2-propanol or related alcohols, known as Meerwein-Ponndorf-Verley (MPV) reduction, is promoted by various metal alkoxides, typically aluminum 2-propoxide [2a,d,281]. The C2 hydrogen of 2-propanol is transferred directly to the carbonyl carbon through a six-membered pericyclic transition state [284], Earlier, a stoichiometric quantity of a metal alkoxide was required for this purpose, but recently, lanthanide [285] and aluminum [286] complexes acting as excellent catalysts have been reported. 

Aluminum ate complexes, (CH3)3SiCH -CHCH2 Al(C2H5)3Li+ (1). The ate complex is prepared by reaction of the lithium anion of allyltrimethylsilane with A1(C2Hs)3. In contrast with the anion of allyltrimethylsilane, which reacts with carbonyl compounds mainly at the y-position, 1 reacts selectively at the a-position (equation T). 

The organometallic chemistry of aluminum is dominated by the chemistry of aluminum(lll), but lower oxidation state compounds are now accessible. The first examples of this class of compounds are carbonyl complexes such as Al(CO), A1(C0)2, and Al3(CO), which were generated upon exposure of aluminum atoms to CO in matrix-isolation experiments near 20 K. The number, relative intensities, and frequency of the carbonyl stretches in the IR spectra, along with isotopic labeling and EPR studies were used to verify these compositions. These complexes exhibit vco values of 1868, 1985 and 1904, and 1715 cm , respectively, indicative of Al- CO 7t backbonding. The carbonyl species are unstable at higher temperatures and no stable carbonyl complex of aluminum, in any oxidation state, has been reported. The monomeric aluminum-alkene adducts A1( -C2H4) and k rf-CeHe) were similarly identified in inert matrices at low temperature. No room-stable alkene complexes of aluminum have been reported. 

Although the Michael addition of metal ynolates to a,/ -unsaturated carbonyl compounds is expected to give six-membered cycloadducts, 1,2-addition to carbonyl groups usually precedes 1,4-addition. The cycloaddition of the lithium-aluminum ate complex of silyl-substimted ynolate 112 with ethyl benzylideneacetoacetate (113), which is doubly activated by the ester and keto functions, gives the y-lactone 114 via a [4 4- 2] type cycloaddition (equation 46). Diethyl benzylidenemalonate (115) affords the uncyclized ketene 116 by reaction with 112 (equation 47). This could be taken as evidence for a stepwise mechanism for equation 46. ... 

All reactions and sample preparations are carried out in an inert-atmosphere enclosure under dry nitrogen. Solvents and reagents are dried in the following manner. Benzene, tetrahydrofuran, and n-pentane are freshly distilled from lithium aluminum hydride pyridine is distilled over barium oxide and tetramethylethylenediamine is distilled over calcium hydride. Solvents used in preparing nmr and infrared samples are degassed by a freeze-thaw technique. Nmr spectra are obtained with torch-sealed nmr tubes. The commercial transition metal carbonyl complexes are recrystallized and vacuum-dried before use. Glassware is routinely flame-dried. 

Carbonyl groups form complexes or intermediates with Lewis acids like AICI3, BF3, and SnCl4. For example, in the Friedel-Crafts acylation reaction in nonpolar solvents, an aluminum chloride complex of an acid chloride is often the acylating agent. Because of the basicity of ketones, the products of the acylation reaction are also complexes. For more detail on electrophilic aromatic substitution, see Section 7. 

The triethylaluminum or triethylborane ate complexes (12) of the (isopropylthio)allyl carbanion react with carbonyl compounds at the a-position (equation 10). In the reactions with carbonyl compounds, very high regioselectivity (for example, butanal 95 5, 3-methylbutanal 99 1, cyclohexanone 92 8 and acetophenone 95 5) was achieved by using the aluminum ate complex. On the other hand, the a-regio-selectivity with ketones decreases if the boron ate complex is used (cyclohexanone 72 28, acetophenone 45 55). It is noteworthy that the stereoselectivity of the a-adduct from an aldehyde is low. Ihesumably the geometry of the double bond in the ate complex (12) is not homogeneous. ... 

It has bmn noted that although the linear geometry is consistently predicted for cationic Lewis acid carbonyl complexes in ab initio calculations, extrapolation of these results to neutral Lewis acid complexes may not be justified. Semiempirical MNDO calculations predicted the bent conformation as the lowest energy structure for neutral Lewis acidic derivatives of beryllium, boron and aluminum complexed with trans-2,3-dimethylcyclopropanone, whereas linear structures were predicted for the cationic complexes of beryllium and aluminum (Table I). ... 

Enolates are undoubtedly the most versatile intermediates for C-C, C-N, C-O bond-forming reactions [36]. Continuous progress has been made not only in fundamental operations involving these anionic species but also during the synthesis of complex natural products. Compared with metal enolates with counter cations of, e.g., B, Si, Li, Na, K, Mg, Ti, Sn, Cu, etc., aluminum enolates have found fewer apphcations, probably because no particular advantages over the other metals have been perceptible. There are, however, still intriguing aspects of novel reactivity and selectivity in the formation and reaction of aluminum enolates. Specifically, very recent development have highhghted pre-formation of Lewis acid-carbonyl complexes by use of bulky aluminum compounds as precursors of aluminum enolates the behavior of these complexes is unprecedented. 

Three major classes of reaction mechanisms can be identified (1) Radical processes for cobalt and manganese carbonyl complexes that give the expected products with little stereoselectivity [11, 12, 49, 50]. Although some claims have appeared of the intervention of radicals in HDS-related hydrogenation, most of the evidence points to other types of surface mechanisms which can be better related to coordinative mechanisms therefore no further mention will be made of radical reactions in this Chapter. (2) Reactions involving Ziegler-type catalysts (a transition metal complex mixed with an alkyl aluminum co-catalyst) these poorly defined systems have proved to be difficult to study in detail [20, 22-25], and they appear rather unrelated to HDS-active... 

Propionylcyclopentadienyliron carbonyl complexes 1.151 (R = MeCH2) form enolates whose aldol condensations are highly selective. Depending on the associated metal, either anti (aluminum) or syn (copper) aldols are predominantly formed at -100°C. The absolute configuration of these aldols depends upon that of the starting complex [408, 522] (Figure 6.87). 

Anion (64) generally reacts with carbonyl compounds at the y-position, whereas it reacts with alkyl halides at the a-position. The triethylaluminum ate complex of (64) exhibits reversed regioselectivity propanal and 2-methylpropanal give exclusively the a-adducts. The anti isomers are produced predominantly. Organic halides and trimethylsilyl chloride do not react with the aluminum ate complex. 9,11-Dodecadien-l-yl acetate, a pheromone of Diparopsis castanea, is prepared by condensation of 9-oxonon-l-yl acetate via the ate complex (Scheme 43). 

Ketenes generated by dehydrobromination of acyl bromides undergo trans-selective [2-f2] cycloaddition with carbonyl groups, using the chiral catalyst bis-pyridinium aluminum-salen complex, to form p-lactones (Scheme 7.51). 

Metal carbonyl complexes can be prepared in some cases from the bulk metal and from the reduction of complexes in higher oxidation states. For example, the highly toxic Ni(CO) can be prepared directly from nickel metal and carbon monoxide at one atmosphere pressure, and small amoimts of Fe(CO)j are found in carbon monoxide that has been stored under pressure in steel cylinders. However, most metal carbonyls are best made in an ordinary laboratory by reductive carbonylation, as shown in Equations 2.1 and 2,2. The reducing agents used include electropositive metals and aluminum alkyls, as well as carbon monoxide itself. 

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