Alkynyl aluminum

One of the severest challenges of asymmetric synthesis is the direct enantioselective construction of quaternary stereogenic centers. Brian Pagenkopof of the University of Texas has reported (Chem. Communications 2003 2592) that alkynyl aluminum reagents will open a trisubstituted epoxide such as 10 at the more substituted center, with inversion of absolute configuration. As the epoxide 10 is available in high from 9 by the method of Yian Shi of Colorado State (J. Am. Chem. Soc. 119 11224, 1997), this opens a direct route to quaternary cyclic stereogenic centers. 

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

The palladium-catalyzed systems seem quite flexible with regard to the nature of the organoaluminum, since aJkyl-, alkenyl- and alkynyl-aluminum reagents were used successfully (entries 8-14, Table 18). Furthermore, the acyl chloride substrates include alkyl, aryl and alkenyl substituents. 

In addition to their reactions with trlmethylsilyl enol ethers, (propargyl1um)Co2(C0)g complexes react with a variety of other mild carbon nucleophiles including activated aromatic compounds, g-dicarbonyl compounds, other enol derivatives (enol acetates and ketones directly), allylsilanes, and alkyl- and alkynyl-aluminum reagents. These reactions provide a flexible means to introduce the synthetically versatile propargyl function. Key features of propargylations using these complexes are 1) ready... 

More recently, the same group found that alkynyl aluminum reagents undergo conjugate addition to cyclic enones in the presence of chiral Ni-bisphosphine complexes [55]. They found that the use of binol-based phosphine L9 provides high yields and enantioselectivities (up to 90% ee) for a broad range of cyclic enones. Interestingly the scope of the reaction is not limited to TMS-protected acetylides but it can be extended to aryl-acetylides (Scheme 10). 

Scheme 9 First enantioselective conjugate addition of alkynyl aluminum reagents...

All these advances provide a basis for further research. For instance, the possibilities offered in the allylic alkylation reactions need to be therefore further exploited (there are only few publications on this topic). Thus, for instance the expansion toward the use of other leaving groups rather than phosphonates would be welcome. With regard to the conjugate addition of (hetero)aryl- and alkynyl aluminum reagents very few examples have been reported. Consequently, these areas are open to further development, and this will provide interesting findings that... 

Aluminum alkynyls have recently been reviewed Zheng, W Roesky, H. W. J. Chem. Soc., Dalton Trans. 2002, 2787. 

Aldehydes and ketones have also been prepared by nucleophilic cleavage of resin-bound O-alkyl hydroxamic acids (Weinreb amides [744]) with lithium aluminum hydride [745] or Grignard reagents (Entries 1 and 2, Table 3.41). Similarly, support-bound thiol esters can be cleaved with Grignard reagents to yield ketones [272], or with reducing agents to yield aldehydes (Entry 3, Table 3.41). Polystyrene-bound sele-nol esters (RCO-Se-Pol) react with alkynyl cuprates to yield alkynyl ketones [746]. 

Aluminum-alkynyl covalent bonds, characteristics, 9, 249-250 Aluminum-aluminum bonds in A1(I) compounds, 9, 261 in Al(II) compounds, 9, 260 Aluminum aryloxides, reactivity, 9, 254—255 Aluminum(III)ates, in organic group-selective transfers, 9, 279 Aluminum(I)-boron bonds, characteristics, 9, 263 Aluminum(III)-boron exchange, process, 9, 266 Aluminum-calix[4]arene catalyst, for alternating epoxide-CC>2 co-polymerization, 11, 617... 

In contrast to the behavior of comparable aluminum, gallium, and indium compounds, the hitherto known alkynyl thallium(III) compounds (170) of the type R2T1C=CR show no tendency to dimerize via n complexation. Molecular weight determinations performed in aniline show the monomeric character of these species, which behave as weak electrolytes due to partial dissociation into R2T1+ and C=CR" (170). 

Cycloadditiom. This reaction provides an enantioselective synthesis of a cyclooctane-containing terpenoid (5). The optically active precursor (3) was obtained by reduction of the r-alkyl alkynyl ketone 1 with lithium aluminum hydride... 

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