Ate complexe

AH implantable medical devices ate complex in design, materials, and implementation procedures. The biocompatibiUty, biodurabiUty, and efficacy of medical devices are the subject of extensive research by biomaterials scientists, device manufacturers, and health care professionals. 

The bicyclic amine 11-methyl-l l-azabicyclo[5.3.1]hendecanc (71) provided a model system in which the hydrogens on the equivalent a-tertiary-carbon atoms cannot be trans to the nitrogen-mercury bond in the mercur-ated complex and in which epimerization at these a carbons is impossible (77). This bicyclic system is large enough to accommodate a... 

Azaferrocene is methylated by n-butyllithium with subsequent treatment with methyl iodide resulting in formation of 25-27 [83JOM(251)C41]. The acetyl-ated complex Ti -(3-acetyl-2,4-dimethylpyrrolyl)cyclopentadienyliron was also described [74JOM(77)69]. The stmcture of 2-methylazaferrocene was studied extensively (69AG150 96JOC7230 97JA1492 97JOC444). 

The initial formation of an ate complex by attack of the nucleophile on the aluminum reagent, followed by reaction with the ketone, is unlikely since treatment of the ketone with a mixture of MAD and the organometallic reagent gave results comparable to those obtained with the organometallic reagent in the absence of MAD. 

In contrast to the intermediate hydroxystannanes, O-protected stannanes 7 are stable compounds which can be distilled or chromatographed and stored under nitrogen for months. Treatment of 7 with butyllithium in tetrahydrofuran at — 78,JC results in rapid tin/lithium exchange (< 1 min). No products resulting from Wittig rearrangement or formation of an ate complex 8 could be detected9. 

Z)-2-Butenylpotassium is generated from 4.5 mL (50 mmol) of (Z)-2-butene, 2.8 g (25 mmol) or /-BuOK. and 10.8 mL (25 mmol) oT 2.3 M butyllithium in THF for 15 min at —45 JC. This solution is cooled to — 78 C and 30 mmol of a 1 M solution of methoxy(diisopinocampheyl)borane in diethyl elher is added dropwise. The mixture is stirred for 30 min at — 78 °C, then is treated with 4mL (33 mmol) of boron trifluoride-diethyl ctherate complex this removes methoxide from the intermediate ate complex. This solution is immediatelv treated with 35 mmol of an afdchyde. Isolated yields of homoallylic alcohols are 63-79%. 

The only preparative limitation to this method is the occasional coproduction of alkenyl-boronates that presumably arise via a-elimination pathways of the ate complex generated upon addition of the organometallic reagent to the a-haloalkylboronate4,29-30. This problem is illustrated in the synthesis of 5-(rm-butyldimethylsilyloxy)-2-pentenyl-substituted dioxaborolane30. 

The reactions of aldehydes with 2-butenylboron ate complexes generated, for example, by the addition of 2-butenyllithium to triethylborane, have also been investigated. These reagents exist mainly as the E-isomer and display moderate selectivity (68-85%) for the antf-diastereomer40. 

Conceptually related methods have been employed in the synthesis of a-substituted allylboron reagents 85 and 106. In the case of 8, a benzimidazoleoxy leaving group is introduced as part of the a-alkoxy-2-butenyllithium reagent. Fragmentation of the ate complex must be conducted at — 100 °C in order to avoid the isomerization of 8, which occurs readily if 8 is allowed to warm to — 78 °C for one hour before addition of an aldehyde5. 

An extremely attractive feature of the route outlined at the beginning of this section for the transformation of boronates 3 or 4 to a-substituted allylboron compounds 5 is that reagents with very high enantiomeric purity (> 90% ee) may be prepared when precursors such as 3 and 4, and therefore also ate complex 1, contain a suitable diol chiral auxiliary17. The following syntheses of (S)-68, lib9, and 1310 illustrate this feature. 

These reagents are not isolated but are used directly in reactions with aldehydes, after generation of ate complexes via the addition of an alkyllithium reagent or pyridine11. 2-(2-Propenyl)-1,3,2-dioxaborolane is also metalated upon treatment with lithium tetramethylpiperidide, but mixtures of a- and y-substitution products are obtained upon treatment of this anion with alkylating agents20. Consequently, this route to a-substituted allylboron compounds appears to be rather limited in scope. 

In a second set of examples, it was shown that the stereoselectivity of the aldehyde allylborations of 9-[( T)-l-trimethylsilyl- or l-trimethylstannyl-2-butenyl]-9-borabicyclo[3.3.1]nonane is controlled to a significant extent by conversion to an ate complex by treatment with butyllithium, MT-butyllithium or pyridine19. 

The heteroatom-substituted ate complexes 3 (Y = S-z-Pr, SeC6H5 or TMS) have also been examined, but their reactions with aldehydes are not as regio- or stereoselective as those of the alkoxy-substituted reagents 8b. 

In the presence of alkoxides or amides, e.g., liberated during the preparation, ate complexes are reversibly formed and may modify the reactivity of the a-titanated 2-alkenyl carbamates17. 

The application of the spiro-titanate 5 to the same reaction leads preferentially to the opposite enantiomer115. Presumably in both cases, an ate complex is involved. Unlike alkylations115,116 which proceed catalytically, stoichiometric amounts of the additive are required. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

In this case the use of the Sm(II) "ate" complex Na[Sm N(SiMe3)2 3] as starting material afforded yet another novel C-substituted amidinate complex resulting from y C-H activation of a N(SiMe3)2 ligand (Scheme 194). All new... 

The ate complexes are analogous to the onium salts formed when a Lewis base expands its valence, for example. 

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