Cross metathesis productive

They correspond to the cross-metathesis of propylene with the neopentyli-dene fragment (Scheme 18), and their relative ratio corresponds to a photograph of the active site as they are formed. Depending on how propylene will approach the carbene, it will generate different metallacyclobutanes, whose stabilities can direct the relative amounts of cross-metathesis (and selfmetathesis) products. This model is based on the following the favoured cross-metathesis product arises from the reaction pathway, in which [1,2]-interactions are avoided and [1,3]-interactions are minimized (here shown with both substituents in equatorial positions) [83]. 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

Using an equimolar quantity of allyl methyl sulphide and ds-pent-2-ene resulted in incomplete reaction of the allyl sulphide and some self-metathesis of the sulphide substrate. When an excess (4 equiv) of but-2-ene was used, however, the desired but-2-enyl sulphide was formed in a good yield at ambient temperature. In this case, the large quantities of unwanted hydrocarbon starting material and self-metathesis products were gaseous alkenes and therefore easily removed. Using a large excess of one alkene to improve the yield of the desired cross-metathesis product in this way is obviously only viable if this alkene is inexpensive and both it and its self-metathesis product are easily removed. 

The two alkenes were so similar electronically and sterically, with the ester group too far away to have any affect on the double bond, that there was very little cross-/self-metathesis selectivity. An approximately statistical mixture of ester 13 and diester 14 was isolated. The high yield of the cross-metathesis product 13 obtained is due to the excess of the volatile hex-l-ene used, rather than a good cross-/self-metathesis selectivity. Although not as predominant as in the reactions involving styrene, trans alkenes were still the major products. 

During the past 2 years several research groups have published research that either uses or expands upon Crowe s acyclic cross-metathesis chemistry. The first reported application of this chemistry was in the synthesis of frans-disubstitut-ed homoallylic alcohols [30]. Cross-metathesis of styrenes with homoallylic silyl ethers 15, prepared via asymmetric allylboration and subsequent alcohol protection, gave the desired trans cross-metathesis products in moderate to good yields (Eq. 15). 

Barrett and Gibson also reported the application of the molybdenum catalysed cross-metathesis reaction to the elaboration of (3-lactams [31,32]. Protected allyloxy p-lactams 16 were successfully cross-metathesised with a selection of substituted styrenes to yield trans cross-metathesis products (Eq. 16). 

As with the allylsilane cross-metathesis reactions, significant quantities of allyl stannane self-metathesis were not detected in any of the reactions and the trans isomer predominated in the cross-metathesis products. Identical reactions were carried out using allyltributyl stannane, in place of allyltriphenyl stannane, but the yields of the cross-metathesis products were consistently lower and in many cases dropped below 25%. 

Blechert and co-workers have also reported the application of cross-metathesis to the synthesis of jasmonic acid derivatives containing modified alkene side chains [28]. Molybdenum or ruthenium catalysed cross-metathesis of acetal 20 with various alkenes gave the desired cross-metathesis products in high yields (Eq. 21) (Table 2). 

Using the ketone instead of the acetal 20 caused a considerable drop in the yield of the cross-metathesis products, which was due in some cases to competing olefination of the ketone (Scheme 4). 

Although the metathesis reaction with allylglycine 23 did not go to completion, a moderate yield of the desired cross-metathesis product was isolated. Very recently, Blechert has reported two similar cross-metathesis reactions of an allylglycine 25 using the ruthenium catalyst [44]. In these cases higher yields of the cross-metathesis products were isolated, presumably due to the higher reaction temperatures employed (Eq. 26). 

Ring-opening cross-metathesis of unsymmetrical cyclobutenes was also accomplished, although an extra complication arises due to the possible formation of two regioisomers (for example 39 and 40) of the desired cross-metathesis product (for example Eq. 32). 

Snapper proposed that the selectivity for the formation of cross-metathesis products 41 observed in these reactions was due to the differing reactivities of the various ruthenium alkylidene species formed in the catalytic cycle (Scheme 6). 

Scheme 6. Catalytic cycle leading to the formation of the desired cross-metathesis product 41...

Replacing hex-3-ene with trans-1,4-dimethoxybut-2-ene resulted in slightly slower reactions, but gave comparable yields of cross-metathesis products. The desired reactions did not take place, however, when ris-but-2-ene-l,4-diol was used as the acyclic substrate. 

