Chain transfer reagents

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

Our review of the use of organoboron compounds in radical chemistry will concentrate on applications where the organoborane is used as an initiator, as a direct source of carbon-centered radicals, as a chain transfer reagent and finally as a radical reducing agent. The simple formation of carbon-heteroatom bonds via a radical process is not treated in this review since it has been treated in previous review articles [3,9]. 

In a preliminary study, in situ generated B-alkylcatecholboranes were allowed to react with PTOC-OMe under irradiation with a standard 150 W lamp. The S-pyridyl products coming from primary, secondary and tertiary alkyl radicals were isolated in moderate to good yields [88]. Based on these initial results, a procedure for conjugate addition to various activated alkenes was developed. A one-pot procedure involving hydroboration of an alkene with catecholborane followed by irradiation in the presence of five equivalents of an activated alkene and three equivalents of the chain transfer reagent PTOC-OMe was developed (Scheme 36) [88]. 

Scheme 35 Barton carbonate PTOC-OMe, a radical chain transfer reagent able to convert a C-centered radical into an O-centered radical (Eq. 35.1) and a radical initiator (Eq. 35.2)...

The rhodium-catalyzed hydroboration has opened the way to cyclization reactions starting from dienes [92], For instance, rhodium-catalyzed hydroboration of the terminal alkenyl group of an os/Tunsaturated lactone followed by reaction with the PTOC-OMe chain transfer reagent afforded the bicyclic a-S-pyridyl lactone in 63% yield (Scheme 39). After oxidation of the sulfide with m-CPBA, thermal elimination of the sulfoxide afforded the corresponding a-methylene lactone in 65% yield. Interestingly, such bicyclic a-methylenelactones are substructures that can be found in many natural products such as mirabolide [93]. 

Scheme 49 Triethylborane as a chain transfer reagent for the conversion of R - Y to R - A...

Sect. 3.2). However, this approach is limited to the few trialkylboranes that are easily available and cheap since only one of the three alkyl group is transferred. By using a triethylborane as a chain transfer reagent, the reaction could be extended to alkyl iodides as radical precursors. Bertrand [94,114] and Naito [95,97] reported both the use of triethylborane for the tin-free addition of alkyl iodides to imines. A typical example for a tentative of asymmetric addition to a glyoxylate imine is depicted in Scheme 50 (Eq. 50a). More recently additions to isatin imines were reported (Eq. 50b) as well as addition to 2H-aziridine-3-carboxylates by Lemos [100] and Somfai [101] (Eq. 50c). 

Enantioselective radical addition to AT-acyl hydrazone using triethylborane as chain transfer reagent has been reported by Friestad. Enantiomeric excesses up to 95% were obtained in the presence of copper(II)-bisoxazolines Lewis acid (Scheme 51) [115]. 

Tandem processes mediated by triethylborane involving conjugate addition to enones followed by aldol reaction are reported (Scheme 52, Eq. 52a). More recently, a tandem process involving addition of an isopropyl radical to an o ,/3-unsaturated oxime ether afforded an azaenolate intermediate that reacts with benzaldehyde in the presence of trimethylaluminum. The aldol product cyclizes to afford an isopropyl substituted y-bulyroloaclonc in 61% overall yield (Scheme 52) [116]. In these reactions, triethylborane is acting as a chain transfer reagent that delivers a boron enolate or azaenolate necessary for the aldolization process. 

PO proceeded in a living manner to yield highly regioregular polyethers with narrow MWDs. These authors also developed the immortal polymerization of epoxides where polymers with narrow MWDs were obtained with the number of polymer chains exceeding the number of initial aluminum-porphyrin complexes (Scheme I). The key in the immortal polymerization is a reversible chain transfer, which is much more rapid than the chain propagation. In the presence of an alcohol (R OH) as a chain-transfer reagent, an aluminum-porphyrin complex with a growing species reacts with R OH reversibly, so that the polymerization takes place from all the molecules of aluminum-porphyrin complex and R OH. 

Immortal polymerization of epoxides with la and an alcohol is also accelerated by co-use of bulky Lewis acid 2a. The polymerization of PO with la/2-propanol system ([PO]/[la]/[2-propanol] = 1000/1/49) in the presence of 2a ([PO]/[2a] = 1000/1) proceeds rapidly to achieve 86% conversion in 1.5 h, while the polymerization in the absence of 2a requires 380 h to reach 84% conversion (Table 1). The polyether produced in the presence of 2a has an of 900 gmoP and an MJM of 1.10, which indicates that almost all of la and 2-propanol participate in the initiation of the polymerization. Other protic chain-transfer reagents, such as methanol, benzyl alcohol, and 4-/ r/-butylphenol, are also applicable to the high-speed immortal polymerization to give similar results as 2-propanol. As a substrate, ECH is also employable. Polymerization of ECH ([EGH]/[la]/[2-propanol]/[2a] = 1000/1/49/1) gives a polymer with and/n of 1100gmol close to the value estimated from the conversion and [PO]/([la] + [2-propanol]) ratio, and a narrow M IM of 1.10, while the conversion is lower than the case of PO. 

The activity of chain-transfer reagents is a function of the reaction temperature, concentration, and monomer type. 

Another very important way that a growing chain may be terminated is by chain transfer. This stops the chain but starts a new one. Thiols, such as phenyl-methanethiol and dodecanethiol, are efficient chain-transferring agents. The reactions involved are as follows (where M represents monomer and RSH represents the chain-transfer reagent) ... 

A radical eyclization was conducted with 2,2 -a/obisisobutyronitnle AIBN (37) as the radical initiator. Tributyltin hydride serves as the chain transfer reagent. Radical 38 arises from halide 15 through abstraction of an iodine atom, and this in turn cyclizes to radical 39. Compound 39 then abstracts a proton from tributyltin hydride. The resulting tributyltin hydnde radical reinitiates the radical mecha msm, in that it abstracts an iodine atom from another halide molecule 15 (see Chapter 14). 

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