Aluminum, electron-deficient compounds

Heteroaromatics are subdivided, according to the electron influence of the heteroatom, into w-electron-deficient compounds and compounds with an excess of it electrons on the ring carbon atoms. The typical ff-electron-delicient compound pyridine has so far been made to react only in one case the reaction of lithium tetrakis(A-dihydropyridyl)-aluminate (LDPA) [112-114), obtainable from pyridine and lithium aluminum hydride, with trifluoromethanesulfenyl chloride in an excess of pyridine affords 3-trifluoromethylmercaptopyridine in low yield (13%) (60). This reaction probably occurs through sulfenylation of the l,2-dihydrop5T idyl moiety of the LDPA with the formation of a 2,5-... 

Beryllium chloride, an electron-deficient compound similar to aluminum chloride, is a Lewis acid. The anhydrous salt is used as a catalyst in organic reactions. Its applications, however, are limited. 

LiAlH4 reacts with the aluminum chloride dimer, the aluminum hydrides, or alanes, result. These are among the many electron-deficient compounds to be discussed in Section 14.5. 

The principal characteristic that distinguishes the Group 3A elements from the rest of the representative elements is the existence of electron-deficient compounds. You may recall earlier references, in this book and elsewhere, to compounds of this type. It is not unusual for boron, aluminum, gallium, and occasionally beryllium and lithium to form compounds in which the metal is surrounded by less than an octet of electrons. One should of course be wary of such phrases as electron-deficient. It seems to imply that there is something wrong with such compounds. In fact, it is... 

Electron-deficient compounds are formed by boron, aluminum, gallium, and indium as well as lithium, beryllium, and magnesium. Such compounds are... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

A reaction in which an electrophile participates in het-erolytic substitution of another molecular entity that supplies both of the bonding electrons. In the case of aromatic electrophilic substitution (AES), one electrophile (typically a proton) is substituted by another electron-deficient species. AES reactions include halogenation (which is often catalyzed by the presence of a Lewis acid salt such as ferric chloride or aluminum chloride), nitration, and so-called Friedel-Crafts acylation and alkylation reactions. On the basis of the extensive literature on AES reactions, one can readily rationalize how this process leads to the synthesis of many substituted aromatic compounds. This is accomplished by considering how the transition states structurally resemble the carbonium ion intermediates in an AES reaction. 

The structure for the (Me2AIH)2 dimer (XIII) has been determined from gas-phase electron diffraction studies (4, 9). Examination of the data in Table III and those in Table IV, for both the electron-deficient bridged systems and the normal bridged aluminum dimers, show that this compound has a typical electron-deficient bridged structure. 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

XXXVII we also see a bridging group with Al—C distances very close to that observed in other bridged aluminum compounds. The distance between the metal centers in this compound is similar to that observed in the simpler aluminum derivatives but greater than the sum of the covalent radii of the two metals (2.54 A), which may be an indication that Ti—Al interactions do not increase the stability of the bridged system. Structures on Cp2MMe2AlMe2 (M = Y, Er, and Yb) have been recently completed and clearly show stable electron-deficient bonds between the aluminum and the transition metal moiety (XXXVIII) (12). 

Sinn and Patat (59) drew attention to the electron-deficient character of those main group alkyls that afford complexes with the titanium compound. Fink et al. (51) showed by 13C NMR spectroscopy with 13C-enriched ethylene at low temperatures (when no alkyl exchange was observed) that, in the more highly halogenated systems, insertion of the ethylene takes place into a titanium-carbon bond of a titanium-aluminum complex. 

An operational description is that one reactant (the more ionic compound with the more electropositive metal) transfers alkyl anions to the other. Thus the four methyl groups in Li2BeMe4 form a distorted tetrahedron around the beryllium, with longer distances to the lithium ions. However, this description is oversimplified. The low-temperature nuclear magnetic resonance (NMR) spectrum of Li3MgMe5 has three different methyl resonances, suggesting structure (14), related to the MeLi tetramer. Ate complexes with zinc and aluminum compounds also form. Electron-deficient bridge-bonded structures, exemplified by the X-ray structure of... 

Of course, these compounds are also dimeric, containing electron-deficient bonds with the aluminum atoms in four-coordinate tetrahedral environments. It is significant that the AI-X-AI angle in alkylaluminum halides is considerably widened from that observed in Me6Al2 (75°) to -90°. 

Carbocations occupy a unique place in the world of electron-deficient cations. Electron-deficient nitrogen and oxygen compounds (nitrenium and oxenium ions, respectively) are extremely unstable and are rarely or never seen. Electron-deficient boron and aluminum compounds are quite common, but these species are not cations. Silenium ions (R3Si+) are extremely unstable kinetically still, it is useful to think of the R3Si+ group as a large, kinetically more stable H+. 

---