Reaction of this same cyclobutene substrate 48 with a terminal alkene (TB-DMS protected pent-4-en-l-ol) gave a good yield (84%) of the cross-metathesis products, but with very little regioselectivity (3 2 mixture of regioisomers). 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Note also that, in contrast to classical heterogeneous catalysts, the initiation step of [=SiORe(=CtBu)(=CHtBu)(CH2tBu)] is well defined and corresponds to the cross-metathesis of the alkene with the neopentyhdene ligand. In fact, in the metathesis of propene, 0.7 equiv of a 3 1 mixture of 3,3-dimethyl-l-butene and 4,4-dimethyl-2-pentene is formed (Figure 3.27) the nearly quantitative formation of cross-metathesis products is consistent with a real single-site catalyst. Moreover,... 

ALkyne cross-metathesis is also achieved using I52/CH2CI2. Even in the case of a 1 1 mixture of two different alkynes, cross-metathesis product 147j is produced in 71% yield using I52/CH2CI2 catalyst ... 

As described above in Eq. 43, simple allylboronates can be transformed into more elaborated ones using olefin cross-metathesis. " Treatment of pinacol allylboronate 31 with a variety of olefin partners in the presence of Grubbs second-generation catalyst 142 smoothly leads to formation of 3-substituted allylboronates 143 as cross-metathesis products (Eq. 104). Unfortunately, these new allylic boronates are formed as mixtures of geometrical isomers with modest E/Z selectivity. They are not isolated but rather are treated directly with benzaldehyde to give the corresponding homoallylic alcohol products in good yields (Table A). 

Scheme 17 Diels-Alder reaction of cross-metathesis products.

Cross-metathesis of hex-l-ene with a four-fold excess of tetradec-7-ene, catalysed by WCl6/Et20/Bu4Sn at 50 °C, results in a 90% conversion of hex-l-ene and a selectivity of 90% for the cross-metathesis product dodec-5-ene153. 

Cross-metathesis of equimolar amounts of styrene with symmetrical alkenes occurs on Re207/Al2C>3 at 50 °C. 85-90% of the styrene is converted to 1-phenylalk-l-enes and only 10% to the self-metathesis product stilbene171. Likewise, the reaction of styrene with 0.5 equiv oct-l-ene catalysed by 8 gives >85% of the cross-metathesis product (>95% trans) and <4% of the self-metathesis product of oct-l-ene172. 

The results of some cross-metathesis experiments for a series of nitriles CH2=CH(CH2) CN reacting with c -hept-3-ene are summarized in Table 4. No crossmetathesis occurs with acrylonitrile (n = 0). For n = 1, 2, 5, 8, 9 cross-metathesis products are formed in substantial amount, but for n = 3, 4 very little reaction occurs, an effect which is attributed to intramolecular coordination of the nitrile group to the metal centre in [Mt]=CH(CH2) CN (n = 3, 4), thereby reducing its metathesis activity or causing its destruction. With n > 5 the nitrile group has little influence on the reaction and its self-metathesis is preferred over that of hept-3-ene, whereas the reverse is true for n = 1,2. 

In a similar way, 126 was converted into the corresponding disilane metathesis product 127 (Equation 19) <19950M2556>. The reaction of equimolar amounts of bis(isopropylidene)disilacyclobutane 128 and 3,4-benzo-1,2-disilacyclobutane 126 in the presence of a catalytic amount of Pd(PPh3)4 gave the cross-metathesis product 129 accompanied by a minor amount of homo-metathesis product 127. The structure of 129 was assigned by X-ray structure analysis (Equation 20) <1996JOM335>. 

Grubbs found that (4 a) ring-opened cyclic olefins, and then react with an acrylate to produce end functionalized linear olefins, giving a ring-opened cross metathesis product (ROCM) with two olefins with differing reactivity (equation 19). Key to the distribution of products was the relative rates of ring-opening and cross metathesis with the functionalized olefin. 

A later report describes AROM/CM of norbomyl alkenes and styrene coupling partners to create asymmetrically functionalized cyclopentanes with alkenyl groups that can be further elaborated. High yields (>98%) of trans (>98) cross metathesis products (predominantly the desired ring-opened, A-B metathesis product) can be achieved using (97a) (equation 22). 

The vinylboronates represent an especially useful set of cross partners [42]. These electron-deficient olefins give excellent yields of cross metathesis products, and also... 

The alkyne cross metathesis and metathesis polymerization can be carried out both thermally and photochemically. The nature of the catalytically active species in the thermally and photochemically activated systems is unknown. The mechanism shown below accounts for the formation of the alkyne cross metathesis products, but none of the currently proposed mechanisms are supported by solid experimental evidence. 

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