Of heteroatom-substituted alkenes

The reaction of heteroatom-substituted alkenes with electrophilic carbene complexes can lead to the formation of highly reactive, donor-acceptor-substituted cyclopropanes. This type of cyclopropane usually undergoes ring fission and rearrangement reactions under milder conditions than do unsubstituted cyclopropanes (Figure 4.22). 

Figure 7. General trends in the hydroformylation of heteroatom substituted alkenes...

The preparation of heteroatom-substituted alkenes from the corresponding gem-dihalides and aldehydes was also mediated with chromium(II) chloride. In Scheme 8.46, representative examples of the preparation of alkenylborane, [51] -silane, [52] and -stannane [53] are shown. In each case, the high -selectivity is observed. As these compounds are very important substrates for Suzuki-, Hiyama-, and Stille coupling, the stereoselective formation of these compounds heightens the value of the chromium(II)-chloride-mediated reactions. 

The major problem remains control of regioselectivity in favor of the branched regioisomer. While aryl alkenes as well as heteroatom-substituted alkenes favor the chiral branched isomer, for aliphatic alkenes such an intrinsic element of regiocontrol is not available. As a matter of fact branched-selective and asymmetric hydroformylation of aliphatic alkenes stands as an unsolved problem. In this respect regio- and enantioselective hydroformy-... 

Dipolarophiles D12. Heteroatom substituted alkenes of general formula D12 have been sparingly used as dipolarophiles when compared to vinyl ethers Dll (see Fig. 2.33). Comparative studies between common heating and microwave... 

Bach, T. The Paterno-Buchi reaction of 3-heteroatom-substituted alkenes as a stereoselective entry to polyfunctional cyclic and acyclic molecules. Liebigs Ann. Chem. 1997,1627-1634. 

A number of heteroatom-substituted dilakylaluminum compounds (R2AICH2-X) can undergo apparent a-, aP-, or ay-eliminations. The apparent a-elimination, when halomethylaluminum compounds cyclopropanate alkenes, is actually a combination of carboalumination and elimination [Eq. (6.87)]. Such eliminations involve hypercarbon intermediates or transition states. 

Rotation around C-X bonds in heteroatom-substituted alkenes The preferred orientations of substituent X in the R2C=CR -X systems relative to the vinyl moiety are also interesting. Whereas the flat conformation is preferred for X=0, S to allow the perfect alignment of the p-type lone pairs atX with the it-systan, the varying degrees of pyramidalization can be observed for X=N as already discussed. When the second lone pair is present (X=0, S), rotation around the 0-R and S-R bonds is controlled by stereoelectronics in a well-defined manner that arranges this lone pair antiperiplanar to the orbital. As with aldehydes, esters etc., this situation again illustrates the interplay of different effects that manifest themselves in different observed properties 7i-effects (n -) n ) are important for reactivity whereas sigma effects (n o ) define shape of the molecules. 

Figure 3.12 Examples of versatile transformations involving a regioselective Mizoroki-Heck arylation sequence with heteroatom substituted alkenes.

Reactions of carbonyl compounds with these gem-dizinc species gave the corresponding heteroatom-substituted alkenes. In these reactions, the addition of a titanium salt was necessary. jS-TiCls was used, except in the case of reaction of a-boryl-substituted gem-dizinc, where TiCU was used instead (Table 5.4). 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

Many chiral diphosphine ligands have been evaluated with regard to inducing enantioselectivity in the course of the hydroformylation reaction [25,26]. However, a real breakthrough occurred in 1993 with the discovery of the BI-NAPHOS ligand by Takaya and Nozaki [65]. This was the first efficient and rather general catalyst for the enantioselective hydroformylation of several classes of alkenes, such as aryl alkenes, 1-heteroatom-functionalized alkenes, and substituted 1,3-dienes, and is still a benchmark in this area [66,67]. But still a major problem in this field is the simultaneous control of enantio-... 

Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptor-substituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. 

Non-heteroatom-substituted vinylcarbene complexes are readily available from alkynes and Fischer-type carbene complexes. These intermediates can undergo the inter- or intramolecular cyclopropanation reactions of non-activated alkenes. Cyclopropanation of 1,3-butadienes with these intermediates also leads to the formation of cycloheptadienes (Entry 4, Table 2.24). 

J Non-Heteroatom-Substituted Carbene Complexes Table 3.21. Preparation of alkenes and dienes by cross metathesis. 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

Electron-deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, which usually undergo further conversions. Thus, treatment of thiiranes with substituted carbene often gives the corresponding alkenes in good yields by electrophilic attack by carbene on the sulfur atom (75JA2553). 

Alkanes can be prepared by the addition of carbon radicals to C=C double bonds (Figure 5.4). The highest yields are usually obtained when electron-rich radicals (e.g. alkyl radicals or heteroatom-substituted radicals) add to acceptor-substituted alkenes, or when electron-poor radicals add to electron-rich double bonds. These reactions have also been performed on solid phase, and polystyrene-based supports seem to be particularly well suited for radical-mediated processes [39,40]. 

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