My answer: it depends.
To whom the purchasing function reports to can be different from company to company, and that’s OK.
To simplify it, if the company sees Purchasing’s primary role as delivering cost savings, the company generally positions Purchasing under Finance, reporting up to the Chief Financial Officer. If the company sees Purchasing’s primary role as supporting operations (through assuring continuity of supply, reducing risk, etc.), then the company generally positions Purchasing under Operations/Supply Chain Management, reporting up to the Chief Operations Officer or VP of Supply Chain.
As another generalization, in manufacturing I see Purchasing more commonly reporting to Operations/Supply Chain Management and in service industries Purchasing more commonly reporting to Finance.