This is not legal JSON. First ' needs to be converted to ". Second, you have extraneous u characters before each key, unless that is a paste issue.
If you have control over how the data gets to JSON, you may wish to read this Splunk answer, written by Martin Mueller, as it discusses JSON extractions at some length. If you do not control the process that produces the JSON, you may want to consider pre-processing the data outside of Splunk. Some things are simpler to do with a script than with Splunk.