Is there any ethical issue which prevents me from paying high quality authors monetary compensation, to agree to write papers with me?

I think the other answers are way too nice. As for myself, I have never heard of such a insulting / distorted idea about what a PHD is. Do you want to buy yourself a PHD? There are countless diploma mills for that, so you could hang a useless paper on your wall to claim you have a PHD without doing all the necessary work / research yourself. Do you want to buy yourself an academic career by hiring established professors to write papers for and with you? That is even more insulting. If that was the case, every rich kid could tell his / her dad. "I want a PHD for Christmas. Buy me some high impact authors to write papers with me. Pleaaase..".

It does not work that way. And the fact that you are at the beginning of your PHD and you are even considering this, is even more infuriating for people who have busted their behinds to get a PHD (and some of the people in Academia SE still do to this day). Reconsider why you want a PHD and what you want a PHD for. I am mostly sure you want it for the all the wrong reasons and not because you actually want to do research. In that case, you should probably spare yourself the agony, because without a passion for research your PHD road will be a long, rocky one.

UPDATE: In retrospect, the tone of my answer is pretty hostile. Still, the upvotes showed that my initial reaction to the OP's question also expressed an important part of the Academia SE community. In this sense, my answer might still be useful if it makes the OP reconsider his "thought" of hiring "high-impact" authors to write papers with him, since such an action will probably cause much more damage to him than my "harsh" words.

Moreover, since we had a previous question about "Is it ethical to hire a programmer for ... my CS PHD" and now "Is it ethical to hire high-impact authors ... to write papers with me", I sincerely hope that in the future we do not get any other questions like "Is it ethical to hire a professor to write my PHD thesis if I still do the typesetting in Latex and all spell checking" or "Is it ethical to hire a professor to do my PHD defence, since I will still bring the pizza and the drinks".

If you are simply writing scientific papers, I don't think it is inherently wrong about hiring a collaborator. I do it all the time when I offer a student an RAship to work on a project and the student ends up as a co-author. It's a bit unusual that the student would be paying the professor, but I don't think there's an ethical problem there. (And in fact I do know of a few cases where a well-off individual has worked out an arrangement of this sort, albeit with a postdoc rather than someone more senior.) The only ethical issue I can see is that if the authors are required to disclosure their funding, the hired author might have to disclose that you had provided the funding. My student RAs always do so, after all -- we acknowledge the grant that supported them.

If the work is to be part of your thesis, things get more complicated. You would want to be very clear that you had not violated any university requirements for the preparation of the thesis. My gut feeling is that paying high-impact coauthors to help with the chapters crosses a line, but I can't nail it down to some particular rule. And obviously (I hope), paying anyone on your PhD committee would almost certainly create a conflict of interest that would be a significant breach of ethics.

All of that said, there most definitely is a reputational issue here. Word will get out that you are paying people to collaborate. Depending on your goals, this may not be in your advantage.

Is there any ethical issue which prevents me from paying high impact authors monetary compensation, to agree to write papers with me?

Assuming you are honest and open about what you are doing, and you don't violate any of the usual ethical rules about collaboration and authorship, I doubt you can get in any trouble for doing this.

However, your collaborators might get in trouble if they accept your proposal. If they are already being paid to do this work, then their employer or funding agency will be unhappy to hear that they are making side deals for extra money, so you'll have to be careful in how this is set up. Furthermore, being paid by a student to collaborate looks terrible, since it suggests they are exploiting you or extorting money from you. Even if they insist it was your idea, I'm sure administrators will be unhappy with them.

In any case, this would be bad for everyone's reputation, as Corvus pointed out at the end of his answer. You'd effectively be announcing that you can't convince people to work with you without offering them money, and your collaborators would be announcing that they sold out and decided to earn money via a project they wouldn't otherwise consider worth their time. Unless you work in a field with an exceptionally strong commercial ethos, neither side would end up looking good.

Instead of offering money, it's worth rethinking how you are trying to attract collaborators. For example, are you trying to recruit people to work on your own ideas? That can be a tough sell, since they probably have lots of exciting projects already taking up their time (otherwise you wouldn't be interested in them in the first place), and they have little idea of how skilled you are or how good your ideas are. Instead, you might start by seeing whether you could contribute in some way to their current projects. If you impress them by doing so, then they may be much more open to other ideas you have.