Heterosubstitutions

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

The direction of the long-wavelength maximum shift caused by the heterosubstitution or the introduction of substituents is deterrnined by the Forster-Dewar-Knott rule (40—42). Spatial hindrances within the symmetrical PMDs cause bathochromic effects (39,43), whereas the introduction of an acetylenic bond is accompanied by the maximum shift to the short-wavelength spectral region (44). 

Diels-AIder reactions, I, 618 reactions, 1, 617-620 synthesis, 1, 614-616 2-Germoxetanes heterosubstituted germacyclobutanes and, 1, 596 Gewald synthesis thiophenes, 4, 888-889 ( )-Gingerol synthesis, 1, 453 (+)-(S)-Gingerol synthesis, I, 435 Ginkgolide A... 

Numerous variations of the Paal-Knorr eondensation are known. The most popular methods use starting materials that are eonverted to 1,4-diearbonyls in situ and eyelize to yield furan produets without isolating their diearbonyl preeursors. Other more speeialized strategies have been developed for the preparation of heterosubstituted furans. 

The direction of primary intermolecular nucleophilic attack at heterosubstituted acetylenes of type HC —X, where X = R2N, RO, RS, is governed by the nature of heteroatom and can be different. 

Not only the smallest optically active amino acid (alanine), but also leucine, several (substituted) aromatic amino acids, heterosubstituted amino acids (methionine, homomethionine and thienylglydne) and even an iminoacid, proline, are obtainable in both the L- and D-form. 

Other sources of Ru(0) can also be used for this synthesis. For example, it was recently demonstrated that [Ru(arene)(diene)] complexes such as 39 undergo double oxidative addition of heterosubstituted dihalo compounds 40 in the presence of phosphine ligands (Eq. 5) [21]. 

High-pressure and thermally induced asymmetric Diels-Alder cycloadditions of heterosubstituted dienes to homochiral ot, -didehydro amino acid derivatives [82]... 

An interesting variation of this reaction that made use of a three-component, one-pot solventless procedure with the corresponding trialkyl phosphites gave dramatically improved yields of many heterosubstituted glyphosate phosphonate diesters (37). When exactly one equivalent of water, 25, and tris-p-chloroethyl phosphite were mixed and heated under neat conditions for a few hours, nearly quantitative yields of displaced p-chloroethanol and the desired triester product 27 were obtained. If desired, the displaced alcohol was first removed by vacuum distillation, or the mixture could be hydrolyzed directly to GLYH3. Various oxygen, sulfur, nitrogen, cyano, and carboxylate functionalities were similarly accommodated in the trialkyl phosphite. 

In the same scheme, moreover, it is evident that, besides phosphazene homopolymers, the substitution of the chlorines with two (or more) different substituents leads to the preparation of substituent phosphazene copolymers [263] containing different homosubstituted and heterosubstituted monomeric units. Moreover, the cationic polymerization of phosphoranimines [215-217] produces polymers with hving reactive ends (vide supra) from which the preparation of chain phosphazene copolymers (block copolymers) [220,223,225, 229,232-235,239, 240] formed by different polymeric backbones linked together in a unique macromolecule could be obtained. 

Terminal heterosubstituted alkynes X-C=CH such as RO-C=CH [16, 17, 151-153], RSe-teCH (R= alkyl, aryl) [154-157], MejSi-teCH [158] and R P-feCH [159] were regioselectively converted into the ( )-2-ethenylzirconium complexes via hydrozirconation using 1. In marked contrast, with terminal BuTe-C=CH [154]... 

Catalytic asymmetric aldol reactions of a-heterosubstituted substrates such as glyoxaldehyde, and methyl pyruvate have been reported (Scheme 81). High diastereo- and enantioselectivity have been obtained by using combined use of Sn(OTf)2 and bis(oxazoline) or pyridinebis(oxazoline) ligands.341... 

The kinetic dienolate 68a also reacted with the non-heterosubstituted cyclo-propylideneacetates 52b and 75 to give the endo bicyclo[2.2.2]octanes 69a and 76, respectively (Scheme 15) [24]. 

A silver(i) complex having the heterosubstituted 3-methyl-l-(2-pyridylmethyl)imidazol-2-ylidene ligand, [Ag(carbene)2] [I/Agl2] 37, was reported, which was further reacted to give a series of palladium(n) carbene complexes that were demonstrated to be active catalysts toward Heck, Suzuki, and Sonogashira coupling reactions.87... 

This principle of formation proved to be general for quinocyclopropenes of type 61a ( phenylogous cyclopropenones )- The required p-hydroxy-phenyl cyclopropenium cations were available by electrophilic substitution of phenolic components (preferentially 2,6-disubstituted) and heterosubstituted cyclopropenium cations (75 and 109), as the representative examples 11099 11176 112and 113 show ... 

Koenigkramer, R.E. and Zimmer, H., a-Heterosubstituted phosphonate car-banions. IX. Diethyl 1-phenyl-l-trimethylsiloxymethane phosphonate as an acyl anion equivalent a novel method for the preparation of a-hydroxyke-tones, Tetrahedron Lett., 1017, 1980. 

Zimmer, H. and Neue, D.M., Synthesis with a-heterosubstituted phospho-nate carbanions. VI. Desoxybenzoins and indoles, Chimia, 31, 330, 1977. 

---