How voltage is related to frequency in CPU?
Generally, there is none, the frequency is driven by an oscillator, whose frequency may be sometimes changed dynamically.

Consider this :


Higher voltages makes faster transistors (stronger electric fields for moving electrons).
More transitions (1->0 or 0->1) and more voltage makes more power dissipation (static power due to leakage and dynamic power due to charge transfers across resistive material).
There is a limit on the amount of power that may be generated by a chip (there is a maximum working temperature for silicon chips, around 100...150°C)
With higher temperature, silicon becomes more resistive and leaky, reducing efficiency.


All that stuff combines to the fact than, when properly cooled, with a fan, liquid nitrogen or whatever, a CPU may be able to work reliably at higher frequency than without.

The adjustement of voltage and frequency in relation to the temperature and CPU load is generally part of a regulation system where frequency is lowered when the temperature is too high (to reduce dynamic power dissipation) or when the computer is not used (to save energy). With slower frequency, it may be possible to lower the voltage (to save energy, reduce dissipation) as the transistors don't need to be as fast.

Asynchronous designs, where the working frequency is directly related to the performance of transistors, using the propagation time across gates, delay loops, ... are practically never used : Chips are characterized, with some margins (process variations, aging...) to determine a suitable range of voltages, frequencies and power dissipation.