Homo-metathesis

Consider a telomer being formed from a cyclopentenyl polymer growing under the pairwise mechanism (Scheme 12.14) with growth being curtailed by cross-metathesis under two extreme conditions (i) with only pent-2-ene present (C4 C5 C6 = 0 100 0) and (ii) with a fully equilibrated mixture of acyclic monoalkenes (C4 C5 C6 = 1 2 1). Under condition (i), one would expect the formation of only hierarchical telomers (n = 1,2,3,4,5, etc.) of the type (C2)-[(cyc-C5) ]-(C3) as the pent-2-ene is split into a C2 and a C3 unit across the growing cyclo polyene. In contrast, under condition (ii), one would expect each hierarchical telomer to be formed in a 1 2 1 ratio of (C2)-[(cyc-C5)n]-(C2) (C2)-[(cyc-C5) ]-(C3) (C3)-[(cyc-Q)n]-(C3)> depending on whether there is cross-metathesis with C4, C5 or C6 (ratio = 1 2 1). The outcome will thus depend on how quickly the pent-2-ene is equilibrated by homo-metathesis to yield the C4, C5 and C6 mixture. Analysis of the rate of pent-2-ene homo-metathesis showed that it was not fast. Indeed, it proceeded at approximately the same rate as the telomerisation reaction. One would thus expect the telomer product early in the reaction to be essentially pure (C2)-[(cyc-C5) ]-(C3) species. Then, as C4 and C6 increase in concentration relative to C5, formation of the (C2)-[(cyc-C5) ]-(C2) and (C3)-[(cyc-C5) ]-(C3) telomers should increase proportionally. This was not found to be the case. 

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>. 

Cross-metathesis. Functionalization of terminal alkenes by the metathetic method using catalyst 1 has been well established. The reaction between styrene and vinylsilanes gives (o-silylstyrenes, between allylarenes and acrylonitrile leads to 4-aryl-2-butenonitriles. Alternatively, homo-metathesis of two allylarene molecules to give 1,4-diary 1-2-butene is first carried out and the cross-metathesis follows. Also of interest is the homo-metathesis of monosubstituted allenes to symmetrical allenes. ... 

The first conventional catalysts based on tungsten and molybdenum complexes exhibited very low activity in the homo-metathesis of allylic-substituted silanes [2]. 

Starting with -hexane (Cg) metathesis, dehydrogenation should give the corresponding 1-hexene, followed by its homo-metathesis to yield ethylene and decene, which upon hydrogenation, should ideally produce ethane and decane (Cj q products) as the major products (Scheme 2.18, path a). However, this tandem reaction process was not selective since -decane represented <50% of the total primary products of heavy alkanes when the reaction was catalyzed with Ir-2(H2). The authors attributed this unexpected distribution of alkanes to the isomerization of the (x-olefin prior its metathesis, as depicted in pathway b (Scheme 2.18). 

The major products were found to be 1-phenyloctane, 1-phenylheptane, and the homo-metathesis of n-octane, along with other, linear alkanes. TONs obtained from the olefin metathesis and (de)hydrogenation transfer were 31 and 50, respectively. This scope of this reaction was also extended to include branched alkanes and alkyl silanes with lower yields (6-25%). 

Using the olefin metathesis reaction itself to prepare symmetrical olefins is an elegant and straightforward method to synthesize suitable substrates from terminal olefins. Kurzhals and Binder [43] used such a homo metathesis approach to prepare a symmetrical olefin for terminal CM. Barbiturate and thymine polymer end groups were successfully introduced in this way. 

Figure 1. Olefin metatesis RCM = ring-closing metathesis, ROM = ring-opening metathesis, ADMET = acyclic diene metathesis polymerization, ROMP = ringopening metathesis polymerization, CM = cross-metathesis, HM = homo-metathesis.

It was found that vinylsilanes containing two chloro-substituents at sihcon underwent highly stereoselective homo-metathesis in the presence of I or n. The reactions allows for the complete conversion of vinylsilanes and led to moderate to high yields of -l,2-bis(silyl)ethenes (Eq. 10) [16]. 

The reactions were accompanied by competitive olefin homo-metathesis. On the contrary no divinylsilane homo-metathesis was observed. Selected results are summarized in Table 1. 

Similarly, the cross-metathesis of divinyl-substituted disiloxanes with olefins proceeds in the presence of 5-10 mol % of catalyst I, II, or III in boiling CH2CI2 proceeded stereoselectively producing a mixture of monosubstituted and disubstituted vinylsilanes and ethene and a products of competitive olefin homo-metathesis (Eq. 12). Under the optimum conditions the products were obtained with moderate to high yields. Selected results are summarized in Table 2. 

Class 2 eonsists of vinylsilanes that exhibit activity both in Aowo-metathesis and cross-metathesis with olefins. Only one substituent motif , that is H2C=CHSiCl2R (where R = alkyl, aryl, OSiMes) was found to be eharaeteristie of this elass. Cross-metathesis of H2C=CHSiCl2R with olefins eatalysed by n or IH gives cross-metathesis products with high yields and selectivities. For these silanes, care must be taken to choose the proper reaetion eonditions as well as the vinylsilane to olefin ratio because crossmetathesis is aeeompanied by competitive homo-metathesis of vinylsilane and olefin. 

---