Synthesis of a Phenylphosphabenzene Ligand to Improve a Cobalt Catalyst to Improve Dimerization for

Linear α-olefins

Jade Willey, 2017

Linear alpha olefins, or terminal alkenes, are used in the production of synthetic oils, detergents, and low-density

polyethylene. In 2012, North America accounted for 50% of world production. The current method used by industry to

produce linear alpha olefins is oligomerization. Oligomerization is a full-range process that produces linear alpha olefins

4-30+ carbons long. However, the most marketable linear alpha olefins contain 8-18 carbons. Dimerization is one method

being studied for the production of linear alpha olefins; e. g.1-hexene, is coupled with another 1-hexene, to produce 1-

dodecene. Dimerization could be a more efficient process, both economically and ecologically, than oligomerization.

The Broene lab is modifying a cobalt catalyst coordinated to Cp*

(pentamethylcyclopentadienyl), trimethylphosphite, and ethene to be a more efficient

dimerization catalyst (shown top right). The cobalt catalyst has been shown to

produce a ratio of branched to linear alpha olefins of 4.5:1. Our hypothesis is that the

current ligand, the trimethylphosphite, which has a cone angle of 107° and freely

rotates in space, does not favor 2,1-insertion due to the sterics (shown bottom right).

Faculty Mentor: Rick Broene

The ligand we explored to facilitate dimerization is phenylphosphabenzene. The

phenylphosphabenzene has been shown to have a cone angle of 108, which is a

larger cone angle than the trimethylphosphite, however the cobalt can

backbond to the LUMO of the phenylphosphabenzene. This backbonding

hinders rotation, effectively eliminating the cone. By utilizing a ligand that will

be more planar, the complex will be sterically less hindered and will favor 2,1-

insertion, and therefore, more linear alpha olefins will be produced via

dimerization.

This summer, the immediate precursor to the phenylphosphabenzene was synthesized in five steps in 2.7% overall

yield. The phenyl(trimethylsilyl)ethynyl ketone was formed through palladium-catalyzed addition of trimethylsilyl (or

TMS) acetylene to benzoyl chloride in yields up to 100%. Grignard addition of ethynylmagnesium chloride to the

ketone formed the 3-hyrdoxy-3-phenyl-1-(trimethylsilyl)-1,4-pentadiyne in yields up to 84%. The alcohol was

methylated via phase transfer using 18-crown-6 and diemethylsulfate in yields up to 100%. Dibutyltin dihydride was

formed with LiAlH4 by reduction of dibutyltin dichloride in yields up to 47%. The hydride was used in the radical-

promoted (benzoyl peroxide) hydrostannylation of the 3-methoxy-3-phenyl-1,4-pentadiyne to form the 1,1-dibutyl-4-

methoxy-4-phenylstannacycle-2,5-diene (shown in HNMR below).

In the future, more experiments

involving the hydrostannylation

reaction exposed to oxygen will be

conducted to see if higher yields of

the stannacycle can be obtained.

After transmetalation of the tin for

the phosphorous atom and the

phenylphosphabenzene is

synthesized, future work will be to

coordinate the

phenylphosphabenzene to the

cobalt catalyst to see how efficient

the catalyst is at producing linear

alpha olefins.

Funded by: American Chemical Society, Petroleum Research Fund

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