Polyhomologation

The polyhomologation of 1-boraadamantane THF by dimethylsulfoxonium methylide formed a series of new macrocyclic trialkylboranes 24, oxidation generated a three-armed star polymethylenic polymer, based on a c/s,m-l,3,5-trisubstituted cyclohexane core <03JA12179>. 

In the rearrangement reaction, the multistep sequence offers the definite advantage of avoiding the polyhomologation of cyclic ketones, often encountered with diazoalkanes due to the presence of the reagent with both the starting and reananged ketone. 

Polyhomologation. Th triorganoboranes. Besides oxid boron atom by a hydroxylated ( compounds with trident carbon... 

Dimethylsulfoxonium methylide. 14,152 15,147 16,146 17,126-127 18,148 19,139 Polyhomologation. The ylide provides the methylene unit in chain extension of triorganoboranes. Besides oxidative workup to generate alkanols, the replacement of the boron atom by a hydroxylated carbon on reaction with dichloromethyl methyl ether gives compounds with trident carbon chains. 

Scheme 12.15 Polyhomologation of boranes to produce cyclic poly(methylene) [55].

In this review, we describe the polyhomologation reaction, a living polymerization that results in the synthesis of linear polymethylene with no branches, controlled molecular weight, narrow molecular-weight distribution, well-defined topology and functionality. Polymethylene is a unique member of the polyethylene family. The carbon backbone is built up one carbon at a time, in contrast to the two carbons in ethylene polymerization (Jellema et al., 2010). The polyhomologation reaction can provide polymethylene samples for model studies of structure-property relationships and functionalized polymethylene for copolymers that can function as polyethylene compatibilizers. In addition, since many vinyl compounds do not readily polymerize, the polyhomologation reaction can serve as an entry to completely new substances. 

The curve in Figure ll.ld is a plot of a simulated distribution of a theoretical polymer produced by an ideal living polymerization (Zhou, 2002). The plot is generated by a Poisson distribution function (Flory, 1940). The polymer distribution (FDMS) produced by the polyhomologation reaction closely approximates the Poisson distribution. This is strong support for the living nature of the polyhomologation reaction. 

Scheme 11.3 The tris(4-methoxyphenylethyl)borane 4-initiated polyhomologation of ylide 1 produces an intermediate three-arm star polymethylene 6. Oxidation affords linear a-hydroxy-(U-(4-methox3fphe-nyl)polymethylene 5.

The polyhomologation reaction has been used to prepare polymethylene with aMn as high as 354 KDa corresponding to an intermediate tris(polymethylene)borane with Mn> 1.0 x 10 (Busch et aL, 2002). A of 1.06 million for the star tris(polymethylene)borane represents approximately 7.6 x 10 turnovers per boron atom. The reaction is complete in <10 min, from which one estimates a lower limit turnover frequency of 6.4 x lO g of polymethylene (mol boron) h at 120 °C. The turnover frequency for the boron catalytic center is comparable to some of the most efficient homogeneous ethylene polymerization catalysts, such as neutral anilinotropone-Ni(II) (8.8 x 10 g of polyethylene (mole catalyst) h ) (80 °C, 200 psi ethylene) (Hicks and Brookhart, 2001). 

The rate-determining step of the polyhomologation reaction is a 1,2-migration of an alkyl group from boron to carbon in the zwitterionic complex 7 (Busch et al, 2002). Structural analysis of zwitterionic complexes (X-ray crystallography and computational study) provides insight into the rearrangement. 

I. 13. In this example, with a postpolymerization modihcation of a polyhomologated organoborane, initiator/catalyst BEt3 produces three-arm polymethylene architectures with well-controlled molecular weight and low PDI. These materials are not readily available by conventional ethylene polymerization. 

Table 11.3 Light scattering, viscosity, and GPC data for three-arm starpolymethylene 14 (Reprinted with permission from C.E. Wagner, J.-S. Kim and K.J. Shea, The polyhomologation of 1-horaadamantane Mapping the migration pathways of a propagating macrotricyclic trialkylhorane, Journal of the American Chemical Society, 125, 40, 12179-12195, 2003. 2003 American Chemical Society.)...

Figure 11.7 Proposed reaction pathways at the initial stage of the polyhomologation of 1-boraada-mantane-THF 12. The bold arrows indicate the main pathway determined by a combination of calculated activation energies and experimental findings. The compounds in the highlighted boxes (21, 22, 23) are the terminated organoborane intermediates recovered from the polymerization reaction.

Other novel polymethylene architectures can be obtained by related stitching reactions of polyhomologated organoboranes produced from structurally novel initiators. For example, the... 

Figure 11.9 Gas chromatograph of the distribution of macrocyclic ketones 31. HRMS found the most abundant ion at t = 19.4 to be 322.3222, the calculated for n = 21 is 322.3235. (Reprinted with permission from K.J. Shea, S.Y. Lee and B.B. Busch, A new strategy for the synthesis of macrocycles. The polyhomologation of boracyclanes, The Journal of Organic Chemistry, 63, 17, 5746-5747, 1998. 1998 American Chemical Society.)...

---